The Use of a Photometer to Detect Luminance for the Visually
Challenged
BE 310: Independent Laboratory Project
Group M8John Ho
A. Laurance MichaelsAnthony NapoliKarl Orishimo
Introduction:
For this laboratory project, an optoelectric device was constructed to judge an
obstacle’s distance from the user. This device used the infrared spectrum as the light
source from which the distance could be determined. The theory behind this device is
based on the fact that as the person gets closer to an obstacle, the intensity of infrared
being detected by an infrared sensitive detector will increase. Therefore, by using an
infrared sensitive phototransistor, the voltage will increase as the device detects a higher
intensity of infrared light. This device, coupled with a voltage-regulated timer chip,
would produce a discernible sound change when the person gets closer to an object. The
preliminary testing yielded reproducible results of 1.403 ± 0.93 feet. This means that
when a person approaches an obstacle, at 1.403 feet, the device would produce a
significant frequency change from the speaker. This device was developed successfully,
however, there was not ample time to fine tune the device for optimal performance.
Future testing would include incorporating different, perhaps improved filters in the
black box and also increasing the volume outputted so that the frequency change would
not be so faint.
2
Background:
Until now, the only aid used by blind people was the walking stick. By swinging
the stick back and forth and tapping the ground, the person can get a sense of the
obstacles near them. Using this device for long periods of time may become an
inconvenience and even a source of embarrassment to the user. The aim was to construct
a device which essentially serves the same purpose as the walking stick while being a bit
more inconspicuous. This device will allow a blind person to freely move about and
avoid injury without feeling self-conscious.
This device will also benefit non-blind people by helping to dispel stereotypes
and generalizations about blind people. Because the blind person using the device is less
conspicuous, they will integrate more with society. Seeing how independent these people
are, the non-blind members of society will more readily welcome them into normal,
functioning society. With the device, both the blind and the non-blind will share the
benefits of increased interaction and integration.
Although no device like this is currently out on the market, similar devices do
exist. For example, Realtors use an IR-based device to measure the dimensions of rooms.
The device emits an IR beam which strikes a wall in the room and is then reflected back
into the device. A chip in the device measures the time for the IR beam to reach the wall
and be reflected back. Using the speed of light and the time taken for the beam to return
to the device, the distance to the wall can be calculated. This distance is then displayed
via a LED or LCD display.
When dealing with light and reflection, one important issue must be realized and
addressed. Because different surfaces reflect light in different ways and also because the
reflected beam does not always have the same intensity level as the original, one must
attempt to maximize the original signal and minimize interference from other forms of
light. Because the transmitter sends out the beam in almost all directions a lens is needed
3
to focus the beam in a particular direction. Similarly, the receiver, most of the time, is
generalized and functions when any and all light is shined on it. It is of great importance
that the receiver be specialized to receive only the reflected beam and not any ambient
light from the environment. The most common method of doing this is by using a filter
that only passes light which is the same as the originally transmitted beam. Even with the
filter and lens, some difficulty may be encountered in transmitting and receiving the
signal because the reflection off of some surfaces is not always exact. Instead of
completely reflecting the original beam, some surfaces may scatter the beam, hence
losing some of the original intensity.
There are multiple types of measurements for the determination of light, however
relevant photometric quantities are radiance and luminance. Radiance is the density of
light per unit surface area while luminance is the intensity of light per unit surface area.
There is an equation that defines the relationship between luminance and radiance:
L K V Lm e
where KmV represents the luminous efficiency and Le is the radiance. (Le Grand, p.75)
Since this device measures the intensity of light, luminance is a more relevant
measurement
The study of optoelectrics combines both the technologies of optics and
electronics. Optoelectric devices are mainly concerned with the interface between
electromagnetic radiation in the form of visible and infrared light and electronic circuits.
Most optoelectrical devices operate over a light spectrum with approximate ranges of 300
nm to 1100 nm. Included in this range are visible light (400 nm to 600 nm) and infrared
light (700 nm to 1000 nm).
Optoelectric circuits are primarily used for two purposes, emission and detection.
Emission involves converting an electrical signal into a light source while detection
converts light into an electrical signal. A photon is emitted when excited electrons fall to
lower energy levels. This transition from one energy state to another is perceived in the
4
form of light energy. Light emission usually occurs in the semiconducting material of the
optoelectrical component. The materials that are most often used for this purpose, and are
most responsive to light, are Germanium and Silicon. The simplest form of an
optoelectric emitter is a light emitting diode (LED). Like a regular diode, an LED only
passes current in one direction. When current is directed in the forward direction (anode
positive, cathode negative), light energy is given off. There is little current flow or light
emission until the forward-biased voltage is equal to or greater than the forward voltage
drop of the LED. Because little or no current flows in the reverse direction, no light is
produced in the reverse-biased condition. By alternating the input signal to the LED, a
signal emitter or a signal indicator can be produced. One must be careful not to raise the
voltage above the LED’s threshold because when the LED’s threshold is surpassed, an
short circuit is created. A short circuit is when the voltage goes to zero and the current
goes to infinity.
The second type of optoelectric device is an optical detector. These devices
transform light energy into an electrical signal. One of the most common light sensors is
the phototransistor. A phototransistor is the same as a normal transistor with the
exception of a small glass window which allows light to strike the base of the transistor.
In a dark environment or when no light shines on the phototransistor, there is very little
base current flow. Under these conditions the circuit is basically open. When light strikes
the phototransistor, a large current flows through the emitter and the circuit is closed.
The intensity of the light striking the phototransistor controls the amount of current
flowing through the circuit. Using emitter and collector resistors, both positive and
negative outputs can be produced provided that light is striking the phototransistor.
Another important aspect of the device design is the safety precautions that must
be observed during the course of constructing and utilizing the device. The first safety
precaution deals with the power source. Care has to be taken to find two batteries of the
same potential. If a new and a slightly used battery were to be connected, the resulting
5
ground would not necessarily be zero volts. Instead, it would have some voltage equal to
the differences of the volts coming into the point to be used as ground. Therefore, there
should be a warning on the device, such as:
“When using this device, both batteries must be replaced simultaneously.”
In this experiment the voltage was initially about 8.5 volts. (most nine volt batteries are
less then nine volts even when new). However, it is important to note that when the
battery drops to half of its original voltage, not enough voltage will be supplied to the
oscillator because device limitations specify that the 555 timer needs a minimum of 4.5
volts. This limited voltage range, which would result in a limited duration of use, should
also be made clear to the user. Another danger that comes from batteries with different
voltages is the fact that batteries have an affinity to achieve the same voltages, when
connected together. Therefore, if one battery were to have more charge than another,
then the higher charged battery would create a flow of current to balance the lower
voltage battery. This type of charging can cause severe problems, such as leakage of the
battery contents.
Another consideration is that the circuit is placed in a black box. Since black
absorbs electromagnetic radiation and warmth, it is best that this device be kept out of the
sun so that the circuit resistance does not increase and so specific components such as the
555 timer will not be heated any more then necessary. The soldered components in the
circuit need to have their exposed wires taped with electrical tape, so that the circuit does
not have any shorts. As well, this will protect the user of the device in the event that the
batteries need to be replaced.
Since this device is using infrared light, it is necessary to become familiar with
this sort of wave. Infrared radiation is an electromagnetic radiation that has a shorter
frequency than humans can visibly discern. This frequency range falls below the visible
6
spectrum, but above radio waves in the electromagnetic spectrum. Infrared radiation can
be related to electromagnetic radiation using the equations:
fc
E hf
where f = frequency,c = velocity of light = WavelengthE = radiation quantum energyh = Planck’s constant
Kirchhoff derived laws of radiation to describe the nature of infrared light. These
laws are that 1) a good absorber is a good radiator, 2) a good reflector or a transparent
body is a poor radiator, 3) emissivity e can be determined by measuring the absorption a,
4) the absorption cannot exceed unity, since it is impossible for a body to absorb more
energy than the total amount radiating onto it, and 5) all the energy radiating onto an
opaque body is either absorbed or reflected away, and can be determined by 1 = a + R =
e + R (where R = reflectance) (Vanzetti, 16).
The advantages of using infrared radiation over other conventional methods of
detecting obstacles are that it is a non-contact technique and is passive (very little energy
is imposed on the target). Choosing infrared light as the device’s emission was a major
factor in the design process, and is discussed later in the potential problems section of the
report.
Finally, the project utilizes the principles of sound and human hearing. Because
the main output is a sound generated by the device, human hearing had to be taken into
consideration. There are two main aspects of sound which are central to human hearing:
frequency and intensity. The frequency of a sound wave is the distance between the
7
crests of the wave and is measured in cycles per seconds or hertz. The frequency of a
sound wave is directly related to the pitch. The pitch of the sound increases as the
frequency increases. The second aspect of sound that is important to human hearing is
intensity. Intensity is the loudness of the sound and is directly related to the amplitude of
the sound wave. Intensity is measured in decibels where an increase of one dB causes a
tenfold increase in the intensity. The human ear can perceive sound over a frequency
range of 20 to 20,000 Hz. It can also distinguish between pitches that differ by 0.3% in
frequency. The normal range of intensities that can be perceived by the human ear spans
a range of twelve orders of magnitude.
8
System Overview:
There were three basic components that had to be integrated on to a small
breadboard ( JE21 ), 3.3” long by 2.1” wide. These components were the transmitter, the
receiver, and the voltage-regulated oscillator.
The transmitter consisted of two infrared Light Emitting Diodes (LED). Two
LEDs, instead of the original one, were used to enhance the intensity of the infrared light
being returned to the phototransistor. These infrared LEDs were available in the
laboratory. A push-button switch was used to open and close the circuit. A 1KW variable
resistor with a value of 284 W was used to limit the amount of voltage going through the
LEDs. The construction and testing procedure will elaborate on the method for testing
for the correct resistance of the variable resistor. If the correct voltage was not obtained
either the LED would not light, (If the voltage was too low, the LED would not light.
Also, if the voltage was too high, the voltage would go to zero, causing a short circuit,
thereby not allowing the voltage to run through the LED) or the voltage would not be at
its maximum. It was determined that the best results would be obtained with the
maximum voltage through the LED. There were two additional pieces of equipment that
were used to enhance the transmitting and receiving of the IR LED. A Fresnel Lens ( 1”x
1”, A431794 ) was used to focus the infrared light emitted. This allowed for a more
intense emission. The lens would focus the infrared light and make the light easier to
detect. Another piece used was an infrared filter ( 1” diameter, A43948 ). This filter
only allowed infrared light to pass through. This was necessary to ensure that the
phototransistor was picking up the infrared light that was emitted from the device and not
light from an outside source (i.e. ambient light, room light). The final change made
9
between the initial proposal and the final project was the use of the 9-volt battery.
Initially, one battery was used. Yet, as the project began, it became apparent that a
positive, a negative, and a ground terminal were necessary. To accomplish this, two
batteries were used. The negative terminal of one battery was connected to the positive
terminal of the other battery. This “new” terminal was ground. This successfully allowed
all three necessary terminals to be used in conjunction.
The next portion of the device that must be understood is the receiver. A 741
operational amplifier was used to amplify the voltage through the circuit. A particular
voltage was necessary to activate the 555 timer chip correctly. This voltage was
calculated and the appropriate gain was applied. The gain was created using a non-
inverting amplifier. Calculations for the gain can be found in the testing procedure. A
non-inverting amplifier was used instead of the original proposal of a comparator circuit
for a number of reasons. First, a comparator would only provide the circuit with positive
or negative nine volts. Since it was necessary to allow for a change in voltage to vary the
oscillation in the timer chip, the comparator circuit would render the phototransistor
useless because as long as the voltage input to the v+ was larger then zero the same
voltage would be input into the oscillator. Therefore, it was determined that a non-
inverting amplifier was necessary. A non-inverting amplifier was chosen over an
inverting amplifier so that a negative voltage would not be supplied to the 555 timer
chip. The most important piece of the receiver circuit was the phototransistor. The
phototransistor was the same as the one used in Laboratory #8 of the BE 310 curriculum.
The specifications and further information on the phototransistor and the IR LED are
included in the appendix.
10
The last piece of the circuit was the voltage-regulated timer chip and the speaker.
A speaker with a resistance of 8W was used as an output device. Depending on the
voltage input into the 555 timer chip, a certain frequency was emitted from the speaker.
This allowed the user to determine his or her orientation with respect to an object. The
capacitor in the 555 timer chip serves as a voltage threshold for the timer chip to
discharge. The charge on the capacitor will range from 1/3 Vcc to 2/3 Vcc. When the
voltage reaches 2/3 of Vcc, the capacitor discharges. The frequency for this event can be
described by the equation:
frequencyR R C
144
1 2 2 1.
( )
However, it is important to note that the oscillation frequency is independent of
Vcc. The frequency of the chip is dependent, though, on the input voltage (pin 5). As
the input voltage increases, the oscillation frequency decreases. In order to produce a
frequency that the human ear can detect, the frequency must be calculated to fall between
the human hearing threshold. To increase the volume, the resistor in series and prior to
the speaker was taken out and replaced by a 4.7 mF capacitor after the speaker. This was
to increase the volume, as well as to dc-couple out the DC component of the voltage, so
the speaker would not burn out.
After all of the circuitry was complete, the final product was placed inside a black
carrying box ( H2853 ). The black box was 4.9” long by 2.5” wide by 1.5” tall. This
allowed sufficient room for the circuit and the batteries. First, two holes a ½” wide were
drilled in the front to allow openings for the infrared LED and the phototransistor. The
infrared filter was placed in front of the phototransistor. This filtered out all of the visible
light that would be received and might interfere with the phototransistor. There were two
11
theories behind the placement of the Fresnel lens. One theory was that the lens should be
placed in front of the LED. In this case, the infrared light would be focused from the
LED and a compact beam would leave the box. This would allow for a strong output.
The second theory believes that the lens should have been placed in front of the
phototransistor. In this case, the lens would focus the reflected IR that was emitted from
the LED, and focus it. It would then be received by the phototransistor. For this
experiment, it was decided that the lens should be placed in front of the emitter. This
would allow the beam to be focused initially rather then dispersed over a wider angle, as
is common of a LED. Ideally, a second lens would be added before the phototransistor
as well. After the filter and lens were installed, a hole was made for the push button
switch. This switch was placed on top of the box, to simulate a remote control device.
Care was taken to place the switch in such a way as to be ergonomically correct. The
final piece of construction on the black box dealt with the speaker. Because the sound
emitted from the speaker was faint, holes were drilled into the box in the back where the
speaker would be housed. This allowed for a larger volume to be emitted from the box.
After this was complete, testing was begun.
12
Construction / Testing Procedure:
After checking to ensure that all parts for the circuit were available and
undamaged, a testing procedure was defined and carried out. The following outlines the
procedures. Each week, a procedure was laid out beforehand for what was to be
performed in lab. These procedures will be defined along with the points of refinement
in the procedure due to adjustments determined to be necessary as a result of
experimentation.
Week 1:
The first week consisted of organizing the newly acquired materials, completing
preparations for construction of the circuit, and actual construction of the circuit. It was
decided that two members of the group would build the receiver portion of the circuit on
the breadboard to be used in the device, while another two members of the group used a
different breadboard to construct the voltage-controlled oscillator. The LED transmitter
was left out because it would be relatively trivial to build (although room was left for it
on the front end of the breadboard) and could easily be substituted later.
The receiver portion of the circuit is shown below:
13
0.1 uf
100K
741
9V
100K
10K
This circuit primarily consists of three significant areas. First, the phototransistor portion
of the circuit consists of a phototransistor powered by a nine volt battery in series with a
resistor. The resistor lowers the amount of current going into the collector region of the
phototransistor to prevent saturation. The second part of the circuit is the operational
amplifier. It is obvious that the operational amplifier is designed to act as a comparator.
When the voltage going into the inverting voltage input (v- ) is larger then the non-
inverting voltage input ( v+ ), the output at pin 6 is intended to be negative nine volts.
However, when the voltage is larger at v+ than at v- the voltage output is intended to be
positive nine volts. This output is then sent to the third major portion of the circuit, the
10kW variable resistor. This is considered a major portion of the circuit because aids in
varying the voltage going into the oscillator circuit, hence varies the frequency of the
resulting sound.
14
Construction of this circuit was completed, and it was quickly tested using a
previously constructed emitter circuit to see if the circuit was effective. When the circuit
did not work, a testing procedure was begun that consisted of checking various nodes
throughout the circuit to try and determine the error. It was determined that there must
have been a short somewhere in the circuit because the op-amp became extremely hot.
The circuit was slowly deconstructed to determine where the problem was and it was
found to be the operational amplifier. The circuit was rebuilt, with a new operational
amplifier, and tested again. This time the circuit worked, but not well. It would return
very small voltage changes at the emitter end of the diode as a result of the light from an
LED. A multimeter was connected below the diode and the diode was continuously
covered and uncovered to see if the voltage would vary. A varying voltage would show
that there was a varying degree of intensity of light entering the photodiode via the LED.
Even though there was very little sensitivity to the incoming light, the receiver portion of
the circuit was put aside for experimentation on the voltage-controlled oscillator to
determine if this portion of the circuit was effective.
The original design of the voltage-controlled oscillator is shown below. This
design is, for the most part, the same as the voltage-controlled oscillator published by
Radio Shack. (Mims, p. 15)
15
100K
555
220
100K
1K
0.01uf
Output toSpeaker
5V
R1
R2
C1R3
The voltage-controlled oscillator, as well, contains three major portions that need to be
considered. First, the 100 kW variable resistor is an important part of this circuit because
it is what determines the voltage going into pin 5. This is especially important because it
is this voltage which controls the 555 timer and changes the properties of its output,
hence the properties of the sound (how exactly it does this will be explored later).
Second, the right side of the circuit that consists of the two resistors (R1 and R2) and the
capacitor (C1) is what controls the oscillations. Without the speaker and the input into
pin 5, this circuit is almost exactly the same as an astable oscillator. Hence, the frequency
of the output can be expressed by the following equation.
FrequencyR R C
14421 2 1
.( )
16
The variable resistor is very important because it can help vary the frequency of
oscillation, hence the frequency of the sound that is produced by the speaker. By varying
this resistor, one can set the base frequency at which the speaker will produce a sound
when none of the infrared light is being returned to the phototransistor. The third portion
of the circuit is the speaker connected in series with a small resistor to the 555 timer.
The small resistor functions to limit the amount of current going through the speaker.
Since the speaker has a very small resistance, too much current flowing through the
speaker could burn it out. Therefore, a small resistor (big relative to the speaker though,
almost 30 times the size) is connected in series to limit the current and have much of the
voltage drop across it.
The oscillator was checked and then transferred to the breadboard containing the
receiver. Testing was then carried out on the oscillator because it did not seem to
oscillate. Again, certain points in the circuit were tested for their expected voltage drop.
One of the problems was that the variable resistor produced significantly different
resistances when it was in the circuit as compared to what it was set at outside of the
circuit. This had a significant impact on the oscillation frequency, especially considering
the other resistance is one hundredth its size. When the non-inverting op-amp was built,
the gain was calculated to be 22. When tested experimentally, the gain was confirmed to
be 22. However, once the non-inverting op-amp was placed in the receiver portion of
circuit, the gain dropped significantly. This could be attributed to the fact that when
placed in the circuit, the non-inverting op-amp components (mainly the resistors) were in
parallel with resistors elsewhere in the circuit (or even the phototransistor which may
17
have acted as a resistor), thereby changing the actual resistance. When the actual
resistances of the non-inverting op-amps are changed, the gain will be significantly
changed. Therefore, it was removed and replaced by a single resistor. This was simply an
arbitrary resistance, even though it was recognized that this would have an impact on the
frequency of the speaker output. However, the oscillation frequency was still within the
audible range of human hearing, so this was not a problem At this point, there was no
more time left for experimentation, so it was discontinued until week 2.
Week 2:
Experimentation began where it left off in the previous week. Since the receiver
circuit appeared to be working, experimentation continued on the oscillator portion of the
device. Since the frequency of sound output by the speaker decreases as the voltage
increases, adjusting the voltage entering pin 5 was the initial test to see if this might
result in a recognizable oscillation. Experimentation then focused on trying to ensure
that the capacitor was reaching threshold and actually discharging as an oscillator circuit
should. However, before a significant amount of experimentation was done on this
another possibility arose. The speaker may not have been receiving enough voltage in
order to produce an audible signal. In order to increase the volume of the speaker, the
220W resistor was removed and a 1mF capacitor was connected after the speaker at
pin 3, and then connected to ground. This served to increase the volume of the output
from the speaker, and the speaker immediately began to produce an audible output.
Moreover, variation of the potentiometer connected to pin 5 varied the frequency of the
signal output by the speaker. This is necessary because increasing the resistance of the
18
potentiometer would decrease the voltage going into pin 5. This would then increase the
frequency output of the speaker and allow greater control over the output. The new
oscillator circuit looks this like:
555
4.7u
Output toSpeaker
+9V
1u
270
220
R1
R2
C1C2
The circuits were then connected together with the assumption that the receiver
circuit still functioned; however, this was not the case. The system was then debugged in
the following way: Starting with the bottom of the circuit (since it was known that the
oscillator worked because it was producing a noise), the voltages at different nodes were
tested and compared to what might be expected. This meant that the resistance of the
variable resistors associated with varying the voltage going into pin 5 of the timer had to
be tested as well. Experimentation led back to the operational amplifier, when it was
determined that using the operational amplifier did not serve its intended purpose. This
was because the operational amplifier was acting as a comparator, resulting in only a
positive supply voltage or a negative supply voltage. As a result, no matter what the
19
intensity of incoming light to the phototransistor might have been, it would not have
mattered because the voltage leading to the 555 timer would always be the same. Hence,
a non-inverting operational amplifier with a chosen gain replaced the comparator. The
non-inverting amplifier that replaced the comparator is shown below.
200
741
+9V
20022K
New receiver portion
to speaker
R2R1
R3
As well, the capacitor that originally connected to v- was removed. Putting a known
voltage in and measuring the voltage out through use of an oscilloscope then tested the
non-inverting amplifier. Both R2 and the variable resistor, R3, (From the old circuit on
page 14) were removed because they were no longer necessary now that the comparator
was converted to a non-inverting amplifier. The non-inverting amplifier was then
connected to the circuit and it was found that the circuit was still not functioning
properly. Since, at this point, every portion of the circuit had been tested except the
phototransistor, experimentation focused there. Again, the original circuit design
contained an error. By having the resistor above the phototransistor as seen in the circuit
20
diagram, the phototransistor was always connected to ground at the emitter end as well as
connected to the v+. Because of this, when the phototransistor was in saturation or
cutoff it would result in zero volts at v+ either way. It was shown that the
phototransistor was in cutoff because the voltage drop was 7.7 volts across it. If it had
been in saturation, almost all of the voltage (except for 0.2 volts) would have dropped
across the resistor and there would be minimal voltage difference between the collector
and the emitter. Again, the voltage would be zero at v+ but would not coincide with the
result of a 7.7 volt drop across it. It was then determined that the circuit had to be
redesigned to have the resistor after the phototransistor. This way, if the transistor was
ever in cutoff, the voltage would be zero at v+ still, but if it was in saturation it would
approximately be equal to the potential of the battery. At this stage, it was then
determined that testing needed to be done on the phototransistor to determine its active
range. By doing so, the range over which the phototransistor would show an increased
voltage due to increased intensity of light could be determined. This was a very
important step now that the operational amplifier was changed from a comparator
to a non-inverting amplifier. The signal that was produced at the emitter as a result of
the intensity of the light could be amplified and sent into pin 5 to change the frequency
of oscillation of the speaker.
Week 3:
The procedure for this week consisted of four tasks. Since it had been determined
in the previous week that the only things not functioning correctly were the non-inverting
amplifier and the phototransistor, this was the focus of the testing. However, it was first
21
necessary to examine the voltage-controlled oscillator and test it to select a base
frequency. (The frequency at which no light is returned to the photodiode and sound is
based purely on the initially chosen oscillation frequency) The base frequency is one at
which it would oscillate independent of pin 5.
Simply putting a variable resistor in at R1 and varying it tested the oscillator. The
output was recorded via an oscilloscope in place of the speaker. The frequency at which
the circuit was oscillating was simply determined by changing the time/division knob on
the oscilloscope to the most accurate setting and recording the distance between identical
voltages. This was because the same curve was repeated at every pulse. This was done at
frequencies ranging from 16Hz. to 2631Hz. The large frequency range was necessary
because it was yet to be determined what frequencies of sound would be used. The most
important quality was which speaker frequency would be most useful in terms of both the
quality of sound emitted from the speaker and the ability of the listener to discern
changes in frequency due to changes in input voltage at pin 5.
The voltage at pin 5 was then tested to examine how the input voltage would
effect the output of the speaker. Simply using a variable voltage source at pin 5 and
again recording the output frequency from the oscilloscope accomplished this. When this
was completed, the speaker was placed in the circuit and the procedure was carried out
once again to determine the range of frequencies that the speaker could support and the
range of voltages that would produce a sound that was clear and distinct. Varying the
voltage and qualitatively assessing when the voltage corresponded to a discernible sound
accomplished this. This range of voltages would then be used to determine the gain of
the non-inverting op-amp portion of the circuit. An appropriate gain would take the
22
maximum voltage at v+ (when the user is standing the minimum distance away from an
obstruction) and multiply it to obtain the maximum input that can be received at pin 5
that could produce an audible output from the speaker. The same would ideally be the
case with the minimum input voltage to v- so that the minimum input could be received
at pin 5 when there is no obstruction at all.
The next part was the connection of the non-inverting amplifier to the circuit and
testing to ensure that it worked within the circuit. This was to be done by applying a
voltage to the v+ of the op-amp and obtaining a representative gain that would vary the
frequency of the oscillator. However, there was some difficulty in this portion of the
testing because it was found that the circuit was not constructed in such a way as to allow
for both a ground and a negative bus. The ground and negative were assumed to be the
same. This problem was corrected by using two batteries connected in series. This
created a theoretical power source of ±9 volts, or a net potential difference of 18 volt.
This obviously resulted in a positive and a negative terminal but it also allowed for a
ground because the positive of one battery was connected to the negative of another,
effectively canceling the two. When this problem was corrected, the circuit was again
tested and it worked. However, it did not work as it was expected. When the amplifier
was placed in the circuit, the gain that was calculated based on simple use of the op-amp
laws was not obtained. Gain for a non-inverting op-amp is:
GainRR
1 2
1
This problem was to be fixed later because the objective was primarily to ensure that the
non-inverting amplifier could produce a high enough voltage so that a varying sound
could be produced based on the input to pin 5. Theoretical gain was secondary to the
23
actual output of the sound. It did not matter whether the experimental gain correlated
with the theoretical gain, as long the audible sound from the speaker was heard. This
was achieved so the next step was to integrate the transmitter and the phototransistor
portions of the circuit onto the protoboard.
Before the phototransistor and transmitter could be integrated, they had to be
tested separately to determine their optimal operating region. The transmitter had to be
tested to determine the voltage at which the largest intensity of light could be produced.
The phototransistor had to be tested to find its active region, the region at which the
voltage varies with the intensity of light received.
Finding what maximum voltage the LED would allow tested the transistor. It was
assumed that the highest intensity of IR was emitted at the highest voltage. This was
assumed because a red LED was tested first. Until approximately 1.90 ± 0.02 volts the
LED would not light. Afterwards, the LED emitted light at higher intensities with the
increase of voltage. At approximately 5.90 ± 0.04 volts, the voltage became too high.
Since the device used a 9V battery, the correct resistor had to be inserted into the circuit
to allow for the appropriate voltage drop. It was determined that a 284 W resistor was
needed. Since the lab did not have 284 W resistors, a 1kW variable resistor was used,
and set at 284 W. This allowed for the maximum intensity to leave the IR LED.
The next piece of the receiver circuit was the phototransistor. A phototransistor is
similar to a normal transistor in that there are three terminals: the base, collector, and
emitter. However, the phototransistor does not have a separate pin for the base terminal.
Instead, the infrared sensitive base varies the voltage based on the intensity of the
infrared light absorbed. The collector voltage is the input power of the transistor, which
24
in the circuit, was approximately nine Volts. The emitter voltage was connected to the
non-inverting op-amp and amplified to fall within a pre-determined range for the timer
circuit so that the optimum frequency output could be produced.
Week 4:
Since week 3 ended with obtaining a functional circuit, week 4 was primarily
based on constructing the circuit casing, finishing up other extraneous testing, and
making further improvements on the circuit to try and make the circuit work as a
function of distance.
One of the more important parts of the laboratory was designing the black box so
that it could hold the circuit and function properly. This involved drilling two holes in
the front of the box for the emitter and the phototransistor. The infrared filter was then
placed in the hole for the phototransistor and the IR lens was placed in the hole for the
emitter. Each one of these was tested previously to ensure that the circuit would work
when these filters were used. Simply putting the filters in between the emitter and
phototransistor and checking to ensure that a signal was still produced did this. Further
drilling then produced a hole on the top of the box so that a push button could be placed
in the circuit. Finally, a series of holes were drilled in the back of the box where the
speaker was to be placed so that the sound would not be dulled by being a complete
enclosure.
The next step was to check the voltages of certain portions of the circuit that had
not been checked in the previous week. These included such things as the gain of the op-
amp compared to the theoretical prediction, and the voltage input to pin 5.
25
Once this further testing was completed, it was necessary to try and use all of the
accumulated data to make the circuit work as originally designed. However, one
immediately came across the problem that most of the values obtained separately (when
each component was tested on its own) were different than the values that were being
found when the circuit was completely connected. For example, the initial testing of the
voltage-controlled oscillator and the phototransistor showed that a gain of 22 would be
necessary to change the maximum voltage input to the non-inverting op-amp to the
optimum frequency range. However, when the whole circuit was connected, it turned
out that this gain was too high for the 555 timer and it had to be lowered. As well,
because of this new addition of voltage to pin 5, the base frequency had to be recalibrated
to a frequency of higher quality output from the speaker. At this stage, simple guess and
check methods were employed to vary the R2 in the oscillation portion and the gain was
varied until the appropriate sound was received. The sound that was selected was one
that corresponded to high quality output from the speaker, and was most aesthetically
pleasing to the listener. The device was then tested to see that it worked and changed
frequency based on distance. Testing showed that this was the case, but the range at
which the device worked was very small; the reflecting object had to be within a foot for
the speaker to produce any output. The gain and the base frequency were changed some
more to see if this would make a difference and it was found that it did have a slight
effect of increasing the object reflection distance recognition to approximately a foot. At
this point, the only other alternative reason for this lack of signal was that the infrared
emitter was not putting out a strong enough signal. To counter this, another emitter
was placed in parallel with the first one to see if this might have an effect on the
26
object reflection distance by enabling the phototransistor to take in more light.
This procedure only increased the object reflection distance a few more feet, and at this
point further testing could not be completed because of time constraints.
Another obstacle in the working of the circuit as a whole was that when the
circuit began to produce a sound when it was within the object reflection distance, this
sound did not vary much at all. In fact, the sound initially produced seemed to have
three frequency stages. First, the base frequency would appear very faintly prior to the
object distance. Then, when the object distance was reached, the frequency would make
a distinct change. The third and last stage of frequency change seemed to be when the
device approached a few inches of the reflecting surface where it quickly changed once
again. The continuously varying frequency that was expected in theory was not obtained
in practice. The technique of altering some of the circuit components to see if this would
have an effect on the frequency change was tried, and this did not seem to matter except
the three frequencies would all be similarly affected. For example, if the circuit were
altered so that the frequency would be slightly lower, all of the three stages would be
slightly lower in terms of voltage, and therefore the net change in voltage would not
change with respect to distance.
Overall, it was found that the circuit worked as a voltage-controlled oscillator, but
it worked as a very weak one. The voltage would vary but only in discrete stages. One
last attempt was made by putting the circuit in the box because it was believed that other
factors such as ambient light could have affected the output. However, when the circuit
was put in the box, which now included the filters, the signal was even more faint, yet
the three stage outputs seemed to become more continuous. It was later found that the
27
phototransistor only had a viewing angle of 16 degrees. Since the phototransistor was
inside of the hole and not sticking slightly outside of it, this could have explained the
much more faint signal. However, because of time constraints, modifications of the
black box and the filters to allow for the phototransistor to receive as much of the 16
degrees as possible could not be made so this theory could not be tested. However, the
device was shown to be operational and most of the original problems were resolved.
28
Addressing original potential problems:
There were certain potential problems that needed to be addressed when
constructing the transmitter and receiver portion of the circuit. First of all, the issue of
how to calibrate the circuit to the correct distance needed to be resolved. This was done
by choosing values for the resistors that would cause the sound to shut off when the
device was too close to an obstacle. Then this distance would be measured, and this was
the value for the resistors at a certain distance from the obstacle.
Another issue that needed to be resolved was whether or not the light was
rebounding back to the device. This was not a major issue, since infrared light is
invisible light and can be detected with the appropriate detector. Even at night, when it
is extremely dark, infrared light can be reliably utilized (as in night vision goggles),
which makes infrared light a very attractive waveform to use in this circuit design.
The final issue that needed to be resolved was how the infrared light would fare
on black objects. In theory, the color black absorbs all visible light, and reflects none of
the light back. However, the black only absorbs the visible light spectrum, which
infrared light does not fall under. Therefore, infrared can be used without diminished
effects.
Improvements:
Although the device was functional, there were many improvements that could
have been made if time and money allowed. Some of the major obstacles encountered in
designing, building and testing the device included focusing and filtering the IR signal
and supplying sufficient power to the components of the circuits. When the IR signal
29
strikes the surface of an obstacle, some of the light is scattered and the reflected signal is
not as strong . While nothing can be done to improve the IR emitter, a Fresnel lens can
be placed in front of the emitter in order to focus the beam. Also, an IR filter can be
placed in front of the phototransistor to decrease any interference by other types of light.
With the lens focusing the outgoing beam and the filter improving the reception of the
reflected beam, the signal into the non-inverting op-amp will be improved. This can also
be accomplished by using a more sensitive phototransistor. A longer lasting power
supply that weighed less would also be a great improvement. One could also try another
speaker to get a larger possible voltage range. Aesthetically, decreasing the size of the
device so that it is less noticeable and easier to carry would make the device
ergonomically superior.
30
Operational Specifications:
For a device as complicated as this, hours of testing, reconstructing, and re-testing
each component part are necessary. As a result, specific experimental values for each
part independently must be acquired so that they may be applied to the circuit as a whole.
In examining these components parts, the first portion to be tested was the
oscillator. The following values for the error in the resistors is based on the given 5%
error and is calculated for each resistor individually. The error in taking the frequency
readings off of the oscilloscope is half of the smallest division. The change in frequency
that this produced was the combined with the error in the frequency purely due to
inherent errors in the components. This procedure was carried out to determine the
optimum base frequency as well as examine the inherent errors in the oscillator.
Table 1: Theoretical vs. Experimental Frequency of oscillation for the Voltage-Regulated Oscillator
Resistor 1 Resistor 2 Theoretical Frequency
Experimental Frequency
% Error
100 kW ± 5000 1 kW ±50 14.1 Hz. 16.1 Hz. ± 4.7%
12.4
47 kW ±2350 1 kW ±50 29.4 33.3 ± 3.5 13.233 kW ±1650 1 kW ±50 41.1 50 ± 3.5 21.622 kW ±1100 1 kW ±50 60 83.3 ± 4.6 38.815 kW ±450 1 kW ±50 84.7 100 ± 5.5 18.010 kW ±500 1 kW ±50 120 153.8 ± 2.9 28.25.1 kW ±255 1 kW ±50 202 294.1 ± 3.6 45.61 kW ±50 1 kW ±50 480 555.6 ± 3.9 15.8470 W ±23.5 1 kW ±50 583 666.6 ± 4.2 14.3220 W ±11 1 kW ±50 648.7 714.3 ± 4.5 10.11 kW ±50 220 W ±11 1000 1333.3 ± 2.4 33.3470 W ±23.5 220 W ±11 1582.4 2083.3 ± 2.4 31.6220 W ±11 220 W ±11 2181.8 2631.6 ± 2.7 20.6
31
Assuming that the error in the resistor is 5% and the error in the capacitor is 10%,
the error for the experimental output can be calculated via the following differential
equation:
fR
R R CR
R R CC
R R C
144
22 88
2144
21
1 22
1
2
1 22
1
1
1 22
12
. *( )
. *( )
. *( )
This result can then be put into the following equation that takes into account both the
errors found above and the error from reading the frequency off of the oscilloscope. It is
calculated via the following equation:
totalerrorff
dOO
2 2
100*
where dO is the error of the reading from the oscilloscope and O is the actual value. The
R2 value measures how the data corresponds to the best fit line. A R2 equal to one is an
exact fit. The blue diamond shaped points represent theoretical data while the pink boxes
represent actual data. According to the following relationship between the variable
resistor and frequency,
frequencyR R C
14421 2 1
.( )
the data should behave logarithmically. The results show this is the case, with a high
degree of precision (The graphical program used for the two figures included in this
laboratory will not plot individual error bars. It was therefore necessary to average the
percent error and apply this as a representative for each. This average was 3.7%.
32
Unfortunately, we could not activate the Excel functions in Word. Therefore, we were
unable to add error bars to this graph.)
Figure 1: Theoretical vs. Experimental frequency variation due tovarying resistance at R2
y = -131.26Ln(x) + 1428.2R2 = 0.9656
-100
0
100
200
300
400
500
600
700
800
0 10000 20000 30000 40000 50000 60000 70000 80000 90000 100000
Resistance (ohms)
Frequency(Hz.)
The following data demonstrates the relationship between the voltage input to pin
5 and the output frequency as a result of this input. This was necessary in order to
examine the maximum voltages that would correspond to an audible sound output as well
as an aesthetically pleasing output. The error for the voltage measured via the
multimeter is ± 0.3% plus the last digit.
Table 2: Effects of input voltage on output frequency
33
Voltage Input to pin 5 Frequency output0 volts 0 Hz. 0.50 ± 0.025 01.00 ± 0.04 01.50 ± 0.055 02.00 ± 0.07 1041.7 ± 21.3 Hz.2.50 ± 0.085 1000 ± 19.63.00 ± 0.10 961.5 ± 18.13.50 ± 0.115 862.1 ± 14.64.00 ± 0.13 833.3 ± 13.64.50 ± 0.145 793.7 ± 12.55.00 ± 0.16 769.2 ± 11.65.50 ± 0.175 740.7 ± 10.86.00 ± 0.19 714.3 ± 10.16.50 ± 0.205 689.7 ± 9.47.00 ± 0.22 666.7 ± 8.87.50 ± 0.235 645.2 ± 8.38.00 ± 0.25 625 ± 7.78.50 ± 0.265 606.1 ± 7.39.00 ± 0.28 0
Figure 2: Frequency as a function of input voltage
y = -65.877x + 1127.9R2 = 0.9532
0
200
400
600
800
1000
1200
0 1 2 3 4 5 6 7 8 9
Voltage input (V)
Frequency(Hz)
One can see that the graph of voltage vs. frequency is linear. This is helpful to
the experimentation not only because of the high degree of accuracy with which the
oscillator follows this linear relationship, but also because it becomes easier to determine
the frequencies that are desired by simply applying the correct gain to the non-inverting
34
op-amp. It should be noted that this graph was utilized primarily in the higher frequency
range because it was found that the higher frequencies produced a better quality sound
and would produce a more discernible frequency change. The table above shows that
there were a number of frequencies at which a frequency output is no longer produced.
This range was from 1.8 - 8.85 volts, or 588.2 - 1041.7 Hz. As well, there was a sound
range at which the speaker produced a clearer sound. Because of the quality of the
speaker, there was a limited range at which the speaker produced a clear, consistent
sound. This sound range was from 2.6 - 5.1 volts, or 769.2 - 1000 Hz.. This is important
because it shows that even with the oscillating circuit isolated from the rest of the circuit,
there was a limited voltage and frequency range at which optimum output characteristics
desired were displayed.
The clear data shown above was different when the full circuit was fully
integrated with all the components. The voltage range at which the speaker produced a
clean, acceptable frequency output diminished. At this stage, the circuit as a whole was
quite variable. The same circuit could be induced to return a certain base frequency but
it was found that the voltage at pin 5 varied little when the infrared light was obstructed
at closer distances. In fact, even though the phototransistor produced a maximum of a
0.3 Volt variation when separated from the circuit and tested in conjunction with the
infrared LED, the range of audible frequency outputs was diminished. This was despite
the fact that a non-inverting op-amp was placed after the phototransistor for the sole
purpose of amplifying the signal to obtain a wider voltage range entering pin 5. For
example, with the initial estimation of a gain of 22 (experimental testing verified the gain
was 22.2) a voltage range from 1.8 to 6.6 volts could theoretically be obtained. Even
35
though the phototransistor could drop to zero volts when no infrared light was coming in,
it was found that the low voltage is 1.8 because of the fact that it will not produce a
frequency output below this voltage at pin 5 (see table 2). This data, in conjunction with
the data on the optimum voltage range at pin 5 for the oscillator discussed earlier, should
have produced a frequency change much like that found in the earlier testing of the
oscillator alone. However, this was not the case.
When qualitative testing showed a very minor frequency change with the distance
of the obstruction, the qualitative assessment was made that the base frequency was too
low. This was based primarily on the earlier testing which showed that the speaker
performed better at higher frequencies, and experimental testing supported it. The base
frequency was increased from 464 to 1000Hz. The gain of the op-amp was then varied
as well until a sound was obtained that was clearer, corresponded to the better end of the
speakers performance, and was more aesthetically pleasing. This was all done
qualitatively because quantitative experimentation could only do so much as to give the
range at which the individual component parts worked, but it was the subjective ear that
determined where in this range the optimum results were produced for the circuit as a
whole.
At this point it was found that there was a very limited range at which the speaker
produced the type of sound that was being sought. More importantly, the frequency
range corresponding to the object reflection distance was very small except at very small
distances, such as six inches. A considerable effort was made to improve the accuracy
and capacity of the circuit to vary the frequency at higher distances, but as the table
36
below demonstrates, a discernible change in the frequency could only increase to a
maximum of 1.403 ft. ± 0.093.
Table 3: Maximum distance of frequency variation discernabilityTrial Number Reflection distance (in.) Reflection distance (ft.)1 16.00 ± 0.0625 1.332 16.25 ± 0.0625 1.353 17.00 ± 0.0625 1.424 16.50 ± 0.0625 1.385 17.89 ± 0.0625 1.496 19.00 ± 0.0625 1.587 15.13 ± 0.0625 1.268 16.00 ± 0.0625 1.339 17.13 ± 0.0625 1.4310 17.50 ± 0.0625 1.46
The above data was tested by having one of the experimenters stand with the
circuit outside of its housing and slowly walking towards a blue obstruction. When the
first noticeable frequency change to the observer occurred, he stopped and the distance
was measured.
37
Device Specifications:
The following table is a summary of all of the important components of the final
device.
Table 4: Device SpecificationsParameter RangeSupply Voltage 4.5 - 15 VLED Voltage range 1.9 - 5.9 VOscillator Base frequency 1000 Hz. Minimum Reflection Distance 1.403 ftOptimum Operating Voltage 2.6 - 5.1 VOptimum Operating Frequency 769.2 - 1000 Hz.On/Off Oscillation Range 1.8-8.8 VOn/Off Oscillation Frequency 588.2 - 1041.7 Hz. Circuit Gain 22.2Applied LED Forward Current 30 mAOperating Temperature 0° to 70° CEstimated Battery Replacement Time 2.67 days
The supply voltage is based on the individual component parts. The limiting
piece seemed to be the 555 operational amplifier, which is why the supply voltage range
of the circuit corresponds to the supply voltage range of the operational amplifier.
Note that the Optimum Operating Frequency is begun at the upper limit. Since an
increase in voltage will decrease the frequency of the impending sound (as shown by
figure 1) the problem of exceeding the On/Off Oscillation Range is removed. Because of
this, the circuit will always be in its operating range. This is because the gain is set so as
to limit the voltage going into pin 5 so that there is never too much voltage going into the
oscillation portion of the circuit so as to exceed the operating range. Since the functional
active operating range of the phototransistor is known, one could theoretically set the
gain so that the maximum voltage out of the phototransistor would correspond to the
38
maximum voltage of the Optimum Operating Voltage range. The forward current for the
LED was determined by finding the resistance of the variable resistor and using a voltage
input of 8.5 volts (since a battery was never found that could exceed this). The 30mA is
much less then is necessary to ensure that the current is below the maximum forward
current (which is 100mA as detailed in the appendix).
The operating temperature of the device is adapted from the specification sheets
of the circuit’s components. It is important to note that the operating temperature could
be higher than the ambient temperature for the device because it is enclosed in a black
box that absorbs electromagnetic radiation of the whole visible light spectrum. However,
there are holes in the back of the device that not only act to increase the volume of the
sound that reaches the users ears, but it also acts to cool the device.
The estimated battery replacement time was determined in the following way.
During testing it was noted that the circuit was supplied with the two nine volt batteries
for approximately a half-hour (not continuously, but over the course of the lab). During
that time, the batteries went from approximately 17 volts to 14 volts. This is a loss of 1.5
volts per battery. If the batteries were to begin at 8.5 volts and drop to 4.5 volts (this
being the lower limit of the supply voltage range), the person would have an estimated
operation time of 80 minutes for the device. Using an upper limit estimate for a highly
active visually impaired person of about 30 minutes of device activity per day, this would
result in an estimated battery replacement time of 2.67 days. This is why it would be
recommended that rechargeable batteries be used with this device because the cost of
batteries alone could exceed the cost of the device in the long run.
Appendix:
39
The following specifications are provided as supplementary as well as additional
information to both demonstrate the parameters portions of the circuit are restricted to,
and for those who may desire further modifications of the circuit.
The cycle repeats independent of the supply
voltage and the frequency is given by the
following equation:
frequencyR R C
144
21 2 1
.( )
The 555 Operational amplifierFunction Pin
numberGround 1Trigger 2Output 3Reset 4Control V 5Threshold 6Discharge 7Vcc 8
40
Note, this frequency is the frequency output of a basic astable circuit. The voltage-
controlled oscillator is extremely similar to the astable oscillator. In fact, the only
tangible difference in the two circuits is the addition of an input across pin 5 and ground.
It is this input voltage that controls the frequency of the output signal. The equation
above simply describes the base frequency at which the 555 op-amp will operate at
without application of a voltage to pin 5. The following parameters for the 555 op-amp
were adhered to for this circuit,
although the table shows that there is
some variability in the range of device
limitations. Further research might
include varying such things as the
supply voltage to examine exactly what voltage resulted in the optimum volume output.
Many of the limitations described above apply to the 741 operational amplifier as
well. However, the 741 operational amplifier was not used as part of an oscillating
circuit in the photometric device, but rather it was used as part of a non-inverting
amplifier.
The 555 Device SpecificationsSpecification RangeSupply Voltage (Vcc) 4.5 to 15 VSupply Current (Vcc = 5V) 3 to 6 mASupply Current (Vcc = 15V) 10 to 15 mAOutput Current (maximum) 200 mAPower Dissipation 600 mWOperating Temperature 0 to 70°C
The 741 Operational AmplifierFunction Pin numberOffset Null 1Input ( - ) 2Input ( + ) 3Negative Voltage Supply 4Offset Null 5Output 6Positive Voltage Supply 7Unused 8
41
Note that the 741 op-amp was initially used as a comparator, in which case the important
inputs to pay attention to are the negative and positive power supply inputs (pins 4 and 7)
because these inputs determine the voltage that input into the oscillator. It is also
important to note that the input voltage should not exceed the supply voltage. However,
this was not a consideration in this circuit because the gain occurred across the op-amp
and not before it. Had the gain occurred before the op-amp (for example, if another op-
amp preceded this one) then the input voltage could have exceeded the supply voltage,
especially as the batteries began to lose their charge. This is another reason the batteries
would have to be changed quite often. It is therefore recommended that rechargeable
batteries be supplied with this device because of the frequent necessity of restoring
maximum voltage supply to the circuit for a number of reasons.
Few of the operational
characteristics played an
integral of role in the design of
the device. However, some of
these limitations, such as the
maximum supply voltage, input
voltage, and operating
temperature are important to
keep in mind when testing and
using the device.
Operational Limitations and characteristicsMaximum Ratings Supply Voltage ± 18 VPower Dissipation 500 mWDifferential Input Voltage ± 30 VInput Voltage ± 15 VOperating Temperature 0 to 70° CCharacteristics (typical)Input offset voltage 2 to 6 mVInput resistance 0.3 to 2 MWVoltage Gain 20,000 to 200,000Common-mode rejection ratio 70 to 90 dBBandwidth 0.5 to 1.5 MHzSlew Rate 0.5 V/msecSupply Current 1.7 to 2.8 mAPower Consumption 50 to 85 mW
42
The specifications below for the NPN silicon phototransistor and the Infrared
LED have been selectively taken from the specification sheet provided by the supplier.
If further specifications are desired one can consult the BE310 Laboratory Manual.
There are a couple of things to note about these characteristics below.
First, it is important to note that if any soldering is to be done on the phototransistor then
it should be applied for no longer then five seconds. The experimenter can also provide a
heat sink to redirect the heat from the phototransistor. Also, since the viewing angle of
the phototransistor is only 16 degrees, this is going to have a significant impact on the
phototransistor’s ability to receive the infrared light emitted by the LED. Because of
this, it is necessary to place the phototransistor so that it slightly emerges from the black
box in order to utilize as much of the receiving angle as possible. This was not done in
experimentation, and would be a further modification of the circuit. As well, since there
is such a small viewing angle, if the LED were to emit light and that light was to fall on a
surface that would diffract the light, then a small amount of it would return to the
phototransistor. This is why it is necessary to have the maximum amount of light
emitted from the LED, maybe even place two of them in the device.
If the infrared emitter were put in parallel with another they could emit the same
strength signal, resulting in a higher intensity of light received by the
NPN Silicon Phototransistor Parameter Maximum RatingsCollector-Emitter Voltage 30 VEmitter-Collector Voltage 5 VOperating Temperature -50°C to 100°C Soldering Temperature 260°C for 5 secondsPeak emission wavelength 940 nmViewing angle 16 °
43
phototransistor. Also, note that
the longer end of the emitter is
the cathode so the anode should
point in the direction of current
flow. This is important because
diodes are nonlinear devices and
only operate properly when there is a forward voltage drop.
TLN110 Infrared EmitterCharacteristic Maximum RatingForward Current 100 mAPulse Forward Current 1 AReverse Voltage 5 VDiode Power Dissipation 150 mWOperating Temperature range -20° to 75°CForward Voltage 1.5 VPeak Emission Wavelength 940 nm
44
Pictures:
45
References:
Hughes, Fredrick W. Illustrated Guidebook to Electronic Devices and Circuits. Prentice-Hall, Inc. Englewood Cliffs. 1983. p 101-104
Le Grand, Yves. Light, Colour and Vision. Chapman and Hall Ltd. London. 1968, pg. 75.
Mims, Forrest M. Engineer’s Mini-Notebook: 555 Timer IC Circuits. Siliconcepts, 1984. pp. 4-7, 15.
Mims, Forrest M. Engineer’s Mini-Notebook: Formulas, Tables and Basic Circuits. Siliconcepts, 1988, p. 37.
Mims, Forrest M. Engineer’s Mini-Notebook: Op-amp IC Circuits. Siliconcepts, 1985. pp. 8, 12-13.
Mims, Forrest M. Engineer’s Mini-Notebook: Optoelectronics Circuits. Siliconcepts, 1986. pp. 6-11, 19-21, 26-27, 38-39.
Tooley, Michael. Electronic Circuits Handbook. Butterworth-Heineman Ltd. Oxford.1993. p 185-195
Vanzetti, Riccardo. Practical Applications of Infrared Techniques. New York: John Wiley and Sons, 1972.
46