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What is that little black arachnid

that was in your baggie?

IC 555

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• I can calculate the charging and discharging time for a capacitor.

• I can calculate the period and frequency for a 555 Timer Oscillator.

• I can describe how the 555 Timer Oscillator can be used to produce temperature information.

What is the IC 555?

The IC 555 is an 8-pin Integrated Circuit (IC) that is capable of producing accurate time delays and/or oscillations.

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DIP chip (Dual-Inline package)

IC 555 – Why 555?

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Control Voltage (5)

Trigger Voltage (2)

Ground (1)

Vcc (8) Discharge (7)

Reset (4)

Output (3)

-

+

-

+RESET

SET

Q

Q

Comparator 1

Comparator 2

Flip-Flop T1

4

Threshold Voltage (6)

5 kΩ

5 kΩ

5 kΩ

IC 555 timerTime Delay Mode

• In the time delay mode, the delay is controlled by one external resistor and capacitor.

Example: Turn a light on in a delayed amount of time.(Just turn on or off once)

Oscillator Mode

• In the oscillator mode, the frequency of oscillation are controlled with two external resistors and one capacitor.

Example: Can make a light flash a specific rate.(Can turn on and off repeatedly)

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This presentation will discuss how to use a 555 timer in the

oscillator mode.

IC 555 timer

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Oscillator mode = astable multivibrator mode

Translation: in this mode the IC 555 will continue to put out pulses until you remove the battery. (your choice of resistors and capacitors determines the vibration frequency)

The 555 timer is a two state device: HI and LOYou can be a two state device too……

IC 555 timerAstable multivibrator mode schematic

Notice: 2 resistors and 1 capacitorOUTPUT is square wave pulses

LESSON 15 “How the 555 Timer Works”

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Thermistors

What do you predict will happen to the resistance of the thermistor as it ascends into the atmosphere?

Temperature dependent ResistorsR = resistanceT = temperaturek = temperature coefficient

Two types of Thermistorsk = positive (PTC)K = negative (NTC)

Our resistor is an NTC thermistor with a range of

10 kΩ – 80 kΩ

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Experiment a bit

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Experiment by using the Thermistor in place of R2.

You may use the spray coolant to cool the thermistor – but be careful it can get super cold!

1. How does reducing the temperature of the thermistor change the output of the IC 555 timer?

2. What evidence do you have to support this?

NTC thermistor

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Output = High

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tHIGH : Calculations for the Oscillator’s HIGH TimeTHE OUTPUT IS HIGH WHILE THE

CAPACITOR IS CHARGING THROUGH RA + RB.

CRR0.693 BA HIGHt

IC 555 12

C0.693RBLOWt

tLOW : Calculations for the Oscillator’s LOW Time

5v

3.333 v

Vc 1.666 v

0 v

tLOW

Output

HIGH

LOW

THE OUTPUT IS LOW WHILE THE CAPACITOR IS DISCHARGING THROUGH RB.

Output = Low

Monday July 14, 2014

IC 555 timer

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Visit this link to view a simulation of the IC 555 in astable modehttp://www.williamson-labs.com/pu-aa-555-timer_slow.htm

How does the charge and discharge of the capacitor relate to the blinking LED?

Period The Period is the total time of an

on/off cycle and depends on the values of RA, RB, and C

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C 2 693.0

693.0 693.0

693.0

693.0

BA

BBA

LOWHIGH

BLOW

BAHIGH

RRT

CRCRRT

ttT

CRt

CRRt

Calculate the period of the flashing light.

FrequencyThe frequency of an oscillation (or anything that exhibits a repeating pattern) is inversely proportional to the period

C R2R 693.0

1F

T

1F

BA

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Calculate the frequency (or blinking rate) of the flashing light.

Unit of Measure:

cycles/second = Hertz (Hz)

Practice

Calculate the

period and frequency of the

blinking LED.

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μ = 10-6

Solution

Period

mSec 534.3T

F8.6 1802390 693.0T

C R2R 693.0TBA

Frequency

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Hz 282.941 FmSec 534.31

F

T1

F

Practice

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Calculate the period and frequency of the blinking light assuming the resistance in the frequency is 10 kΩ.

Assume Elsa from frozen just walked in and room temperature just dropped to 0 C ̊� and the resistance on the thermistor is 30 kΩ. PREDICT – will the light blink faster or slower?

Calculate the period and frequency of the blinking light now.

CricketSat Schematic

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Key Points

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The 555 timer works with 2 resistors (one of which is temperature dependent) and 1 capacitor to

establish an oscillation in the circuit.

The 555 timer output has two modes: ON and OFF and it turns whatever device it is connected to

ON and OFF at a rate that depends on the changing resistance of the thermistor (which

depends on temperature)!!!

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