IEEE TRANSACTIONS ON EDUCATION, VOL. 33. NO. 2, MAY 1990 179

Real-Time Demonstration of Electronic Circuit Operation

Abstract-Several techniques for the demonstration of electronic cir- cuit operation before a large group are described. Working schematics t:<:tt are projected with an overhead transparency projector are illus- trated. Also discussed are large screen displays of analog meter move- aents and digitizing oscilloscope displays of waveforms.

I. INTRODUCTION HE DEMONSTRATION of electronic circuit opera- T tion before a large group has always been difficult due

to the small physical size of the electronic components involved and the lack of suitable large screen displays of the results. This paper will describe several different tech- niques that have been used for groups as large as 60 peo- ple. The demonstration techniques described here have been used to illustrate transistor, op-amp circuit, and transducer operation. These techniques are used in a three- day short course presentation on solid-state electronics.

A sense of cause and effect is often missing in the dem- onstration of electronic components before large groups due to the small physical size of the components. When the instructor moves a measurement probe from node po- sition to node position, the audience normally cannot see the action that is producing the new displayed result. To increase the visibility of such experiments, one can use a videotape of the demonstration that he has prepared in advance, as has been done for the demonstration of the practical application of statics [l]. This tape would be similar to commercially available demonstration tapes of specific test equipment [2]. While the tape allows close- up viewing of the devices, it lacks the sense of immediacy of a live presentation where questions from the audience can determine which demonstration options to explore.

Another alternative is live, closed-circuit TV display of the demonstration circuit. This does maintain the real-time nature of the discussion; however, the logistics of such a setup can be complicated. For example, at The Pennsyl- vania State University, the use of a large screen projection color TV requires the scheduling of an operator in ad- vance. Smaller green-CRT projectors are more portable and do not need an operator, but their screen brightness is very limited so the observing group must be very small. Large screen television monitors may be sufficient, de- pending on the video detail required, but a closed-circuit

Manuscript received September I , 1988; revised February 19, 1989. The author is with the Communications and Space Sciences Laboiatory,

Department of Electrical Engineering, The Pennsylvania State University. University Park, PA 16802.

IEEE Log Number 9034652.

camera must be taken along for each demonstration. Also, television displays may already be allocated for the pre- sentation of oscilloscope waveforms.

Some additional demonstration approaches are possi- ble. Digital logic has been demonstrated with large “elec- tric blackboards” that used large incandescent lamps on a large board driven by the TTL chips that were being illustrated [3]. More recently, the demonstration of digital circuits has been updated to overhead transparency pro- jectors through the construction of a transparent plastic board which contained logic block diagram symbols and lamps driven by a small microprocessor [4]. This trans- parent circuit board approach can be extended to analog circuitry for the illustration of various amplifier charac- teristics.

11. WORKING SCHEMATICS While video technology can be used to provide close-

up views of electronic circuits, a much more portable ap- proach is to build a “working schematic” directly on a plexiglass sheet, which is then placed directly on an over- head transparency projector. Each component and the neatly laid out wiring that interconnects the components produce a shadow that is projected on the screen. Com- ponent values are labeled beside each component. Fig. 1 shows the assembly used to demonstrate op-amp opera- tion. The actual shape of the op amp package and socket is masked out by a triangle-shaped cardboard cutout. Each node of the circuit has a small jack pressed into a clear plastic sheet, and each component is terminated with a mating pin plug. In this way it is very simple during the demonstration to unplug a component and replace it with a different element, illustrating cause and immediate ef- fect. Figs. 2 and 3 show the projected silhouettes that re- sult from variations of this op-amp demonstration. Through this working schematic, the goal of demonstrat- ing the ease and simplicity of circuit modification for ob- taining a desired system gain is met.

Another advantage of this approach becomes apparent during the demonstration of a simple one-transistor am- plifier. In this demonstration an oscilloscope probe is moved from one node to the next. The instructor’s hands do not disappear into an out-of-view circuit for each change. Instead the shadow of the oscilloscope probe can be seen touching the node on the working schematic, and the result appears on the oscilloscope screen. The sense of cause and effect is maintained.

0018-9359/90/0500-0179$01 .OO 0 1990 IEEE

180 IEEE TRANSACTIONS ON EDUCATION, VOL. 33, NO. 2, MAY 1990

Fig. 1. Working schematic constructed on clear plastic sheet for projection by an overhead projector.

Fig. 2. Projected image of a noninverting amplifier obtained by repatching the working schematic of Fig. 1.

Fig. 3 . Projected image of a bandpass amplifier obtained by repatching the working schematic of Fig. 1.

111. LARGE SCREEN DISPLAYS The display of the circuit performance also requires

special attention. DC or slowly moving values can be shown in a projection analog voltmeter. This is a variation of the 35 mm projection meters that were in use many years ago. The basic idea has been modified for use on

the overhead transparency projector. A small analog me- ter movement has been disassembled and mounted in a plexiglass box which is in turn mounted just above a 1 /4- in-thick glass plate. (This assembly is shown in Fig. 4.) The plane of the meter needle is just above the glass sur- face which is also the plane of focus for the working sche- matic. In order to place the needle just above the glass surface, the sense of “upscale” deflection is reversed. The mechanical zero of the meter has been adjusted to the original 100% deflection position. “Upscale” deflection (clockwise on the projected image) now occurs as the needle is driven back to its original 0% position. A trans- parency has been made from the original meter scale and is added just below the needle. The meter range selection switch and scale setting resistors are located to one side and masked from projected view. An earlier version of this projection meter used a plexiglass bottom plate. However, static charges could easily accumulate on this bottom surface and create apparent “stiction,” which locks the needle to one position. This difficulty has been minimized by the use of the 1/4-in-thick glass bottom plate. Two small holes have been drilled into the glass plate just above the meter movement. Each has a small stud epoxied into the hole. The working schematic plex- iglass sheet has two corresponding holes which align over these studs so that the working schematic does not shift relative to the meter display. Thus, both the working schematic and the meter display are shown at the same time. For typical projector-to-screen distances, the re- sulting image of the working schematic is 0.9 X 0.7 m with the meter display filling an additional 0 . 8 X 0.3 m area.

Digital voltmeter projection display also requires mod- ification of the original meter assembly. Removal of the reflective foil backing from the LCD display of a DVM converts the display to a transmission type. In an inex- pensive DVM such as the Radio Shack Cat. No. 22-188, the LCD display attaches to the main PC board via a flex- ible ribbon cable which can be extended through a slot in the case. Excessive heat from the transparency projector can temporarily darken the display, so it is important to allow air to pass between the LCD and the projector sur- face. Either the analog or digital voltmeter displays are suitable for showing static meter values in the demonstra- tion of transistor biasing or meter loading effects.

When rapidly changing time-varying waveforms must be displayed, an oscilloscope type display is needed. One option is to dedicate a closed-circuit TV camera to view a standard oscilloscope screen, which means that both the oscilloscope and TV camera must be set up in the class- room. Another option is to use a commercial digitizing oscilloscope to take snapshots of the waveforms. A read- out device such as a small computer is then needed to transfer the stored image to a format that is compatible with TV displays or that can be shown with a projection LCD computer display. Again, a fair amount of equip- ment must be set up. Also, the data transfer rate from the digital oscilloscope can be rather slow, even using the GPIB bus.

CROSKEY: ELECTRONIC CIRCUIT OPERATION

Fig. 4. Projection analog meter and working schematic of a one-transistor amplifier.

Since the three-day short course for which these tech- niques were developed is presented in many different lo- cations, the amount of transported equipment must be kept to a minimum. Rather than carrying along a closed-circuit TV camera and oscilloscope, large screen displays can be accomplished through the use of a custom-built digital sampling oscilloscope assembled from standard compo- nents. In this unit the digitized waveforms are constantly refreshed from memory to form a composite video signal that can be sent either to a large screen TV projector or a set of distributed TV’s. Most short-course sites have available locally either TV’s or monitors. The perfor- mance requirements of the digitizing oscilloscope are not extreme. A large amount of detail in the display is not needed. The one-part-in-256 resolution that results from an 8-bit A/D convertor is sufficient and fairly compatible with a noninterlaced TV scan rate. A maximum data sam- pling rate of 100 kHz has proven sufficient for many dem- onstrations and allows straightforward construction from off-the-shelf parts.

IV . DIGITIZING OSCILLOSCOPE The first generation of the digitizing oscilloscope used

standard TTL logic and thus required a fairly heavy power supply. The second generation unit which is now used for the real-time classroom demonstrations utilizes CMOS logic and is more compact and lighter. The complete unit has a volume of 0.006 m3 and has a mass of 2.9 kg. Fig. 5 shows a block diagram for this unit.

Two analog channels are each sampled by an eight-bit A/D convertor at a common rate; however, each channel can be “frozen” or blanked off by a front panel switch. Each analog input is scaled or amplified as selected by an input attenuator and preamplifier. A Schmitt trigger com- parator generates a “sync” signal for triggered sweep

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Fig. 5 . Digitizing oscilloscope which produces an output that is compati- ble with television monitors.

mode operation. During free running or “auto” sweep mode operation, the timing generator is retriggered auto- matically at the end of each sweep.

The timing generator that determines the sampling of the input data consists of the following blocks which are shown in Fig. 5 . A “time base select” contains a master oscillator and determines the sample rate of the A/D con- verters. A “divide by 128 burst counter” sets the number of samples during one sweep. A “start stop flip flop” ter- minates the A/D conversions at the end of the sweep. The ‘‘update rate” free-running multivibrator, along with the synchronizing signal from the ‘‘Schmitt trigger” deter- mines the time of the next sweep cycle. This timing ar- rangement triggers the A/D converter in a burst mode, that is, 128 samples are acquired and stored in the mem- ory at the “sweep rate” chosen on the front panel. This is followed by an adjustable hold-off (dead time) before the next data burst is taken. A front panel control allows adjustment of the update or data burst repetition rate. An update rate of two to five bursts/second usually produces a satisfactory display. The digital divider circuits of the time base select are arranged in a divide by two, five, or ten sequence to duplicate the standard oscilloscope sweep choice. The newly acquired data values from the A/D converter are stuffed directly into the buffer memory, in- dependent of the condition of the TV refresh timing cir- cuits. While this does produce a small amount of “spar- kle” in the display as these random access memories (RAM’S) are updated, the visual effect is minimal.

182 IEEE TRANSACTIONS ON EDUCATION. VOL 3 3 . NO 2 . MAY 1990

Fig 6 Digitizing oscilloscope display for a one-transistor amplifier

At the end of each conversion by an A/D converter, its output is enabled, placing its digitized result on the data lines leading to the corresponding memory element. At the same time, the 2 : 1 multiplexor is driven to connect the address of the sample within the sweep (obtained from the burst counter) onto the address lines of these RAM’S. At all other times, this multiplexor connects the memory address to the “DIV 128” counter of the master TV re- fresh generator. Thus, most of the time the address to the memories is scanned as the TV trace proceeds from left to right across the screen. Each data value read from the channel memory is compared to the value of the vertical line counter (divide by 256) of the TV refresh generator. If the data value (which is Y variable to be plotted) matches the line number currently displayed, the A = B output of the comparator goes high for a short time and a bright spot is produced on the TV screen.

The overall timing of the TV display readout circuits is arranged so that the composite video is similar to the U.S. standard. An overall master oscillator determines the pixel interval of about 0.5 ps. The following divide-by- 128 counter sets the horizontal repetition rate at about 15.4 kHz. (Some of the data samples are lost during video blanking of the horizontal retrace interval.) An additional divide-by-256 counter sets the vertical repetition interval and serves as an address bus for the display readout. Due to the vertical retrace time, the vertical visible screen res- olution is about 230 lines (noninterlaced). These counters have been chosen as straight binary division ratios to min- imize the number of chips needed. A minor touchup of the overall master timing oscillator frequency produces a stable picture with most televisions or monitors.

The demonstrated amplifier input and output can be compared with the dual-trace display (Fig. 6). Additional circuitry not shown optionally generates graticle divisions for amplitude and time interval measurements. Both com- posite video and modulated RF signal outputs from the digitizing oscilloscope are available. One or two 19-in diagonal television/monitors have been proven sufficient for the “oscilloscope” signal to be seen by the audience.

Fig. 7. Working schematic of a one-transistor amplifier, resting on an overhead projector, with the resulting projected image and TV-displayed waveforms.

commercially available digitizing oscilloscopes form Tektronix, Hewlett-Packard, or Heath, it fills a special- ized need for a compact unit that has an output directly compatible with standard television displays.

V. CONCLUSION Real-time classroom demonstrations have been devel-

oped for use in the presentation of a three-day short course on solid state electronics. Working schematics are pro- jected by an overhead transparency projector. As com- ponent values are changed or an oscilloscope probe is moved from node to node, cause and effect are easily ob- served. Large screen display of the dc results of the dem- onstrations are shown with a projection analog meter or DVM. AC waveforms are displayed with a digitizing os- cilloscope that produces a composite video signal that can be easily shown with a television or monitor. While these techniques have evolved with ease of equipment transport as a prime concern, they are also useful in the more tra- ditional classroom environment.

REFERENCES [ I ] A. Pytel, “Videotaped demonstrations and problem solutions to en-

hance the instruction of statics,” in Proc. 1985 ASEE Annu. Conf., 1985, pp. 1371-1373.

[ 2 ] “2230 digital storage oscilloscope operation,” Customer Service Training. Textronix, Beaverton, OR, p. 14. Apr. 1986.

[3] A. Longacre, “An electronic blackboard for use in teaching digital logic,” lEEE Truns. Educ.. vol. E-18, pp. 206-207. Nov. 1975.

[4] H. Panagratz, “A module for demonstrating digital circuits by over- head projection,” f E E E Trans. Educ., vol. E-25, pp. 113-144, Nov. 1982.

Charles L. Croskey (S’65-M’7S-SM’89) re- ceived the B.S . degree in 1967, the M.S. degree in 1970, and the Ph.D. degree in 1976, all in elec- trical engineering. from The Pennsylvania State University, University Park.

In addition to teaching in the Department of Electrical Engineering, The Pennsylvania State University, he is Course Director and Lecturer for a three-day short course on solid-state electronics. His research interests involve measurements of the electrical properties of the ionosphere by rocket-

Fig. 7 shows a typical arrangement of projector, working borne payloads. As an Associate Professor in the Communications and Space Sciences Laboratory (formerly the Ionosphere Research Labora- tory). he is also currently involved in microwaveimillimeter-wave radi- ometry of atmospheric constituents.

schematic, digitizing oscilloscope, and TV display. While the overall performance of this unit is far from that of