John T. Stock
University of Connecticut Storrs, 06268
Simple Units for Demonstrating Digital Circuitry
A n understanding of the electronic aspects of chemical instrumentation is best acquired by actual experimentation with the basic units or modules. Suitable commercial equipment is available,' together with appropriate texts%nd other litera t ~ r e . This ~ does not solve the problem of squeezing yet another laboratory course into an already crowded curriculum. One alternative is to offer the course as an elective, with the consequent risk of being bypassed. Another is to reduce contact hours by operating on a lecture-demonstration basis. This ensures that all students can receive some exposure to the subject and is, in fact, a good route by which to move towards the desirable full-course treatment. Elective associated laboratory work, taken initially by only a few of the students, allows the respective merits of various forms of equipment to he compared and a suitable program of experiments to be developed. The ready availability of operational amplifier assemblies and associated equipment greatly facilitates demonstration of the basic principles of analog circuitry. Malmstadt and Enke's text4and the associated equipment have done much the same for digital circuitry. In the initial stages of course development, some aspects of digital circuitry can he simply and inexpensively demonstrated with a few easily constructed circuit board units. In most caqes, a small 3-V battery is ample as a driving source. Figure 1shown the circuit board that is used to demonstrate the inverting action of a common-emitter switching transistor and the logic associated with NOR andNAND gates4 The components are mounted on an 11.1/8 X 8.5/8-in. panel of l/cin. thick plywood. This has X a/n-in. wood strips around the underside of the perimeter, to provide space for underpanel wiring, downward projections of sockets, etc. After the essential circuitry has been drawn on a sheet of typing paper, this is slipped between the leaves of a transparent report cover5 and secured to the panel by thumbtacks, as ehown. Resistors, transistors, and any capacitors are mounted alongside the corresponding circuit symbols by passing the leads through small drilled holes. Before drilling the large holes needed for mounting sockets or pushbutton switches, it is advisable to cut through cover and paper with a sharp corkborer of appropriate size. This prevents tearing. Power, ground, and any take-off connections are madc by means of inexpensive 14in. color-coded jumper cables: the alligator clip ends of two of which are shown in Figure 1. Each actual connection point is the projecting shank of a 6-32 X 3/rin. machine screw that is inserted through a solder tag on the underside of the panel and secured by a nut and washer on the upper side. The 2N5129 NPN transistors (National Semi516
/
Journal of Chemical Education
Figure 1.
Circuit board for NOR ond NAND gates.
conductor Co.) shown cost about a quarter each. Suitable substitutions can be made. The transistors can be mounted in sockets, or the leads projecting from the underside of the panel can be directly soldered to 24gauge connecting wires. Direct soldering should he done rapidly with a hot fine-tipped iron, then each joint should be quickly cooled by application of a large nail or similar cold object. The "high" or "logic 1" state of the output X is indicated by the lighting of a 2-V GO mA lamp (type 45 screlv base, or type 49 bayonet base). This lamp is indirectly operated from X by one of the 2N5129 lamp drivers shown a t (a) and (b) in Figure 2. The two-transistor version (b) draws very little of the output current, so that most is available ,for energizing other devices. The lamp and the pushbutton switches that are used to energize inputs A and B are fed internally from the common V + supply. Once the basic logic of these two simple discrete-com'A. R. F. Products, Ine., Gardner Rd., Raton, N. M. 87740; Heath Co., Benton Harbor, Mich. 49022; MeKee-Pedersen ~nstrurnen&,P.O. Box 322, Uanville, Calif. 94526; and others. 2 EWING, G. W., "Andytical Instrumentation," Plenum Press, C. G., "ElecNew York, 1966; MALMSTADT, H. V., AND ENICE, tronics for Scientists," W. A. Benjamin, Inc., New York, 1963. 3 MPT Annlimtionq Nntns. Vol. 1., -~ 1966 and later issues. McKee. r r . . . . . . . . . . . . . . , . Pedersen Instruments, P.O. Box 322, Danville, Calif. 94526. H. V., AND ENKE,C. G.,"Digital Electronics AMALMSTADT, for Scientists," W. A. Benjamin, Inc., New York, 1969, p. 160. 5 %Z Slide. Cook's Inc., Blackwood, N . J.; Kleer-Vu, Kleer-Vu backbone Industries. 1"c.. New ~ & k N. . Y. 10016. The spring is not used. 6 Lafayette Radio Electronics, Jericho Turnpike, Syosset, N . Y. 11791; Part no. 99E00572. ~~~
.
.
~
~
~
Figure 3. Layout of board for demonrtroting the JK flip-flop. chronous clocking orrmgemenk; a, divide b y 4; b, divide by 3.
Syn-
Figure 2. Ancillary circuitry. a, Single-transistor lomp driver; b, twatran.irtor lamp driver; c, device for producing rapid-fall clwking signal.
ponent gates has been grasped, their use to build up AND, OR, and EXCLUSIVE OR gates follows quite easily.? This buildup, requiring from two to five identical "simple" gates, provides a natural lead into the advantages of integrated circuit (IC) devices. I n fact, Lancasters has described suitable circuitry that enables a quad 2-input NAND gate IC to be used to demonstrate the logic of the six common gates. Similarly constructed circuit boards with discrete components are used to demonstrate the circuitry and function of astable and monostable multivibrators, and of RS and clocked RS multivibrators or "flipflops." Clocked flip-flops require a very sharply defined operating signal, which can be supplied by the simple "bounceless" switch described by Genter.# This device shown at (c) in Figure 2, makes use of a pL 914 IC dual 2-input NOR gate and a single-pole double throw tapping key or pushbutton switch. Although the very valuable JII flip-flop can be built up from discrete components, the resulting maze of connections between a dozen or so transistors has no educational value in an introductory course. It is cheaper, more instructive, and much simpler to use an IC J K flip-flop such as Fairchild RTL 923 or Motorola HEP 583. After outlining the nature of the internal circuitry, the device is treated merely as a "black box" with one clocking and three controlling inputs, plus two complementary outputs. Incidentally, almost any IC device, faulty or not, is useful for stressing the quite complex arrangements that can be accommodated on a tiny silicon "chip." After cutting open the case to expose the "chip," the device is suitably mounted for projection or for viewing under a lowpower microscope. Fimlre 3 shows the lavout of the circuit board that is used to demonstrate the action and a typical use of the JK flip-flop. The board carries three IC's, the actual positions of which are indicated by the large black circles. IC3, operated by pusbbutton T, is connected as shown at (c) in Figure 2 to provide the rapidly-
-
falling signal needed to "clock" the inputs TI and TI of J K flip-flops IC1 and IC2. Pushbutton P is used to bring outputs Q1 and Q to the low or "logic 0" state by temporarily joining the "preset" connections to the V + supply. The smaller black circles are the shanks of the screws to which power and other connections can be made. Banana plugs, attached by short flexible leads to inputs C1 and Sl of flip-flop 1, can be inserted into color-codcd sockets that enable either or both of these inputs to be grounded ("low" or "logic 0") or connected to the V + supply ("high" or "logic 1"). For example, when both C, and S1 are grounded, Q1 and Q1 change state with every pulse a t TI. The alternate lighting and extinguishing of lamp Ll illustrates the "divide by two" action of the flip-flop in this configuration. Jumper cables can be used to ground inputs C2 and S2 and to connect &I to T2. The lighting of lamp L then illustrates the "divide by four" action of a "ripple" or "asynchronously clocked" divider circuit. Another "divide by four" arrangement shown at (a) in Figure 3 takes advantage of the fact that a J K flip-flop is disabled (i.e., cannot respond to clocking) when both the C and S inputs are "high." I n this arrangement, the clock input is applied synchronously, i.e., directly to both T inputs. The "divide by three" arrangement shown a t (b) in Figure 3 enables other a-pects of the "steering" provided by inputs C and S to be illustrated. The connection to Sl is made by attaching a jumper cable to the plug after this has been withdrawn from its socket. Division by 5, 7, 10, etc., involving the use of more than two flip-flops, is readily appreciated when the action of these simplcr assemblies hari been examined. Figure 4 shows the circuit board for demonstrating the Schmitt trigger, a valuable device for controlling and signal shaping. A 5-Kohm potentiometer, fed from the V+ supply through a 4.7-Kohm fixed resistor, enables the input to be varied between the limits 0 and 1.5 V. With the components shown, the output 7
See p. 169 of reference in footnote 4. LANCASTER, D., Pop. Eleclrm., 29 (12), 41 (1966) GENTBR,R., Radio Electron., 39 (12), 57 (1968). Volume 49, Number
7,July 1972 / 517
Figure 4.
The SchrniW trigger.
Q goes "high," and the lamp lights, when the input reaches +O.SO V. However, Q does not return to "low" until the input has been decreased to below +0.70 V. This characteristic "hysteresis" has been utilized in a device to demonstrate the working range of acid-base indicators.'" The voltage-to-frequency converter is a key cnmponent in pH meters, coulometers, and other devices that yield a digital readout from a continuous, or "analog," input signal. Various forms of converters are extensively described by Malmstadt and Enke." A circuit board to illustrate the principle of one method of conversion is shown in Figure 5. This device is merely a unijunction transistor relaxation oscillator. Connection screu7sallow the wave forms at the emitter and a t base 1 (the output) to be displayed on an oscilloscope. The actual "readout" is an audible one, as a
51 8
/
Journal of Chemical Education
Figure 5.
Board to illudrate voltage-to-frequency conversion.
succession of "clicks" from a built-in %in. diameter speaker unit. The input voltage is obtained from the V+ supply through a 1.5-Kohm potentiometer. This is connected in series with a 5-Kohm variable resistor that has one end grounded and is mounted on the underside of the panel. Biasing is necessary because the unijunction requires an input considerably above zero in order to operate. With the V,. potentiometer set a t zero, the variable resistor is adjusted until the device is just not oscillating. Becau~ethe output frequency is not linear with respect to the input voltage, no V,. scale is provided. However, good linearity can be obtained with more refined circuits of this type.'? STOCK, J. T.,J. CHEM.EDUC.,47, 311 (1970). Chapter 7 of reference in footnote 4. '*GE Transistor Manual (7th ed.), General Electric Co., Syracuse, N. Y., p. 346. lo
" See
