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Build This Basic Logic Trainer And learn all about digital ICs This Basic Logic Trainer from Dick Smith Electronics is just the shot for teaching digital electronics and demonstrating digital logic concepts. It’s easy to build, easy to operate and runs from a 9V DC plug­pack supply. Design by REX CALLAGHAN As shown in the photograph, the Basic Logic Trainer is built around a central prototyping board. The trainer provides the necessary power supply rails (5V DC), clock signals and logic inputs to this board, while a number of LEDs are used to indicate logic outputs. The connections to and from the prototyping board are made using single strand telephone cable, as are the connections bet­ween IC pins on the board itself. You can make the test 12  Silicon Chip circuit as simple or as complicated as you like – anything from just one digital logic IC to 10 or more ICs. Two large banana plug sockets are used for the power supply terminals and these are located directly above the prototyping board. This regulated 5V supply is current limited and is there­ fore protected against short circuits. Because it has no heatsink or securing bolt, the regulator will thermally shut down some­where near its rated current if there is an overload. The choice of a single 5V DC supply makes this unit suit­able for use with 74 series TTL integrated circuits (74xx, 74LSxx, 74HCxx, 74Cxx, etc) and with 4000 series CMOS logic ICs. The latter will operate over a supply range from 3-15V DC and therefore will work from a 5V supply without problems. All logic inputs to the test circuit are buffered and these are set by four switches immediately to the left of the prototyp­ing board. When a switch is in the up position, the corresponding logic input is high. Conversely, when a switch is in the down position, the corresponding logic input is low. These buffered inputs are labelled B0-B3 and are brought out via a 5-way vpin header socket. The fifth terminal on the header socket provides the clock pulses from an additional circuit hidden behind the front panel. The pulse Fig.1: IC1 (a TLC555 timer) is used to provide the clock pulses, while IC3a and IC3b form a window comparator to provide the logic probe function. IC2b-f and IC4a-d buffer the logic signals to and from the test circuit. is high or low, or is alternating between these two logic states. How it works output provides either a single pulse if its associated switch (at top, left) is pushed down momentarily, or a stream of clock pulses if the switch is in the clock position. At the other end, the logic output(s) from the test circuit are fed to a 4-way pin header socket. Each output is then fed to a buffering circuit and these in turn drive four LEDs (labelled Q0-Q3) to show the logic states at up to four different points on the test circuit. By the way, the fact that the outputs from the test circuit are “buffered” means that they do not need to be driven with the full LED current. That’s taken care of by the buffering circui­try. Each buffer stage has a high input impedance, to avoid loading the outputs of the test circuit. Logic probe Another very worthwhile feature is the provision of a simple logic probe. This uses a standard multimeter test lead which plugs into a 3.5mm socket on the front panel. The probe can be used to establish the logic states at various points on the test circuit. Connecting the probe to a logic 0 level will cause a green LED to light. Conversely, connecting to a logic 1 level will illuminate a red LED. The two indicator LEDs are immediately to the right of the probe socket. They simply indicate whether a point Refer now to Fig.1 for the circuit details of the Basic Logic Trainer. As stated above, power for the circuit comes from a 9V DC plugpack supply. Diode D1 provides protection against reverse supply polarity. Its output feeds 3-terminal regulator REG1 which produces a regulated +5V DC rail at its OUT terminal. LED 1 provides power on/off indication, while R101 limits the current through the LED. The output from the regulator is also connected directly to the +5V output terminal and it supplies the ICs. The negative output terminal connects to the negative supply line. IC1, a TLC555 timer, is used to provide clock pulses. It is wired as February 1996  13 Take care with the orientation of polarised components (ICs, diodes, LEDs and electrolytic capacitors) when assembling the PC board. The LEDs and pin header sockets are soldered after the board is secured to the front panel. an astable oscillator, the frequency of which is deter­mined by the total resistance present between pin 3 and the 0.47µF capacitor (C2) on pin 2. Normally, when SW1 is in the centre-off position, pin 3 of buffer stage IC2a is held low by R1 and so pin 4 (reset) of the timer is also held low. This effectively holds IC1 in the reset state, with its output at pin 3 remaining low. When SW1 is in the CLOCK position, pin 4 of IC1 is pulled high via SW1a and IC2a and the reset is released. At the same time, SW1b shorts out R3 and so the timing for the circuit is set by R2, R4 and C2. This causes IC1 to oscillate at a 2Hz rate. Conversely, when SW1a is in the PULSE (spring loaded, momentary contact) position, R3 is switched in series with the timing circuit. As a result, IC1 runs much more slowly than before, to produce one pulse about every 1.6 seconds. Thus, by momentarily flicking SW1 to the PULSE position, IC1 outputs a single clock pulse. Diode D2 ensures that C2 rapidly discharges when SW1 returns to its centre-off position. The output from IC1 is fed to pin 14 of non-inverting buffer stage IC2b. This in turn drives the PULSE terminal, to provide either a continuous clock signal or a one-shot pulse signal. Switch logic The PC board is mounted on the rear of the lid using 12mm tapped spacers and secured using short machine screws. Note how the 1000µF electrolytic capacitor is mounted. The remaining gates in IC2 (IC2cIC2f) are used to buffer the logic setting switches (SW2-SW5). When a switch selects the +5V rail, the output of its corresponding buffer is high. Con­versely, when ground is selected, the output of the buffer is low. The RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 1 1 2 1 1 8 1 1 1 1 4 6 14  Silicon Chip Value 1MΩ 470kΩ 390kΩ 150kΩ 130kΩ 100kΩ 62kΩ 47kΩ 36kΩ 10kΩ 2.7kΩ 220Ω 4-Band Code (1%) brown black green brown yellow violet yellow brown orange white yellow brown brown green yellow brown brown orange yellow brown brown black yellow brown blue red orange brown yellow violet orange brown orange blue orange brown brown black orange brown red violet red brown red red brown brown 5-Band Code (1%) brown black black yellow brown yellow violet black orange brown orange white black orange brown brown green black orange brown brown orange black orange brown brown black black orange brown blue red black red brown yellow violet black red brown orange blue black red brown brown black black red brown red violet black brown brown red red black black brown buffered logic outputs appear at pins 10, 6, 12 & 4 of IC2 and are fed to the B0-B3 terminals respectively. LEDs 2-5 are used to indicate the logic states. These LEDs are driven using inverting buffer stages IC4a-IC4d via 220Ω current limiting resistors. Normally, R13-R16 hold the inputs to these buffer stages low. This means that their outputs are all normally high and so the LEDs are all off. However, if any of the Q0-Q3 inputs is pulled high, the corresponding buffer input is also pulled high and so its output switches low and lights the relevant LED. Resistors R9-R12 protect the inputs of the 4049. Logic probe IC3a and IC3b form the logic probe circuit. These two op amps are wired in a standard window comparator configuration and drive two logic indicator LEDs (LED6 & LED7). Resistors R22-R26 set the bias voltages on the op amp in­puts. As indicated on the circuit, pin 5 is biased to +2V, pins 6 & 3 to +1.4V and pin 2 to +0.9V. As a result, the non-inverting input of each op amp is normally above the inverting inputs and so the op amp outputs are normally high and the LEDs are off. If, however, the probe input is connected to a logic high (ie, 2- 5V), pin 6 of IC3a will also be pulled high. As a result, pin 7 of IC3a switches low and this lights LED7 (red) to indicate that the high logic state has been detected. IC3b will not change state and so LED6 will remain off. Conversely, if the probe input is connected to a logic low (ie, less than 0.9V), pin 3 is also pulled low and the output of IC3b switches low instead. This lights LED6 to indicate that a logic low has been detected. Diode D3 is there to clip any large negative-going pulses that might be picked up via the probe input, to prevent damage to the op amps. Construction Construction of the Basic Logic Trainer is easy, since virtually all the parts mount onto a single large PC board. The exceptions are the banana sockets which mount directly onto the front panel and the 3.5mm panel socket for the plugpack supply. Refer to Fig.2 when installing the parts on the PC board. Begin by installing the resistors, followed by the Fig.2: install the parts on the PC board as shown here. Note particularly that the 1000µF electrolytic capacitor is mounted on the copper side of the board. February 1996  15 PARTS LIST 1 console case 1 front panel 1 prototyping board 1 PC board (© DSE) 1 test lead (for logic probe) 1 9V 200mA plugpack supply 4 SPDT miniature toggle switches 1 DPDT centre off, momentary on toggle switch 1 red banana socket (large) 1 black banana socket (large) 1 yellow banana socket 4 12mm tapped spacers 1 3.5mm DC panel socket 1 14-pin wire-wrap socket 4 self-tapping screws (to secure front panel) Semiconductors 1 TLC555 timer IC (IC1) 1 4050 hex non-inverting buffer (IC2) 1 LM393 dual op amp (IC3) 1 4049 hex inverting buffer (IC4) 1 78M05 3-terminal regulator (REG1) 1 1N4004 silicon diode (D1) 2 1N4148 silicon diodes (D2,D3) 6 5mm red LEDs (LED1-5, LED7) 1 5mm green LED (LED6) Capacitors 1 1000µF 16VW electrolytic 1 0.47µF monolithic 5 0.1µF ceramic 1 .01µF ceramic Resistors (0.25W, 1%) 1 1MΩ 1 62kΩ 1 470kΩ 1 47kΩ 2 390kΩ 1 36kΩ 1 150kΩ 1 10kΩ 1 130kΩ 4 2.7kΩ 8 100kΩ 6 220Ω Wire & cable 1 200mm-length 0.71mm tinned copper wire (for links) 1 500mm-length single strand telephone cable 2 400mm-lengths of hook-up wire, red & black WHERE TO BUY A KIT A kit of parts for the Basic Logic Trainer is available from Dick Smith Electronics stores & by mail order from PO Box 321, North Ryde, NSW 2113. Phone (02) 888 2105. The cost is $129 + $8 p&p. Please quote catalog number K3010 when ordering. Note: PC artwork copyright © Dick Smith Electronics. 16  Silicon Chip ceramic ca­pacitors, the diodes and the ICs. Note that D1 is a 1N4004 type, while D2 and D3 are 1N4148s. The five wire links can be installed now, using the off-cuts from resistor leads. This done, install the 7805 3-terminal regulator, noting that its leads are bent through 90° so that its metal tab sits flat against the PC board. The 1000µF electrolytic capacitor is installed on the underside of the PC board. Its leads are also bent through 90°, so that it can be laid flat against the board surface. Be sure to install this part the right way around. The five toggle switches are mount­ ed directly on the PC board. Push them right down onto the board before soldering their leads and note that S1 must be oriented with its momentary (ie, spring-loaded) position towards the bottom. The switch nuts can be either omitted or screwed all the way down. By this stage, the board will be complete except for the LEDs and the pin headers. The LEDs can be installed now (the green one is LED 6) but do not solder their leads yet, as they must first be adjusted for height when the front panel is in­ stalled. Take care with the orientation of the LEDs – the cathode (K) lead will be the shorter of the two. In addition, the cathode lead is adjacent to a flat edge at the bottom of the LED body. The 4-way and 5-way pin headers are obtained by cutting down a single 14-pin wire-wrap socket. To do this, first cut the wire-wrap socket in half using a pair of sharp sidecutters, to obtain two 7-pin sockets. The unwanted pins can then be removed and the socket bodies carefully trimmed and filed to size to that they fit the slits in the front panel. Do not mount the pin headers yet; that step comes later when the front panel is fitted. Hardware assembly Begin the hardware assembly by attaching the prototyping board to the front panel using double-sided tape. This done, fit the three banana sockets to the front panel. Use the red socket for the positive terminal, black for the negative terminal and yellow for the logic probe terminal. Do these sockets up tight and connect appropriately coloured leads (eg, red for positive, black for negative) to their solder lugs. These leads should all be about 60mm in length, so that they can be run to their respective points on the PC board. The 3.5mm DC socket is mounted on the bottom lefthand hand side of the rear panel (as viewed from the rear). This socket is wired via two 150mm long leads (red for positive, black for nega­tive) to the plus (+) and minus (-) inputs on the PC board. Twist these leads together to keep things neat and tidy. Note that the positive lead must go to the tip terminal of the DC socket, while the black lead must go to the collar (or sleeve) terminal. Now for the final assembly. First, fit the four 12mm-long spacers to the PC board and secure them with the short screws supplied. This done, fit the 4-way and 5-way pin headers to the PC board, then fit the front panel and secure it in position. The various LEDs and the pin headers can now be pushed through their respective holes on the front panel and carefully aligned. When everything looks OK, solder the leads to secure them in position. Finally, fit the lid to the case and secure it using the four self-tapping screws supplied. Testing When power is applied, the red LED next to the +5V socket should illuminate. If it doesn’t, then the LED is either in backwards, there is a fault in the regulator circuit, the supply polarity is incorrect, or the regulator output is short-circuited to ground. In particular, check that D1 and REG1 are correctly oriented. The 5V DC output terminals are a convenient place to check the 5V supply at any time. The power on/ off LED will provide a handy quick visual indication of the state of the 5V supply; eg, you may see this LED dim if a heavy load is placed across the supply, or extinguish if the power supply is inadvertently short-circuited on the protoboard. Assuming that the power supply is OK, the next step is to check out the logic probe circuitry. To do this, simply plug the probe in and touch the positive and negative supply terminals in turn. Check that the red LED (High) lights when the positive terminal is touched and that the green LED (Low) lights when the negative terminal is touched. If the logic probe doesn’t work, first check that the vol­tages on pins 2, 3, 6 & 5 of IC3 match those marked on the cir­ cuit. If they don’t, then it’s likely that one of the bias resis­tors (R22-R26) is incorrect or D3 is back to front. Check also that D3, IC3 and the two LEDs are correctly oriented. Once the logic probe is working correctly, it can be used to check the rest of the circuit. For example, by touching the probe on the PULSE terminal, you can check the clock circuitry (IC1). The green LED should light when S1 is in the centre-off position, while the red LED should flash at a 2Hz rate when S1 is in the CLOCK position; ie, the logic probe should show the indi­ vidual positive going clock pulses as they occur. You should get a much slower rate if S1 is held in the PULSE position (ie, one pulse about every 1.6 seconds). If you don’t get any clock pulses, check the circuitry around IC1. Similarly, use the logic probe to confirm that SW2-SW5 can be used to set their corresponding outputs (B0-B3) high or low. The output indicator circuitry can be tested by setting B0 high and connecting a lead from this terminal to Q0, Q1, Q2 & Q3 in turn. In each case, the corresponding output LED should light. If it doesn’t, check the resistor values at the inputs to IC4a-IC4d. Using the trainer By this stage, you have checked that the individual compon­ents of the Basic Logic Trainer are functioning correctly. Having done this, you will have a basic knowledge of how to use these components; ie: (1) the logic switches (SW2-SW5) are used to set the logic states on B0-B3 (high or low); (2) the Q0-Q3 terminals are continually monitored and their status indicated by individual LEDs; (3) the clock/pulse generator switch can provide either a train of clock pulses or individual pulses as required; and (4) the logic probe can be used at any time to check the logic state at different positions on the test circuit. We recommend using the insulated single-strand wire to make the various wiring connections. One length of 500mm can be cut into many smaller lengths, each of which should have about 5mm of insulation stripped back SC at either end. Basic Logic Trainer Demonstration As an example, we’ll wire up a common CMOS flipflop and exercise it. The device is the 14-pin CMOS 4013 which is a dual D-type flipflop. Its pin connections are shown in Table 1. Table 1: 4013 Pin Connections Function Pin No. Function Pin No. Q1 1 Vdd (+) 14 Q1 2 Q2 13 Clock 1 3 Q2 12 Reset 1 4 Clock 2 11 Data 1 5 Reset 2 10 Set 1 6 Data 2 9 Vss (-) 7 Set 2 8 The step-by-step procedure is as follows: (1) Insert the IC into the proto­ board, such that pin 1 is at the top left and pin 14 at the top right. The IC should be inserted so that the two vertical columns of pins are either side of a channel in the prototyping board, so that they are not shorted together. (2) Using suitable leads, connect the +ve and -ve supply termi­nals to the +ve and -ve supply buses on the prototyping board. A short lead can then be connected from the +ve bus to pin 14, while a second lead can be connect from the -ve bus to pin 7. You can check that these two connections are correct by using the logic probe. This should show a low state at pin 7 and a high state at pin 14. (3) Configure flipflop 1 by connecting: (i) a wire from pin 6 to B0; (ii) a wire from pin 4 to B1; (iii) a wire from pin 5 to B2; (iv) a wire from pin 1 to Q3; (v) a wire from pin 2 to Q2; and (vi) a wire from pin 3 to PULSE/ CLOCK The above procedure connects the four inputs and the two outputs of flipflop 1 on the 4013. Table 2 shows these various test connections. Now let’s explore the basic operation of the flipflop: (1) set B0-B2 to logic 0; (2) push the Pulse switch once. LED Q3 should be off (logic low or 0) and LED Q2 should be on (logic high or 1); (4) set switch B2 to a logic 1; and (5) Push the Pulse switch once. This should clock the new data (ie, a logic 1 from B2) through to the outputs and so the logic states on Q3 and Q2 should reverse (ie, Q3 now on, Q2 now off). Further pulses should not alter this, until the DATA 1 input (B2) is altered again. This shows the basic operation of a D-type flipflop; ie, the logic level on the Data input is transferred through to the Q output (pin 1) on receipt of a clock pulse. What this means is that you can get the flipflop to au­tomatically toggle on the receipt of each clock pulse by connect­ing its Q-bar output to its Data input. To do this: (1) remove the wire between B2 and pin 5 of the 4013; and (2) connect a wire between pin 2 and pin 5 of the 4013. Now when you work the pulse switch or switch to the CLOCK position, the two output LEDs should flash on and off alternately. Further experiments (1) Connect an additional wire between pin 3 of the 4013 and LED output Q1. This will allow you to observe the relationship bet­ween the clock input to the 4013 (on LED Q1) and the 4013 Q and Q-bar output levels (on LED Q2 and LED Q3). Because the Q-bar (inverted) output of flipflop 1 is fed into the Data input, each clock pulse will change the exist­ing state of the Q output. This is the classic divide-by-two configuration of the basic flipflop and is used in many circuit applications. Table 2: Test Connections Protoboard Connections IC Pin Function Pin No. Clock/pulse Switch Clock 1 3 Switch B1 Reset 1 4 Switch B2 Data 1 5 Switch B0 Set 1 6 LED Q3 Q1 1 LED Q2 Q1 2 February 1996  17