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Build this low-cost 3-digit counter module Looking for a cheap module for event counting or to be used as part of a larger project? If so, consider this 3-digit counter module. It uses only two low-cost CMOS ICs and can be put together ..in a couple of hours. By DARREN YA TES Over the past few years, there have been quite a few designs for 3 or 4-digit counters which have either used the 74C92X-series 4-digit counters or a string of 4029 CMOS single digit counters. The basic problem with these, designs is that they are too expensive or physically too big. The 74C92X-series counters are a good example of the former. While they are very compact 54 SILICON CHIP devices and require very little PCB space, at up to $17 each, they are too pricey for the average experimenter. On the other hand, a counter based on a string of 4029 single digit counters has the problem of going the other way. While 4029s are quite cheap, they require a dedicated 7-segment display driver for each digit. For a 3 or 4-digit counter, this ends up being 6-8 ICs and uses up board space like it was going out of fashion! The circuit described here strikes a balance between these two problems. It uses the sometimes forgotten 14553 CMOS 3-digit counter IC from Motorola (also available as the GD4553 from Goldstar and sold by Novocastrian Electronic Supplies, PO Box 8 7, Broadmeadow, 2292). This device is quite cheap at about $4 and requires only a single 4511 7-segment display driver plus 3 transistors to produce a 3-digit event counter. 4553 functions To understand this IC, you'll need to read the text and follow the block diagram in Fig.1. Looking at the diagram, the IC contains three BCD counters whose 4-bit outputs are each passed through a four-bit register or quad latch. These allow us to store a certain count and have it displayed at the output, while the counters themselves are still counting. We'll talk about some practical uses of this feature a little later. The way in which the digits are displayed is achieved by using a technique known as multiplexing. The outputs from the three quad latches are fed into a quad 3-input mulitplexer. A multiplexer is just like the input selector switch on your stereo amplifier except that instead of you rotating the switch between the different input sources, it is done automatically and continuously, so that each of the sound inputs is heard for a short time at the loudspeakers several times a second. This is basically what the counter IC is doing with each of the digits. The scan oscillator drives a scan counter whose outputs, Q0 to Q2, continually select each digit in sequence at a reasonably fast rate. The 4 bits of that digit then appear c,.b .---- - ® r-----, '© --i_ ®- ~ T r.: •ULSE GENERATOR LSD ® Bio <at> CP 1 >-of><>-1,-.of COUNTER 0 CP "o 1--- -+-i 01 D1 QUAD 01 LATCH 02 a 2 02 03 D3 03 QUAD 3-I NPUT MULTIPLEXER If CP1 is kept low and the clock signal is fed into CP0, then the counter increments on the negative or "falling" edge of the clock signal. If, however, CP0 is held high and the clock signal is fed into CP1, then the counter is triggered by the positive or "rising" edge of the clock signal. OK. That's how the 4553 IC works. Now let's take a look at the circuit diagram in Fig.2. The circuit As you can see from the circuit diagram of Fig.2, the module uses two ICs, a handful of resistors, a couple of 7-segment displays and Oo a, oo, l~UT~~ a, a 2 t--1---+-i , a, 03 03 D3 PARTS LIST "o MR a, 1--1-----1 Dt QUAD 0t a,1--1-----1 02 LATCH 02 03 03 l--i.----1 a, z, }, a, a. a,® o, MSO Voo = Pin 16 V55 • Pin 8 0 = Pin Number TC MR <at> ® Fig.1: inside the 4533 3-digit counter. It contains three BCD counters, with each counter driving a 4-bit latch. The latch contents in turn drive a quad 3-input multiplexer. at the output pins, Q0 to Q3. The end result is that the 4 bits of each digit appear at the output pins at small discrete time intervals. In order to recreate the three digit number, the outputs of the scan counter are provided at pins 2, 1 and 15 respectively. These outputs, marked "DSO0" to "DSOZ", are used to drive each of the 7-segment displays in turn via a switching transistor at the correct time interval. Even though each 7-segment display is only switched on for a third of the time, our eyes cannot detect this fact. This is because the speed at which they are switched is faster than the eye can respond. As a result, the display appears to have a constant brightness and does not flicker. The speed of the switching is set by the external capacitor fitted to the scan oscillator at pins 3 and 4. The larger its value, the slower the output cycles between digits. The IC also has several usercontrolled inputs which make this counter a very versatile unit. The MEMORY RESET input (pin 13), when taken high, resets each of the three counters back to zero. The LATCH ENABLE input at pin 10, when taken high, stores the current value of each of the three counters in the corresponding latch and continues to send this count to the output and the displays. The TERMINAL COUNT output at pin 14 is an overflow output which goes high for one clock cycle when a count of "999" is reached. The counter then resets back to "000". There are also two input clock pins, labelled CP0 and CP1. By correct selection of these, it is possible to make the IC increment on either the positive or negative edge of the clock signal. PC board, code SC04309901 , 11 8 x 80mm 7 PC pins Semiconductors 1 4553 CMOS 3-digit BCD counter (IC1) 1 4511 CMOS 7 -segment decoder driver (IC2) 1 7805 5V regulator 3 BC328 PNP transistors (01 ,02 ,03) 3 FN_D500 or equivalent common-cathode 7 -segment displays Capacitors 1 1 OOµF 16VW electrolytic 1 .001 µF metallised polyester (greencap) Resistors (0.25W, 5%) 3 1 OkO 5% 1 220fl 1 % 7 6800 5% 1 1500 1 % Miscellaneous Hook-up wire, solder. little else. It is basically a "barebones" job in that the board itself contains only the basic parts necessary for it to operate. This allows the builder to add on other "extras" as they are needed rather than pay for something they may not use. To make external connections easier, the controlling inputs are brought to the bottom of the board for easy access. IC1 is the 4553 3-digit counter IC. The .001µF capacitor on pins 3 and 4 sets the multiplex scanning rate SEPTEMBER 1990 55 ponents on the board. Follow the wiring diagram of Fig.3 when assembling the components onto the board. Next, solder in all the wire links. Once you've done that, wire in all the resistors. This should be an easy job but be careful about solder splashes shorting out nearby tracks; don't use excessive amounts of solder. The electrolytic capacitor goes in next. Make sure you get its polarity right - the positive pin goes to the outside of the board. The .001µ.F capacitor associated with ICl can also be soldered in at this stage. Next, do the three PNP transistors, making sure that they are all oriented correctly - see the photo to double check this point. Finally, solder in the two ICs and the three LED displays. Check the orientation of the two ICs carefully +9V 3 4 16 BL 5 3xFND500 7x68Dn , 13 b 12 LE C11 IC2 4511 .,. 4 d 10 1 e9 a ,,-g-,b ·I /c f 15 d g 14 10 A B C 0 7 1 2 6 7 6 5 01 02 03 15 OS3 TERM COUNT 1 14 TC LATCH ENABLE 2 10 LE 11 CP1 3 12 CPD 4 02 BC558 IC1 4553 CP1 DS2 1 .,. CPD 13 MR MASTER RESET 5 01 BC558 DS1 03 BC558 2 C1A 3 GND6~ ,. .001 i,aD;:;.:UT-------+gv 100 + B EOc 16VWJ VIEWED FROM BELOW GNO .,. LOW-COST 3-DIGIT COUNTER Fig.2: ICl is the 4553 counter IC and this drives 7-segment decoder IC2 and the digit driver transistors (Ql-Q3). IC2 in turn drives the display segments while Ql-Q3 switch the displays. to about lkHz, depending on the exact value of the supply voltage. The 4-bit outputs (Q0 to Q3) are fed into a 4511 CMOS 7-segment decoder/ driver (IC2). The outputs of this driver are connected to the three 7-segment displays which have their corresponding pins connected together. Each display is switched on at the correct time via the display control outputs at pins 2, 1 and 15 (DS1, DS2 & DS3). These are activelow outputs; ie, for a particular digit to light, its displ1;1y control out- put must go low rather than high. These outputs each drive a BC328 PNP transistor (Q1-Q3) via a 10k0 resistor and the transistors in turn switch the common cathodes of the display digits. Building the module The board itself should take no more than a couple of hours to put together. When you etch or buy the printed circuit board, make sure that there are no shorts or breaks in the tracks. If you do find any, fix them before you mount any corn- Fig.3: the unit is easy to wire up but be sure to orient the three displays correctly. The decimal point of each display goes to bottom right. 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Testing Once you're sure that all of the components have been correctly positioned and soldered onto the board, you can test it using the following method: First, if you look closely at the board, you'll see that the output pin of the 7805 regulator [the pin closest to the ICs) has two tracks running from it. One goes to a 2200 resistor and the other goes to a wire link. This wire link will be used as the positive supply line. Now, either use a clip lead or solder in a piece of wire from this link to the CP0 input at pin 4 on the user port. Use two more clip leads to tie pins 2 and 5 to the ground pin [pin 6). Now feed a low-frequency (say about lOHz) clock signal of no more than 9V peak to the clock input at pin 3 of the port. You should now see the counter start counting. If this is the case then all is well. Using the module As mentioned before, the module is designed to work with other devices. Fig.3 shows how the user port at the bottom of the board is arranged from the component side. Pin 1 is the overflow output. If you wish to combine a number of these counters to produce a 6 or . maybe even a 9-digit counter, link this pin to the clock pin [CP0 or CPl) of the next counter module. Pin 2 is the LATCH ENABLE input. This pin is normally held low while 0 ===.ri=::. . --.. SU SC04309901 ~ 00 0 Fig.4: here is a full-size reproduction of the PC artwork. the clock is counting and taken high to store a particular count without having to stop the counting process. This can be used, along with the master reset pin, to turn the counter into a low-cost frequency meter. Pins 3 and 4 are the clock inputs. Note that no buffering or amplification has been given to these inputs. The signal is just fed straight into the IC from the user port pins. Again, the idea of this board was to produce a versatile design. If you intend to use this board with a CMOS project, and provided the supply voltages are the same, you most probably won't need to buffer the signal. If, however, you are using small signal equipment, you'll need to add these on via an external board. The SOUND AUSTRALIA SOUND AUSTRALIA SOUND AUSTRALIA SOUND AUSTRALIA can offer you everything from electronic components to professional public address systems Amplifiers - 100 Watts to 1800 Watts/mixers, speakers, microphones supersoft high quality coloured microphone cable + many more electronic accessories BRING ALONG OR POST THIS AD TO RECEIVE A $5 GIFT VOUCHER Ph (03) 791 1622. 28 Walker St, Dandenong 3175 58 SILICON CHIP choice is left up to you. So too is the choice of either rising-edge or falling-edge triggering by the clock signal. Pin 5 is the MASTER RESET pin. In normal operation, this pin is held low, and taken high to reset the counter at any time. Pin 6 is the ground rail, while pin 7 is the positive supply line. An important point to remember is that the voltage on any of the user port pins should not exceed the regulator output voltage. So, if you're using the 7805 regulator to provide a 9V supply, don't allow any of the input pins to go higher than 9V to prevent damage to ICl. If you want to power this project with from existing CMOS circuitry power supply, you can remove the 7805 regulator and the 2200 and 1500 resistors and just put in a wire link between the two outside holes of the 7805's position on the board. If you do this, make sure that the supply voltage doesn't exceed + 15V. To have a regulated 5V supply, leave the 7805 in place, remove the 2200 resistor and replace the 1500 resistor with a wire link. If you change the supply voltage, you'll need to check that the seven display resistors are not too small or too large in value. A good rule of thumb is that the resistors should be about 3300 for a 5V supply, about 6800 for a 10V supply, and about lkO for a 15V supply. ~