Fullscreen HTML5Med Res (??MB)HTML4

Build this LED digital tachometer Have you ever wondered how many revs your car's engine is doing at lOOkm/h or at any speed for that matter? This digital tachometer will tell you. It works with all ignitions from Kettering to Hall Effect systems and with 4, 6 & 8-cylinder cars. By DARREN YATES Although many new cars feature a conventional analog rev counter, it 's hardly wh at you would call an exciting use of technology! By contrast, this new tachometer features a bright 4-digit readout that indicates from 09900 RPM with a reso lution of 100 RPM. As can be seen from the photographs, th e unit is housed in a neat littl e plastic case that can easily be attached to the dashboard of your car using Ve lcro strips. The unit is very easy to build and uses readily avail16 SIUCO N CIIII' able parts. In fact , you will probably already have most of the parts in your junkbox. Unlike many other tachometers, this unit will work with just about any car ignition system. We checked the prototype on a number of cars ranging from Commodores & Falcons with electronic ignition systems through to a beat-up old VW w ith a pointsswitched (Kettering) ignition. The unit worked perfectly in all cases, although the calibration control does have to be reset when switching between cars with different numbers of cylinders . Only three connections are required to connect the unit to your car: one to the negative terminal of the coil and two for power (+12V and GND). The positive supply is derived from the ignition switch, so that the unit is switched on and off with the engine. Basic principle The tacho circuit basically works as a frequency counter but first we should consider what it is that we are counting. In a 4-stroke design, the ignition coil produces two sparks per revolution for a 4-cylinder engine, three sparks per rev for a 6-cylinder engine, and eight sparks per rev for an 8cy linder engine. So if we have a 4cylinder engine operating at 1500 RPM, then the ignition coil must be delivering 3000 high vo ltage pulses per minute. This corresponds to a frequency of 30007 60, or 50Hz. This frequency of 50Hz also corresponds to 1000 RPM for a 6-cylinder engine and 750 RPM for an 8-cylinder engine. Since the frequency goes up linearly with revs per minute, all we need to do is sample the pulses from the engine coil for the correct amount of time to give the correct display. Because we decided on a maximum count of 9900 with a resolution of 100, we only needed a 2-digit counter. This 2-digit counter is used to drive the two most significant digits (MSDs) of the display, while the two least significant digits are permanently wired to show "0"s. This keeps the complexity and cost of the· project to a minimum. Engine irregularities The other reason for using just a 2digit counter is that a 3 or 4-digit design would be overkill because of engine irregularities. At any speed setting, an engine will typically vary its speed from moment to moment by as much as ±50 RPM and this means that the last two digits of a 4-digit display flicker continuously. This effect applies even to the latest cars with their electronic ignitions and fuel injection controlled by a microprocessor. They are certainly smoother than the older cars with carburettors and Ketterir:i.g ignition but they vary nonetheless. Therefore, it makes good sense to use a 2-digit counter with two extra digits as dummies, to make the display easily read, at a glance. If we intend to fit this counter to a 4-cylinder car, we have to make a 50Hz input appear as 1500 RPM on the · display. However, as we've just pointed out, we are only concerned with the two most significant digits which, in this case, must display "15". This is achieved simply by counting the 50Hz input for 0.3 seconds. But what if you have a 6-cylinder car? Well, the two MSDs must read "10" for the same 50Hz input which means that we only have to count for 0.2 seconds. Similarly, for an 8-cyiinder car, we have to count the 50Hz input for 0.15 seconds to get a display of"7" or "8", which is as close to 750 RPM as we can get. Block diagram Refer now to Fig.1 which shows a block diagram of the circuit. As shown, the input is taken from the negative side of the coil's primary winding (ie, from the points or main switching transistor). Each time the ignition coil INPU TFROM COIL NEGATIVE -- lJl FILTER ANO SCHMITT (IC3d ,f) TT TWO-DIGIT COUNTER (IC4-IC6) CLOCK INPUT EN LATCH R T ENA !LE -EDGE DETECTOR (IC3c) TIMING MONOSTABLE i--(IC1) +EDGE DETECTOR (IC3b) rt • RST 7 5 3 +10 COUNTER CLK EN (IC2) CLK --- SQUAREWAVE OSCILLATOR (IC3a) Fig.I: block diagram of the Digital Tachometer. The high voltage spikes produced at the negative terminal of the coil are applied to a Schmitt trigger/filter stage which produces clean square wave pulses. These pulses are then fed to a 2-digit counter which drives the two most significant digits. The remainder of the circuit produces the necessary timing signals for the counter - reset, clock enable & latch enable. switches, it generates a voltage spike of about 300V, followed by a ringing waveform of decreasing voltage due to the coil's self-resonance. This input signal is filtered and fed to a Schmitt trigger stage (IC3d,f) which produces clean squarewave pulses corresponding to the high voltage spikes. These pulses are then fed into the clock input of the 2-digit counter (IC4, IC5, IC6; more about this later). To produce a display that updates smoothly, we need to store and display the previous count while the counter is tallying up the new one. The rest of the circuit deals mainly with this task. Going back to the block diagram, a squarewave oscillator (IC3a) continually feeds a divide-by-10 counter (IC2) with clock pulses. The CLOCK ENABLE line of this counter is "active low ", which means that for the counter to advance, the line must go low. Because it is a divide-by-10 counter, each ::;j;;}g .- :,I!d• -,,:;-.--:--:.\= C All the parts for the Digital Tachometer are mounted on two PC boards. These two boards are soldered together at right angles & fit neatly inside a standard plastic case. A red perspex window sits in front of the LED displays. AUGUST 1991 17 .... z:,: >:E0U "" 1J~~Ej ~,~~"-~~ s~ ...-'"' ."' N ·~ 0 N ."' 'I• ~ > ~ 7•· .... "' ::, ~~ ...~!:i -, "' ~ "' 0z ,- ~•· 0 Hf--1 I· "' .... ~ ~!::i ~ ~ cc w Iw 0 N -, ~-II· :E 0 "' 0 ::c ~ ~ ,- ~-~ :!: .... ...§;:( ."' ,. ~ I· ~ ~,ci-' N -, "' .. I 0 ::, ,. ~ a) ~ ....... .... M ►M ...-'"' "'1=::1~1 - -:-- ~ ~ ~!::i ~ ...J ~ 0 ct33a, " ~g ww ~ w > .. 0 "' . ... - "'- ~ :: =-- .a ,-"' N ~ "' .a "' ~ ::u ~ ~ M ... "' m ,....-!! liii "' "' N ... ~ ;! = "'..,-~ I~ ~ ~ ·~ "' - - "'....I w .... ;! ~ ' .. ..,_ ..,..,.., - .... ~ ~ - N ~ -~H•· :=! ~ lj = N .... ,,. E' =>u~ ..111: . ~ .:a: Jo ~ SILICON CHIP . z M f!al•· N . N,-. ..,-= ,~ cc: ffi "'l - >< <.> ;! •t--- . cc: ~1[_ cc:= .... cc: !:!~ j> ~ ~ ~~ !;;; ~ c> c> ~ ~HI· "' ~"' .a .., I· c> !: +:f-{1- .... ~~ ~ N~ . ~I• :5 .... l'.'l "'~ ~~ ~ ,-0> /':-:\. "' ~=· -=-Ji· ~ 'I• ~ O...:E;< Q .., c> 7,,- :g _17.., .,,l6HI· c> ....- w -"' ~ ~ ~== -~~r:J'~ z ~ ~ 0 wl O "' ..,<.> ,~ cc: ; ~g 0 a:~1- ~ + - H•· f;i§ "' ~ :: ~ ... I I• r.>. 18 - "' ~I· ~ CJ~ -EE~ .... "' ~..._.z= ; ::Zc, C!:I "' ,~ cc: c> "' . :=! ;! N U ~ ..- ~~ + - - r.;'\ N - ,..., c:, .. ~ ffi ~M . ~ N "' ~, - N Im ~ .., ffi ~ ~ ,_ - ~ \:!.I "' ~~ . .... .... "' "'I - 11 N -"'- . "' a, -.... 0 ~ .,,u1n ---!:?/~ ~~ ,. <.> .a "' "' ...........,: !: "' "' "' :=! ~I• N -. - . . .... > ~ m u ► cc: ~ ~ "' "' Fig.2 (left): all the circuit functions depicted in the block diagram (Fig.1) can be directly related to this main circuit diagram. Ql, IC3d & IC3fform the Schmitt trigger/filter stage & this drives a dual 4-bit BCD counter (IC4b & IC4a). These counters then drive 7segment decoders IC6 & IC5 which h1 turn drive the two most significant digits. IC7a provides leading zero blanking, while ICl, IC2 & their associated Schmitt trigger inverters provide timing signals for the 2-digit counter. c> 0 ~ . cc: successive output goes high in turn as the counter is clocked. The 7th decoded output is fed into the CLOCK ENABLE line, so that when power is first applied, the CLOCK ENABLE is held low, thus allowing the counter to count. When output "3" subsequently goes high, it triggers the LATCH ENABLE of the 2-digit counter. This instructs the 2-digit counter to store and display the pulse count from the coil. During this time, the squarewave oscillator continues to clock the divide-by-10 counter. When output "5" subsequently goes high two clock pulses later, it resets the 2-digit counter so that it is ready to count the next series of pulses on its clock input. This counting period is initiated another two clock pulses later, when output "7" goes high and pulls the CLOCK ENABLE input high to stop the divide-by-10 counter. This high on output "7" also triggers a timing monostable (ICl) via a positive-edge detector (IC3b) , which allows the 2digit counter to count the incoming pulses for a specified period of time . The timing monostable output goes high for 0.3s for a 4-cylinder engine, 0.2s for a 6-cylinder engine and 0.15s for a V8. This output is fed into the CLOCK ENABLE input of the 2-digit counter which now starts counting. At the end of the specified interval, the output of the timing monostable goes low and the 2-digit counter is disabled. This low-going signal is also picked up by a negative-edge detector (IC3c) when then provides a short positive-going pulse at its output to reset the divide-by-10 counter. This allows the divide-by-10 counter to again go through the above sequence of steps; ie, latching and dis- playing the current count in the 2digit counter, then resetting the 2digit counter and allowing it to count the next timing interval. Circuit details Take a look now at the main circuit diagram - see Fig.2. It contains all the circuit elements shown in the block diagram (Fig.1). The input pulses are taken from the negative-side of the primary winding of the coil and fed to a voltage divider consisting of 33kQ and l0kQ resistors. Because of the high voltages involved, the 3 3kQ resistor must be rated at 0.5W and 300V. The signal is then AC-coupled into the base of transistor Ql which acts as a,switch. Each time a high voltage spike is applied to the input, Ql turns on and shorts a 0. lµF capacitor to ground. This in turn pulls the input of Schmitt trigger IC3d low and thus the output of IC3f (pin 4) also goes low. Because the input spike to Ql is very narrow, the transistor quickly turns off again. The 0. lµF capacitor across Ql now charges via an 18kQ resistor and, after a brief period, switches pin 4 of IC3f high again. The RC time constant here is about 2ms, which is longer than the period fo r which hash (ie, ringing due to coil resonance) is present on the input signal. In practice, this means that the input circuit is disabled for about 2ms after the initial spike is detected to prevent false triggering. The output of Schmitt trigger IC3f thus consists of a series of negativegoing pulses, with each pulse corresponding to a plug-firing pulse from the coil. These pulses are now fed into the clock input (pin 1) of IC4b which is half of a 4518 dual 4-bit binary-coded-decimal (BCD) counter. The most significant bit of IC4b is connected to the CLOCK ENABLE input (pin 10) of IC4a to produce a 2-digit BCD counter. The 4-bit outputs of each counter are then fed into separate 4511 7-segment decoder ICs (IC5 & IC6) which in turn drive the two most significant displays. Leading zero blanking To make the display more attractive, we have added leading zero blanking to the unit. This part of the circuit is quite simple and relies on the fact that if the leading digit is "0" , then each of the four bits output from IC4a will be low. These outputs are fed into a 4-input diode OR gate (D3D6), the output of which is fed to Dtype flipflop IC7a. IC7a acts as a memory cell or latch. When the DATA input of IC7a goes low, the Q output also goes low at the next clock pulse. This in turn pulls the BLANKING INPUT (pin 4) of IC5 low and so display 4 is turned off. However, if the leading digit has any value from 1-9, the output of the diode OR gate will be high. Thus, the output of IC7a will also be high and so the blanking function will be disabled. Timing circuit !Cl, IC2 and Schmitt trigger inverters IC3a, IC3b & IC3c make up the timing circuit (see also Fig.1). Schmitt trigger IC3a is connected as a simple square wave oscillator which operates at about 450Hz. Its output at pin 10 drives the clock input (pin 14) of IC2 which is a 4017 divide-by-10 counter. When power is first applied, ICZ's "0" output is high and the remaining outputs are all low. The remaining outputs then go high (and low again) in sequence as the counter is clocked. After two clock pulses, the "2" output at pin 4 goes high and clocks IC7a, which is the leading zero blanking latch. This ensures that if the most significant digit is zero, it will be blanked out for the whole timing cycle. On the next clock pulse from IC3a, IC2 's "3" output (pin 7) goes high. This high is inverted by IC3 e and fed to the LATCH ENABLE (LE) pins of decoders IC5 and IC6. These !Cs then latch the counts at the outputs ofIC4a & IC4b and decode this binary data to drive the two leading 7-segment displays (display 3 & display 4). With the count now latched and displayed, IC2's "5 " output (pin 1) goes high two clock pulses later and resets counters IC4a & IC4b. These two counters are now ready to start counting a fresh sequence of pulses from the coil but this doesn't happen until the clock enable input (pin 2) of IC4b is pulled high. We 'll see how this happens shortly. In the meantime, IC2 continues to count up until output "7" (pin 6) goes high. When this happens, it pulls its own CLOCK ENABLE input (pin 13 high) and thus disables the clock input. As PARTS LIST 1 plastic instrument case , Arista UB14 or DSE Cat. H-2503 1 PC board, code SC05108911 , 112 x 84mm 1 PC board, code SC05108912, 84 x 38mm 1 front panel label , 115 x 40mm 3 6mm standoffs 1 piece red perspex, 55 x 20mm 1 50kQ linear mini vertical trimpot Semiconductors 1 NE555 timer IC (IC1) 1 4017 CMOS divide-by-10 counter (IC2) 1 74C14 CMOS hex Schmitt trigger inverter (IC3) 1 4518 CMOS dual BCD UP counter (IC4) 2 4511 CMOS ?-segment display drivers (IC5,IC6) 1 4013 CMOS dual D flipflop (IC?) 1 7805 +5V regulator 1 BC337 NPN transistor (01) 2 1N4004 power diodes (D1 ,D2) 4 1N914 signal diodes (D3-D6) 4 LTS543 common-cathode ?-segment displays Capacitors 1 33µF 35VW electrolytic 1 2.2µF 50VW electrolytic 4 0.1 µF 63VW 5mm-pitch metallised polyester 1 .047µF 63VW 5mm-pitch metallised polyester 1 .022µF 63VW 5mm-pitch metallised polyester 2 .01 µF 63VW 5mm-pitch metallised polyester 1 .001 µF 63VW 5mm-pitch metallised polyester Resistors (0.25W, 5%) 3100kQ 1 82kQ 1 56kQ 1 47kQ 1 33kQ (0.5W, 300V) 1 18kQ 3 10kQ. 261kQ 1 220Q 1 1500 Miscellaneous Solder, insulated hookup wire , tinned copper wire , screws, nuts & washers. A UGUST 1991 19 ply connecting six of their segments (segment "g", pin 10 is the exception) to the +9V supply rail via lkQ resistors. The common cathode, pin 8, connects to the 0V line. Power for the circuit is derived from the car's battery and passes via the ignition switch to diode DZ which provides reverse polarity protection. DZ then feeds a 7805 3-terminal regulator which, together with its associated 220Q and 150Q resistors , delivers a regulated +9V to power the circuit. The 33µF capacitor on the input of the 3-terminal regulator provides supply decoupling, while the 0.lµF capacitor filters out any high frequency noise. •• DISP1 •• •• LTS543 : ,.•• - •• DISP2 • •• LTS543 : • •• 1• •• DISP3 • •• LTS543 •: 1• •• 8~~\:•• •• • Fig.3: install the parts on the PC boards as shown here & pay particular attention to the orientation of the semiconductors & the LED displays. After assembly, the two PC boards are soldered together at right angles (see text)., Construction a result , the "7" output stays high for a fixed period of time, until ICZ receives an external reset signal. This external reset signal is supplied by 555 timer IC1 which also sets the count time. It works like this. When decoded output "7" goes high, it also triggers a positive edge detector consisting of Schmitt trigger IC3b, a 10kQ resistor and a .047µF capacitor. IC3b thus momentarily switches its output low and this triggers IC1 which is connected as a monostable. When the 555 is triggered, its output at pin 3 goes high for a short period of time , as determined by VR1, resistor Rx and a 2.2µF electrolytic capacitor. This high is fed to the CLOCK ENABLE input of IC4b which is now clocked by pulses from the coil. The counter is subsequently disabled when pin 3 ofICl goes low at the end of the timing period, after which the 4-bit counts are latched by IC5 & IC6 as described previously. Trimpot VR1 allows the monostable period to be adjusted so that the tacho can be accurately calibrated, while Rx is selected to suit the number of engine cylinders (since VR1 only has a limited range). For a 4-cylinder engine, Rx is 82kQ; for a 6-cylinder engine, it's 56kQ; and for a V8, it's 47kil When pin 3 of IC1 goes low at the end of the monostable period, it also resets IC2 via the negative edge detector based on IC3c. Normally, pin 13 of IC3c is held high by a 100kQ pullup resistor. However, when pin 3 of IC1 switches low at the end of the timing period, pin 13 ofIC3c is briefly pulled low via a .00lµF capacitor. Pin 12 of IC3c thus briefly switches high and resets ICZ so that the next cbunting cycle can begin. The two least significant digits in the display are wired to show "0" continuously. This is achieved by sim- OK, we've examined how the circuit works. Now let's build the tachometer. The project is built on two PC boards, one for the circuitry and one for the four 7-segment LED displays. After assembly, the two boards are soldered together at 90 degrees to give a compact assembly that fits into a low-profile plastic case. Fig.3 shows the assembly details. Before mounting any of the parts, care- TABLE 1: CAPACITOR CODES 0 0 0 0 0 0 Value IEC Code EIA Code 0.1µF .047µF .022µF .01µF .001µF 100n 47n 22n 10n 1n 104 473 223 103 102 TABLE 2: RESISTOR COLOUR CODES 0 0 No. 3 0 0 0 0 0 0 0 0 0 20 3 26 SILICON CHIP Value 4-Band Code (5%) 5-Band Code (1%) 100kQ 82kQ 56kQ 47kQ 33kQ 18kQ 10kQ 1kQ 220Q 150Q brown black yellow gold grey red orange gold green blue orange gold yellow violet orange gold orange orange orange gold brown grey orange gold brown black orange gold brown black red gold red red brown gold brown green brown gold brown black black orange brown grey red black red brown green blue black red brown yellow violet black red brown orange orange black, red brown brown grey black red brown brown black black red brown brown black black brown brown red red black black brown brown green black black brown As explained previously, resistor Rx is selected to suit your car's en- gine. Check the bottom lefthand corner of Fig.2 for the correct value for your car. The 0.5mm fixed pitch capacitors can now be installed (see Table 1), followed by the 50kQ trimpot, diodes Dl-D6 and the transistor (Ql). Check that the diodes and the transistor are correctly oriented before soldering their leads. Finally, install the six ICs and the 7805 regulator. Note that the ICs all face in the same direction and that the regulator is oriented so that its metal tab is adjacent to the edge of the PC board. Display board Follow the procedure described in the text when soldering the two boards together at right angles. Note that each LED display must be mounted with its decimal point at lower right. fully check the copper sides of the boards to make sure that they have been correctly etched. When you are satisfied that they are OK, begin the assembly by installing all the wire links on the main PC board (code SC05108911). A worthwhile tip here is to stretch the tinned copper wire to be used for the links slightly before cutting the individual lengths. This will ensure that the links are all nice and straight and prevent them from shorting to adjacent components. Next, install the resistors. Table 2 lists their colour codes but we suggest that you also check each value on your DMM before mounting it on the PC board, since some of the colours 0 ~ ® ,.. TO BASE, 01 12·24VAC o---------GND Fig.4: here's how to use the mains as a 50Hz frequency reference. Adjust VR1 for a reading of 1500 RPM on. a 4-cylinder engine or 1000 RPM on a 6-cylinder engine. On a V8, adjust VR1 until the display alternates between 700 & 800RPM. can be difficult to distinguish. Note that some resistors in the top lefthand corner of the board are mounted end on to save space. -=1._______ DIGITAL TACHOMETER Fig.5: this full-size artwork can be used to mark out the front panel window. This board (code SC05108912) is easy to assemble since it only carries the four LED displays. Push each display down onto the board as far as it will go and make sure that it is correctly oriented (not upside down!) before soldering its pins. You can determine the correct orientation by checking the location of the decimal point - it should be at the bottom righthand corner of each LED display when the display is viewed the right way up. The two PC boards can now be soldered together via their bus connector strips. To do this, temporarily mount the main board in the case on 6mm standoffs and butt the display board against it. Check that the bottom edge of the display board rests on the bottom of the case, then use a pencil to mark the back of the display board where the boards intersect (note: you may have to file the bottom corners of the display board slightly to clear the case mounting pillars). The two boards can now be removed from the case and tack soldered together at each end. This done, check the assembly in the case, adjust the boards as necessary, and solder the remaining connections. Front panel To cut down on glare, a piece ofred perspex is fitted into a hole cut in the front panel, immediately in front of the LED displays. This cutout is best made by using the published artwork as a marking template, then drilling a series of holes around the inside perimeter and knocking out the centre piece. AUGUST 1991 21 1 11 I 11111111111 1\\l\\ \ \\lll correctly, you should get a "000" display with the MSD blanked out. At this stage, it's a good idea to check the supply voltage to the ICs. First, check that the 7805 regulator is delivering +9V, then check that this voltage is present on pin 8 ofICl, pin 14 ofIC3 & IC7, and pin 16 ofICZ, IC4 , IC5 & IC6. If you don't get the correct voltages or the display is incorrect, switch off and check your boards against Fig. 3 for wiring errors. In particular, check for incorrect component placement or orientation and for missed or faulty solder joints. Calibration •II N ,... O') CX) 0 ,... LO l!!filil 1 ~~11 1 fi Fig.6: here are the full-size artworks for the two PC boards. Check your boards carefully before mounting any of the components. The cutout is then carefully filed to shape until the perspex window is a tight fit. Once this has been done, remove the perspex, carefully affix the adhesive label to the panel, and cut away the panel from around the hole using a sharp utility knife. Finally, replace the perspex window and check that it is a tight fit. If the perspex is loose, it can be secured using a spot of adhesive at each corner on the inside of the panel. The board assembly can now be 22 SILICON CHIP installed in the case. Note that the three external leads pass through a small grommeted hole in the rear panel. Tie a knot in the leads inside the case before passing them through the grommet to prevent the wires from coming adrift. Testing Now unit to battery switch for the big test. Connect the a 12V DC supply (eg, a car or a 12V DC plugpack) and on. If the project is working There are two ways of calibrating the Digital Tachometer: (1) you can calibrate it against another tachometer (eg, in another car); or (2) you can calibrate it against a mains-derived 50Hz frequency reference. The first method is the easiest but its accuracy depends on the accuracy of the .reference tachometer. In this case, all you have to do is adjust VRl until both tachometers give the same reading. By contrast, the second method is extremely accurate. Fig.4 shows a suitable mains-derived calibration circuit. This uses a diode to half-wave rectify the 12-24V AC secondary voltage of a rp.ains transformer to provide a 50Hz input waveform. The 4.7kQ resistor in series with the diode provides current limiting, to protect the transistor. Connect this calibration circuit directly to the base of Ql (just solder the input lead to the top of diode Dl) and don't forget the ground connection. Now switch on and adjust VRl until you get the correct reading. This will be 1500 RPM for a 4-cylinder car and 1000 RPM for a 6-cylinder car. For an 8-cylinder car, adjust VRl until you get a reading that alternates between 700 & 800 RPM (ie, 750 RPM) .. Finally, remove the calibration circuit, install the board assembly in the case and mount the unit on the dashboard of your car. Don't forget that the input lead is conrrected to the negative side of the ignition coil primary, while the +12V supply is derived via the ignition switch. In most cars, this switched +12V rail can easily be picked up at the fusebox (use automotive connectors for all connections). Make sure that the power is derived via one of the fuses. SC