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readout * Digital plus bargraph be used as * Can a gearchange indicator drive a rev * Can limiter auto* Display dims at night Keep tabs on engine revs with this: Digital Tacho This versatile Digital Tachometer has a 4-digit LED display plus an analog style bargraph to indicate engine rpm. The displays automatically dim at night and there’s even a limiter output, so that you can limit engine revs. By JOHN CLARKE Tachometers are a “must have” item for driving enthusiasts. If you prefer a manual car, a tacho lets you know when to change gear and can help you keep engine rpm within the best operating range. An accurate tachometer is also a vital tuning aid if you have an old car and you prefer to do the engine tune-ups yourself. 14  Silicon Chip Traditionally, analog tachometers have been circular in shape with a needle (or pointer) which sweeps in a clockwise direction as the engine speed (rpm) rises. The scale behind the needle is usually marked in 100s of rpm and there’s also often a colour scale to indicate the normal rpm range (green), a high rpm range (orange) and an “over-the-limit” range (red). In recent years, digital tachometers have also become quite popular with car enthusiasts. These directly show the engine speed on 7-segment LED displays or on an LCD but they do have one disadvantage – the forbidden red zone, where you can do serious engine damage due to over-revving, isn’t indicated on the display. Instead, it’s up to the driver to remember the where the redline is and drive accordingly. This design overcomes that problem by including a bargraph display. This display operates in conjunction with the digital display and has 10 LEDs – seven green and three red. As the engine speed rises, the seven green LEDs progressively light and then the three red LEDs all light together. In effect, the bargraph has eight steps – seven for the green (normal) range and one for the redline. These eight steps can be programmed to operate at any value within a 0-9900 rpm range, so the new Digital Tachometer can be used with virtually any engine (provided its redline is less than 9900 rpm). By the way, a reading of 9900 is also the limit for the digital readout but that should be more than enough for any normal engine. Beyond 9900rpm, the 7-segment LED displays show a value of “-00” to indicate the over­ range. Basic features The Digital Tachometer is a compact unit which is much smaller than any of our previous tachometers. In fact, it is about as small as you could expect, considering that there are four 7-segment displays and a 10-LED bargraph housed in the case. That’s all been made possible by basing the design on a PIC16F84 microcontroller – the same device as used in the Speed Alarm (November 1999) and the Digital Voltmeter (February 2000). In fact, this circuit completes a trilogy of car project designs based on the PIC16F84 microcontroller. As before, the PIC controller has allowed us to dramatically reduce the required parts count and this in turn makes the unit easy to build. Even the circuits are quite similar – we’ve “simply” made a few hardware changes and rewritten the software that’s programmed into the microcontroller, so that it now functions as a tachometer. The new Digital Tachometer is also very easy to install and calibrate. It connects to the ignition supply and ground for power and obtains its signal from the ignition coil or from an engine management computer. It shows the engine rpm in 100 rpm increments on the 4-digit LED display, while the bargraph indicates engine rpm in an analog format. One nice feature is that the display brightness varies according to the ambient light. In bright light, the display is at its maximum brilliance so that it can be easily seen. However, as the ambient light falls (eg, at night time), the display automatically dims so that it won’t be too bright. Before using the tachometer, you have to select the calibration profile for your particular engine and adjust Main Features • 4-digit LED display showing up to 9900 rpm; 10-LED bargraph with redline indication. • • 100 rpm display resolution. • LEDs 8-10 (red) in bargraph display light up together for redline indication. • LED rpm indication thresholds in bargraph can be individually set (eg, to allow the unit to be used as a gearchange indicator). • Automatic calculation and setting of the LEDs 1-7 rpm thresholds when the LEDs 8-10 rpm threshold is set. • • Optional dot or bargraph display. • • Adjustable rpm hysteresis for limiter output and bargraph display. • • Automatic display dimming during low light conditions. Works with 4-stroke engines with up to 12 cylinders and 2-stroke engines with up to 6 cylinders. Rev limiter output signal (can drive the SILICON CHIP Rev Limiter switcher board described April 1999). Three switches for setting calibration, bargraph and hysteresis values (Mode, Up and Down). Rpm sensing directly from ignition coil or via low voltage signal from engine management computer. the bargraph display range. We have made this process very easy to do using just three pushbutton switches. These switches are located on the circuit board just below the bargraph display but are not accessible when the lid is on since calibration is normally a “set and forget” function. The first time you apply power to the unit, the unit will be ready to display the engine rpm. In addition, the internal program loads a number of default values for the calibration, bargraph display and hysteresis. Initially, the unit is calibrated for a 4-cylinder 4-stroke engine, the redline is set at 4000 rpm and the hysteresis is set at 100 rpm. The first LED in the bargraph lights at 0 rpm but you can change this and the other green LEDs to light at what values ever you like (eg, to indicate gear change-down points). Note that the default values remain in place unless changed by pressing the calibration switches. We’ll tell you how to do this later in the article. Dot or bargraph display In case you’re wondering, the style of the bargraph display can be changed from bar to dot mode – hey, we are using a microcontroller after all! The major difference here is that in the dot mode, only one LED from LEDs 1-7 will light at a time. However, LEDs 8-10 always light together so that aspect remains the same. The Dot mode is selected by holding down the Mode switch while power is applied to the unit (ie, when the ignition is switched on). The display will then show a “d” to indicate dot mode. Similarly, the bargraph mode can be reactivated by again pressing the Mode switch during power up. This time, the display will show a “b” to indicate that the unit is now in bar mode. Note that the adjacent digit will also show a “0”, so the display actually shows “d0” or “b0”. The dot mode can be used to provide some unique display results. For example, if you program more than one LED to light at the same rpm value, then only the LED that’s on the right will light. You can use this feature to set up the tachometer to provide gearchange indication, whereby a series of three LEDs light in sequence to indicate when to April 2000  15 Fig.1: (left): a PIC microcontroller does most of the work in the Digital Tacho. It accepts input pulses from the coil (via a pulse conditioning circuit) or from the tacho output of an engine management computer and drives the LED displays. change up. The lower four LEDs can be blanked out by programming their rpm settings to the same value as for LED 5. The hysteresis for the LED bargraph display in dot or bar mode can also be selected to give the best bargraph display and limiter results. The hysteresis sets the rpm difference between when a LED first turns on and when it is switched off. If the hysteresis is set at 0, then each LED and the limiter output will switch on at the preset rpm and also switch off at this same rpm value. This means that a LED will continually flicker on and off if the rpm remains fairly constant. Adding hysteresis (eg, 100 rpm) ensures that the engine rpm must fall by a preset amount before the LED extinguishes after first switching on. This prevents display flicker which can be distracting. Hysteresis is also useful for the limiter output. This must stay low for a certain length of time to give the ignition limiting circuit a chance to work. The hysteresis is initially preset to 100 rpm and this value should be suitable for most applications. However, if your engine doesn’t maintain a constant rpm value at a given throttle setting, a greater hysteresis value may be required. In practice, you can set it to any value from 0-900 rpm in 100 rpm steps. One feature that is fixed in the software is the display update time. This is nominally set at the count period for the ignition coil pulses and is 0.3 seconds for a 4-cylinder 4-stroke engine. However, engines with more sparks per revolution will have a calibration which gives a faster count period and this would cause the display to become a blur as the digits rapidly changed, particularly the 100 rpm digit. The software compensates for this problem by only changing the display reading at a maximum of once every 0.3s regardless of the count period set by the calibration value. The 16  Silicon Chip bargraph display update time is also fixed at 0.3s. The accompanying calibration table (Table 1) shows the correlation between the number sparks per revolution, the count period and the display update time. Note how the count period becomes very short for 6-12 cylinder 4-stroke engines. Circuit details Refer now to Fig.1 for the complete circuit details. It’s dominated by IC1 which is the programmed PIC16F84P microcontroller. This device accepts inputs from the ignition coil (via a pulse conditioning circuit) or from the tacho output of an engine management computer and drives the LED displays. OK, let’s start with the pulse conditioning circuit. First, the voltage pulses from the ignition coil are attenuated by a factor of three using a voltage divider based on 22kΩ and 10kΩ resistors. The attenuated signal is then filtered by a .056µF capacitor which shunts signals above about 400Hz to ground and then AC-coupled via a 2.2µF capacitor to diode D1 and zener diode ZD2. ZD2 limits the peak signal level to 20V, while D1 allows only positive-going pulses to be fed to the inverting input (pin 2) of IC2a. A 10kΩ resistor between this input and ground holds the voltage low in the absence of any signal via D1. Alternatively, an ignition signal which swings from ground up to a maximum of 20V can be applied to the low input if this type of signal is available on your vehicle (eg, the tacho output of the engine management computer). IC2a functions as an inverting comparator with hysteresis. Each time a positive-going pulse is applied to pin 2, the output at pin 1 swings low. Alternatively, when no signal is present, pin 1 of IC2a swings high to almost 12V. Pin 3 of IC2a is nominally biased to about 1.6V by a voltage divider consisting of 4.7kΩ and 2.2kΩ resistors, while the 47kΩ positive feedback resistor provides the hysteresis. This sets the high-going threshold for the comparator to 1.7V and the low-going threshold to 1.5V and prevents false triggering due to noise. IC2a’s output drives pin 6 (RB0) of IC1 via a 2.2kΩ limiting resistor. Specifications • • • RPM accuracy typically 0.5% plus 100 rpm. • Bargraph rpm LED threshold values and limiter output rpm level can be set at any value from 0-9900 rpm. • Bargraph and limiter output hysteresis (rpm on to rpm off) adjustable from 0-900 rpm in 100 rpm steps. • Limiter output time set at a minimum of 0.3s. Linearity and repeatability within 100 rpm. Tachometer display update time: 0.6s for 2-cylinder 4-stroke calibration, 0.3s for 4-12-cylinder 4-stroke calibration settings. This resistor limits the current flow from IC2a when its output swings to a nominal 12V, while the internal clamp diodes at RB0 limit the voltage on this pin to about 5.6V (ie, 0.6V above the supply). Pin 6 (RB0) of IC1 is set as an interrupt and the internal software responds whenever this input goes low. on and applies power to the common anode connection of DISP3. Any low outputs on RB1-RB7 will thus light the corresponding segments of that display. After this display has been on for a short time, the RA2 output is taken high and DISP3 turns off. The 7-segment data on RB1-RB7 is then updated, after which RA1 is brought low to drive Q2 and display DISP4. Finally, after a short time, RA0 is taken low to drive Q3 and LEDs1-7 of the bargraph. Note that displays DIPS1 and DISP2 always show “00”. These displays have their a-f segments commoned and connected to ground via 150Ω resistors. DISP1 is switched by transistor Q2 and so it lights when DISP4 lights. Similarly, DISP2 is switched by transistor Q1 and lights when DISP3 lights. But why multiplex DISP2 and DISP1 if they always show “00”? Why not just leave them on all the time? The answer is that we multiplex them so that they will have the same brightness as the other displays. This LED displays The 7-segment LED displays and the LEDs1-7 of the bargraph are driven directly from the RB1-RB7 outputs of IC1 via 150Ω current limiting resistors. As shown, the corresponding segments of displays DISP3 and DISP4 are connected together, as are the segments for DISP1 and DISP2. In addition, the cathodes of the first seven LEDs in the bargraph (LEDs17) are each tied to a DISP3/4 display segment. The displays are driven in multiplex fashion, with IC1 switching its RA0, RA1 and RA2 lines low in sequence to control switching transistors Q1-Q3. For example, when RA2 is switched low, transistor Q1 turns Table Table 1: 1: Calibration Calibration Data/Update Data/Update Times Tim es N o. Of Cyls. (4-stroke) 1 N o. Of Cyls. (2-stroke) 2 1 3 4 2 5 Pulses/Rev Count Period Update Time 0.5 1.2 1.2 1 0.6 0.6 1.5 0.4 0.4 2 0.3 0.3 2.5 0.24 0.3 6 3 3 0.2 0.3 8 4 4 0.150 0.3 10 5 5 .06 0.3 12 6 6 .05 0.3 April 2000  17 limiting resistors when the reline has been reached. Second, it provides the limiter output signal. This output is normally at +5V but goes low to drive an external limit circuit whenever the redline is reached. Switch inputs Fig.2: install the parts on the PC boards as shown here. Note that switches S1-S3 on the display board must be installed with their terminals oriented as shown, while the electrolytic capacitors must all be mounted parallel to the board surface (see photo). is particularly important when the displays are dimmed. Multiplexing them also means that we only need six 150Ω current limiting resistors for the two displays rather than the 12 that would be needed if they were not multi­plexed. The output at RA3 performs two functions. First, it switches low and drives LEDs 8-10 via 470Ω current Switches S1, S2 & S3 are all monitored at the RA4 input. The other sides of the Mode, Down and Up switches connect to the RA0, RA1 & RA2 outputs respectively. Normally, the RA4 input is held high via a 47kΩ resistor which connects to the +5V supply rail. However, when a switch is closed (pressed), the RA4 input is regularly taken low by one (and only one) of the RA0-RA2 outputs. The microcontroller then determines which switch has been closed by checking to see which one of the RA0, RA1 & RA2 outputs is low when RA4 is low. For example, if RA4 is low when RA0 is low, then it’s the Mode switch that’s been pressed. Similarly, if RA4 is low when RA1 is low it’s the Down switch that’s press­ed and if RA2 must be low then it’s the Up switch. The 1kΩ resistors in series with the Mode and Up switches are there to ensure that the RA0, RA1 & RA2 outputs can not be shorted if more Capacitor Codes     Value IEC Code EIA Code 0.1µF   100n 104 0.056µF    56n 563 15pF   15p  15 Resistor Colour Codes  No.   1   1   1   2   1   2   2   1   3   2  13   1 18  Silicon Chip Value 47kΩ 22kΩ 22kΩ 10kΩ 4.7kΩ 2.2kΩ 1kΩ 680Ω 470Ω 220Ω 150Ω 10Ω 4-Band Code (1%) yellow violet orange brown red red orange brown red red orange brown brown black orange brown yellow violet red brown red red red brown brown black red brown blue grey brown brown yellow violet brown brown red red brown brown brown green brown brown brown black black brown 5-Band Code (1%) yellow violet black red brown red red black red brown red red black red brown brown black black red brown yellow violet black brown brown red red black brown brown brown black black brown brown blue grey black black brown yellow violet black black brown red red black black brown brown green black black brown brown black black gold brown than one switch is pressed at the same time. This could otherwise produce strange display results. Dimming IC2b is used to control the display brightness. This op amp is connected as a voltage follower and drives buffer transistor Q4 which is inside the negative feedback loop. Light dependent resistor LDR1 controls the voltage on the pin 5 input of IC2b according to the ambient light level. IC2b in turn controls Q4 and thus the voltage applied to the emitters of display drivers Q1-Q3 and to the commoned anodes of the red LEDs in the bargraph. The circuit works like this. When the ambient light is high, LDR1 has low resistance and so the voltage on pin 5 of IC2b will be close to +5V. This means that the voltage at Q4’s emitter will also be close to +5V and so the LED displays will operate at full brightness. Conversely, in low light conditions, the resistance of the LDR will be higher and so the voltage on pin 5 of IC2b is lower than before. In fact, when it’s completely dark, the voltage on pin 5 is determined by VR1 which sets the minimum brightness level. As before, the voltage on pin 5 appears at Q4’s emitter and so the displays are driven at reduced brightness. Note that, in practice, VR1 is adjusted to give the requisite display brightness at night. Clock signals Clock signals for IC1 are provided by an internal oscillator circuit which operates in conjunction with 4MHz crystal X1 and two 15pF capacitors. The two capacitors are there to provide the correct loading and to ensure that the oscillator starts reliably. The crystal frequency is divided down internally to produce separate clock signals for the microcontroller operation and for display multi­ plexing. The crystal frequency is also used to give a precise time period over which to count the incoming ignition pulse signals at RB0. The num­ber of pulses counted in a given time indicates the engine rpm. Power Power for the circuit is derived from the vehicle’s battery rail via the ignition switch. A 10Ω 1W resistor and 47µF capacitor decouple this The display board (in case at top) plugs directly into the pin header sockets on the processor board (above), eliminating wiring connections between the two. Notice how the electrolytic capacitors on the processor board are bent over, so that they lie across the regulator leads and across ZD2. 12V supply rail, while zener diode ZD1 protects the circuit from transient voltage spikes above 16V. The decoupled supply rail is then fed to REG1 to derive a +5V rail and this in turn is filtered by the 47µF and 0.1µF capacitors. The +5V supply rail is used to power all the circuitry except for IC2 which is powered directly from the de­coupled 12V ignition supply. OK, so much for the electronic hardware which is fairly straightforward. As you’ve probably gathered by now, most of the complicated stuff takes place inside the microcontroller under software control. We’ll describe how this software works next month. Construction Fortunately, you don’t have to understand how the software works to build this circuit. Instead, it’s all programmed into the PIC chip. You just buy the preprogrammed chip and “plug” it into the socket on the circuit board. All the parts for the Digital Tacho­ meter are mounted on two PC boards: a processor board coded 05104001 April 2000  19 The pin headers are installed on the track side of the display board using a finetipped soldering iron. Note that it will be necessary to slide the plastic spacers along the leads to allow room for soldering. This view shows how the two boards are stacked together in “piggyback” fashion to make a compact assembly. Make sure that none of the parts on the processor board contact the back of the display board. and a display board coded 05104002. Both boards measure 78 x 50mm. They are stacked together and the connections between them automatically made using pin headers and cut-down IC sockets. Fig.2 shows the assembly details. Begin the construction by checking both boards for shorts between tracks, open circuit tracks and undrilled holes. This done, you can install all the parts on the processor board as shown in Fig.2. First, install all the wire links, then install the resistors using the accompanying resistor colour code table as a guide to selecting the correct values. It’s also a good idea to use a digital multimeter to measure each 20  Silicon Chip one, just to make sure. Note that the 150Ω resistors on the processor PC board are mounted end on. The horizontal trimpot (VR1) can go in next, followed by a socket to accept IC1 – but don’t install the IC yet. IC2 is soldered directly to the board and can go in now. Make sure that both IC2 and the socket for IC1 are correctly oriented. Next, install diode D1 and zener diodes ZD1 & ZD2, followed by transistors Q1-Q4. Be careful here – Q4 is a BC338 NPN type while Q1-Q3 are BC328 PNP types, so don’t get them mixed up. Now for regulator REG1 – this is installed with its metal tab flat against the PC board and with its leads bent at rightangles to pass through their respective mounting holes in the board. Make sure that the hole in the metal tab lines up with its corresponding hole in the PC board. The capacitors can now be installed, making sure that the electrolytic types are correctly oriented. Note that the electrolytics must all be mounted so that they lie parallel with the PC board, as shown in the photograph. The two 47µF capacitors at bottom right are bent over so that they lie across the regulator’s leads, while the 2.2µF capacitor below diode D1 lies across ZD1. Crystal X1 also mounts horizontally on the PC board. It is secured by soldering a short length of tinned copper wire between one end of its metal case and a PC pad immediately to the right of Q1. The three 7-way in-line sockets can now be fitted. These are made by cutting two 14-pin IC sockets into single in-line strips using a sharp knife or a fine-toothed hacksaw. Clean up the rough edges with a file before installing them on the PC board. Finally, install PC stakes at the five external wiring positions (near the bottom edge of the board and adjacent to D1). Once they’re in, trim these stakes on the component side of the board to prevent them from shorting against the display PC board later on. Also, the coil input PC stake needs to be shortened to prevent it from arcing to adjacent tracks on the display board due to its high voltage. Display board assembly Now for the display board. Install the wire links and the resistors first, including the six 150Ω resistors that sit beneath DISP1 and DISP2. The four 7-segment LED displays can then be installed with their decimal points at bottom right. Note that all the displays are mounted slightly proud of the board because of the 150Ω resistors. Make sure that they are all correctly aligned before soldering all their pins. Switches S1-S3 must be oriented correctly, so that there is normally an open circuit between the top and bottom terminals of each switch. These switches have leads which are rectangular in shape and it’s simply a matter of installing them with their leads oriented as shown in Fig.2. The LED bargraph mounts so that the anode leads are to the left. Install Fig.3: follow this diagram when stacking the boards together and be sure to use plastic washers where indicated. Note the small heatsink attached to the brass spacer. Fig.4: the full-size artworks for the front panel and PC boards are shown above and at right. it so that the green LEDs are to the left and the red LEDs to the right and you can’t go wrong. It should also be installed so that its top face is 19.5mm above the PC board, so that it will later sit flush with the front panel. The LDR should be mounted with its face about 1.5mm above the displays. Finally, complete the display board assembly by inserting the pin headers. These are installed from the copper side of the board with their leads just protruding above the board surface. You will need a fine-tipped soldering iron to solder them to the copper pads on the PC board. It will also be necessary to slide the plastic spacers along the leads to allow room for soldering. Final assembly The plastic case requires a minor amount of work before installing the PC boards. First, use a sharp chisel to remove the integral side pillars, then slide the processor PC board into the case and drill two mounting holes – one through the metal tab hole of the regulator and the other below the 0.1µF capacitor near IC2. An oversize drill can then be used to countersink the holes on the outside of the case, to suit the specified M3 x 6mm CSK screws. Two holes are also required at the rear of the base of the case for the power supply wiring and for the ignition coil lead. These holes can be drilled so that they line up with the relevant PC stakes. The next step is to fashion a small heatsink from sheet copper and solder it to the 6mm brass spacer – see Fig.3. This heatsink must be shaped so that the copper sheet cannot make contact with any components on the processor PC board and cause a short. The main component to watch out for here for is ZD1. The display board can now be plugged into the processor board and the assembly secured exactly as shown in Fig.3. Be sure to use plastic washers and spacers where specified and note that you must use an M3 x 15mm Nylon screw on one side of the assembly, while the other side uses a metal screw. Check that the leads from the parts on the display PC board do not interfere with any of the parts on the processor PC board or with the copper heatsink. Some of the pigtails on the display PC board may have to be trimmed to avoid this. The front panel label can now be affixed to the front panel and used as a template for making the display cutouts and for drilling the hole for the LDR. The main display cutout is made by first drilling a series of small holes around the inside perimeter, then knocking out the centre piece and filing the job to a smooth finish. Make the cutout so that the red Perspex or acrylic window is a tight fit. The window can be further secured by applying several small spots of super glue along the inside edges. Similarly, the cutout for the LED bargraph can be made by drilling a row of small holes and then filing so that the bargraph is a neat fit. Test & calibration It’s a good idea to check the power supply before plugging the microcontroller IC into its socket. To do this, first unplug the display board and connect automotive wires to the +12V and GND inputs of the processor board. This done, apply power and use a multimeter to check that there is +5V on pins 4 & 14 of IC1’s socket, using the metal tab of REG1 for the ground connection. If this is correct, disconnect the power and insert IC1 in place, ensuring that it is oriented correctly. Now attach both PC boards together and reapply power. The 7-segment LED displays should show “000” rpm, April 2000  21 Parts List 1 processor PC board, code 05104001, 78 x 50mm 1 display PC board, code 05104002, 78 x 50mm 1 front panel label, 80 x 52mm 1 plastic case utility case, 83 x 54 x 30mm 1 dark red transparent Perspex or Acrylic sheet, 59 x 20 x 2.5 1 4MHz parallel resonant crystal (X1) 1 LDR (Jaycar RD-3480 or equiv.) 5 PC stakes 3 7-way pin head launchers 2 DIP-14 low cost IC socket with wiper contacts (cut for 3 x 7-way single in line sockets) 3 tactile switches (S1-S3) (Jaycar SP-0730 or equiv.) 1 500kΩ horizontal trimpot (VR1) 1 6 x 20 x 0.5mm sheet copper for heatsink 1 400mm length of 0.8mm tinned copper wire 1 2m length of red automotive wire 1 2m length of black or green automotive wire (ground wire) 1 2m length of 250VAC wire for ignition coil connection 3 6mm tapped spacers 2 M3 nuts 2 M3 x 6mm countersunk screws or Nylon cheesehead cut to length 3 M3 plastic washers 1mm thick 1 M3 x 15mm Nylon screw while the first seven LEDs of the bargraph should be lit. Pressing the Mode switch (at far left) selects the first calibration function (or mode). This mode shows the calibration value which is a number ranging from 1-12, corresponding to 1-12 cylinders for a 4-stroke engine. Note that the display also shows the two fixed righthand “00” digits but these are ignored. Initially, the display should read “400” which is the default value for the number of engine cylinders; ie, the default is for a 4-cylinder engine (as previously stated, the two right­hand digits are ignored). The calibration number is changed using the Up button (far righthand side) which selects the next value. 22  Silicon Chip 1 M3 x 15mm brass screw Semiconductors 1 PIC16F84P microprocessor programmed with TACHO.HEX program (IC1) 1 LM358 dual op amp (IC2) 1 7805, LM340T5 5V 1A 3-terminal regulator (REG1) 3 BC328 PNP transistors (Q1-Q3) 1 BC338 NPN transistor (Q4) 4 HDSP5301, LTS542A common anode 7-segment LED displays (DISP1-DISP4) 1 10-LED bargraph (Jaycar ZD1702 or equiv.) (LEDs 1-10) 1 16V 1W zener diode (ZD1) 1 20V 1W zener diode (ZD2) Capacitors 2 47µF 25VW PC electrolytic 1 2.2µF 50VW bipolar electrolytic 2 0.1µF MKT polyester 1 .056µF MKT polyester 2 15pF ceramic Resistors (0.25W, 1%) 1 47kΩ 2 1kΩ 1 22kΩ 1W 1 680Ω 1 22kΩ 3 470Ω 2 10kΩ 2 220Ω 1 4.7kΩ 13 150Ω 2 2.2kΩ 1 10Ω 1W Miscellaneous Automotive connectors, heat­shrink tubing, cable ties, etc. You simply press this switch until the required value appears. So, if you have a 6-cylinder car, press the Up button twice so that the display reads “600”. The Down switch (middle) does not operate for the calibration adjustment. Note that if you are calibrating for a 2-stroke engine, you should select a value that is twice the number of cylinders. Pressing the Mode switch again lights the lefthand LED in the bargraph display. This corresponds to the lower rpm LED setting which is initially “000” rpm. It can be adjusted using the Up and Down switches if you wish to alter the default value. Pressing the Mode switch again cycles to the next LED in the bargraph display and so on until the final 8, 9 & 10 (red) LEDs of the bargraph display all light up. As indicated at the start of the article, the initial pre-programmed redline value is 4000 rpm and this will be indicated on the display. This value should be altered to suit the redline limit for your engine using the Up and Down switches. Once this had been done, the lower rpm settings for LEDs 1-7 are automatically calculated to provide a linear progression. You can go back and check this by pressing the Mode switch until you return to the rpm setting modes (after three Mode switch pressings) for each LED on the bargraph display. Note that you must change the 4000 rpm setting, otherwise the automatic calculation process won’t take place. This means that if you wish to set the redline limit at 4000 rpm (ie, to the default value), you must first press the Up switch and then the Down switch to return to 4000 rpm again. Once this has been done, the automatic calculation will take place. OK, so that’s the basic setup procedure for the Digital Tachometer. Note that all these settings now remain in place unless they are altered using the switches – even if the power is removed. Advanced features While most users will be happy with the basic setup, there are some added features for those who would like to customise their tachometer. One of the obvious changes that could be made is to individually adjust the rpm setting for each LED in the bargraph display. This could be done to compress the rpm range for the middle LEDs where most of the engine action takes place. For example, the lower LED could be set to indicate the engine speed at which to change down, to prevent the engine from labouring. The middle LEDs could then be programmed to light over a narrower range of rpm values compared to the linear progression that is automatically calculated. The only thing to note here is that it is important to adjust the LEDs 8-10 (redline) value first before changing the lower rpm values for the remaining LEDs. If you don’t do this, the settings will be overwritten by the automatic recalculation process that takes place each time the LEDs 8-10 rpm value is changed. Simply cycling through the LEDs 8-10 rpm setting using the Mode switch will not activate the automatic recalculation process, however. Automatic recalculation only occurs when the Down or Up switch is pressed in this mode. In fact, you can cycle through all the modes without changing any of the settings. The hysteresis setting mode is selected by repeatedly pressing the Mode switch until the display shows “H100” (ie, the default is 100 rpm). If necessary, this can be altered using the Up switch. As you do this, the display indicates hysteresis in 100’s of rpm. Note that the Down switch does not operate in this mode. Further tests & installation You can test the dimming feature by holding your finger over the LDR to simulate darkness. Unfortunately, you will need to unplug the display board (with the power switched off) to make adjustments to VR1, so adjustments will have to be done on a trial and error basis. The best time to make this adjustment is at night – just set VR1 to give the correct minimum brightness in the dark. You can further test the Digital Tachometer with a signal generator set to give a 3V rms sinewave output. Attach the signal generator output between ground and the low voltage input of the tachometer. The unit should show a reading of 3000 rpm per 100Hz input (4-cylinder, 4-stroke calibration only). Use automotive cable and connectors when installing the unit into a vehicle. The +12V supply connection is derived via the ignition switch and a suitable connection point will usually be found inside the fuse­box. Be sure to choose the fused side of the supply rail, so that the existing fuse is in series. The ground connection can be made by connecting a lead to the chassis via an eyelet and self-tapping screw. The coil input for rpm sensing can connect directly to the switched side of the ignition coil using 250VAC rated wire. Alternatively, you can use a low voltage signal if this is available from the vehicle’s computer; eg, a low-voltage tachometer output signal. A 0-5V signal will directly trigger the Digital Tachometer if the signal is Using The Rev Limiter Output A S MENTIONED the Digital Tach   ometer limit output can control an engine limiter. This will reduce the number of sparks per revolution at the rpm limit and thus prevent the engine from revving past this limit. We published a suitable Rev Limiter circuit in the April 1999 issue but note that you don’t have to use the whole circuit. Instead, you only have to use the Ignition Switcher circuit which was assembled on a separate PC board. The Ignition Switcher uses a single 555 timer IC and several transistors to drive a high-voltage Darlington output transistor. When the rev limit is reached, this transistor shorts out the main switching transistor in the car’s ignition system for about 50% of time, thus reducing the engine power and thereby limiting the engine rpm to the redline. The two circuits are easy to marry – all you have to do is connect the limit output from the Digital Tacho­meter directly to the terminal marked “From Rev Limit Controller” on the Ignition Switcher. A suitable value for C1 must be chosen for the Ignition Switcher from the table published in the April issue. This sets the requisite number of sparks that are blocked out during the limiting action. Note that if the Digital Tachometer derives its input signal from the coil, it will sense that the rpm has dropped as soon as the coil is prevented from sparking via the limiter action. This means that the limit action may not be as smooth as it would be if the tachometer signal was derived from a different source, such as the tachometer output from the engine computer. However, the limit output from the tachometer will remain low to disable the spark for at least 0.3s, regardless of the input source for the tachometer. This should provide sufficient time for the limit action to take place. The limiter output from the Digital Tacho can be used to drive this Ignition Switcher board (SILICON CHIP, April 1999), to restrict engine revs to the “red-line” setting. connected to the low voltage input. Note that some cars, including late-model Holden Commodores and Ford Falcons, use double-ended ignition coils, with each coil simultaneously firing two spark plugs (ie, three coils are used for a 6-cylinder engine). Similarly, some cars use individual coils for each cylinder and these are usually located at the ends of the HT leads, directly on the spark plugs. Invariably, these types of coils are fully encapsulated and their terminals are not accessible. The answer here is to use the tacho output from the engine management computer. You will need to refer to the wiring diagram for your vehicle to identify the correct lead or check with an auto SC electrician. April 2000  23