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By JOHN CLARKE Don’t blow your engine – fit this rev limiter Do you regularly rev your engine to the red line on your car’s tacho? Have you ever missed a gear change and spun the engine to squillions of revs? Or have you actually blown your engine by over-revving it? If so, you need this rev limiter. It can save thousands of dollars damage to your engine. 80  Silicon Chip This versatile rev limiter works by blocking some of the ignition sparks when the engine exceeds the preset limit. It is not a “hard” limiter which kills the engine RPM by stopping fuel and all the sparks from the ignition system. Instead, it blocks about 50% of the sparks once you exceed the preset rev limit. So instead of suddenly “running into a wall” your engine runs out of puff” and it won’t be damaged. The rev limiter incorporates three indicator lamps, two (green) to indicate that you’re approaching the RPM limit and the third (red) to indicate that “rev limiting” is occurring. Features • • • • • • • Limits engine RPM by ignition spark reduction Uses Hall Effect, points, low voltage signal or reluctor input or ignition coil to measure RPM Adjustable limit for RPM restriction Two prelimiting warning indicators One limit warning indicator Can be used as a gear change indicator Single component selection to suit most engines As an alternative use, this project could be employed as a simple gear change indicator, with or without the bonus of rev limiting. If you have a performance engine in your car and it has a typical 5-speed manual gearbox, you already know how easy it is to spin the engine out to and beyond its red line on the tacho. The red line is not an arbitrary limit but is based on a judgment made by the car manufacturer about risk of damage to your engine. Provided you drive below the red line, your engine should have a long life, all other things being equal. But exceed that limit and you risk doing serious damage and even catastrophic failure, such as putting a con rod through the side of the block. The risk of damage to your engine is much greater if you exceed the red line when the engine is unloaded, as it is if you happen to miss a gear change when accelerating strongly. So if your car is capable of high performance and you are keen to push it to the limit at every opportunity, then you really do need a rev limiter. Of course, some modern cars already have very effective rev limiters built into their engine management systems but the majority of cars do not have this very worthwhile protection. The Rev Limiter comes in two parts. The Rev Limit Con­troller is housed in a small plastic instrument case which can be mounted on your car’s dash panel. It has three lights on the front Fig.1: this block diagram shows the frequency to voltage converter and the three comparators of the Rev Limit Controller. Comparator 3 controls the operation of the Ignition Switcher board. Fig.2: the LM2917 frequency-to-voltage converter monitors the spark rate as a measure of engine RPM. panel and an on/off switch. Four trimpot adjustments set the sensitivity and the RPM thresholds for the three indicator lamps and the rev limit itself. The Rev Limit Controller operates the Ignition Switcher which is a modified version of the Engine Immobiliser circuit which was published in the December 1998 issue of SILICON CHIP. The Ignition Switcher operates by shorting out the engine’s ignition coil switching transistor (or the ignition points) about 50% of the time. This severely restricts engine power and hence limits the RPM. The Rev Limit Controller monitors engine RPM and is connected to the ignition trigger system which can be Hall effect pickup, reluctor pickup or a low voltage signal from the engine management computer to the ignition switching transistor. If you have a conventional points ignition (ie, Kettering not transistor-assisted or CDI), there is a bit of a problem. The engine speed monitoring will take place at the same point as the ignition blocking action and therefore the rev limiting action may be inconsistent and will tend to give quite rough engine operation when limiting is occurring. Mind you, we assume that there will not be too many performance engines which don’t have some sort of high energy ignition system. Block diagram Fig.1 shows the block diagram, embracing both parts of the Rev Limiter. The signal from the ignition pickup is processed in a frequency-to-voltage April 1999  81 Fig.3: the Rev Limit Controller uses the LM2917 and three com­parators to control the indicator lamps and the Ignition Switcher board. Once the red-line limit is reached, the Ignition Switcher cuts out around 8 sparks in every 16, effectively cutting engine power and preventing a further rise in engine speed. converter which produces a DC voltage which is proportional to the input frequency. The frequency-to-voltage converter is the well-proven LM2917 and its block diagram is shown in Fig.2. The output from the frequency-to-voltage converter is fed to three comparators, one of them inside IC1. Two of the comparators drive warning lamps to warn the driver of the onset of rev limiting while the third comparator actually controls the Ignition Switcher board. The Ignition Switcher must be set so that it only blocks out a nominal 8 sparks in every 16. It switches on at a rate which is fast enough to reduce engine power but not produce any noticeable jerking in the engine which would be the case if it switched at a lower rate. The setting to switch out 8 sparks in 16 (a duty cycle of 50%) is fairly critical. If more sparks are switched out, there is a higher risk of backfire, while less sparks cut out will mean 82  Silicon Chip less power reduction and rev limiting will be less effective. Circuit description Fig.3 shows the circuit for the Rev Limit Controller while Fig.4 shows the circuit for the Ignition Switcher. Fig.3 comprises two ICs and a regulator plus several tran­sistors and passive components. There are two input circuits, one for a reluctor pickup and the other for the remaining types of engine ignition triggers. Only one of these should be used at any time. The signal from the ignition points or Hall effect input is fed to a voltage divider comprising 22kΩ and 10kΩ resistors bypassed by a .056µF capacitor. The signal is then AC-coupled via a 1µF capacitor to a 10kΩ resistor and a 4.7V zener diode to provide signal clamping. The 1kΩ input resistor is there to provide a low voltage signal input point such as the 5V signal from an engine management computer. Further filtering is provided at this point using another .056µF capacitor before the signal is applied via a 1kΩ resistor to pin 1 of IC1. The reluctor input uses a 1µF coupling capacitor to provide isolation from the trigger circuit used on the engine ignition while a 100pF capacitor filters out any high-frequency hash. The signal is then applied to the base of transistor Q4 via 47kΩ and 220kΩ resistors and a 470pF speed-up capacitor. The collector of Q4 is normally low and a negative-going reluc­ tor signal switches off Q4 which then has its collector pulled high via the 10kΩ resistor. This signal is applied to pin 1 of IC1. Following the op amp comparator within IC1 is a charge pump. This basically switches charge from the .033µF capacitor at pin 2 to the 2.2µF capacitor connected to pins 3 & 4. This occurs on each comparator detection of a signal on pin 1. The 10kΩ resistor and trimpot VR4 at pin 3 discharge the 2.2µF capacitor to provide a time constant for the charge pump circuit. VR4 provides the calibration adjustment for the circuit. A second comparator within IC1 monitors the voltage at pins 3 & 4. The inverting input of this internal comparator (pin 10) connects to trimpot VR1 which sets the threshold voltage. The comparator output (pin 8) is an open-collector transistor and this output drives transistor Q1. When pin 3 of IC1 goes above pin 10, pin 8 goes low and this turns on transistor Q1 and Lamp 1 then lights up. Comparators IC2a & IC2b also monitor the pin 3 output of IC1. IC2a’s output goes low when its pin 2 goes higher than the preset voltage from trimpot VR2 at pin 3. When this happens, transistor Q2 turns on and this lights Lamp 2. IC2b operates in a similar manner to IC2a and has a threshold set by trimpot VR3. It drives Q3 which lights Lamp 3 and it also goes low to drive the Ignition Switcher circuit shown in Fig.4. Power for the circuit comes from the car’s ignition switch, switch S1 and a 10Ω resistor to a 16V zener diode which provides protection from any spike voltages. From there it goes to a 3-terminal regulator REG1 which provides a 5V supply for IC1. IC2, the transistors and the Lamps run from the +12V rail. Ignition switcher As noted above, the Ignition Switcher circuit in Fig.4 is an adaptation of the Engine Immobiliser circuit which appeared in the December 1998 issue of SILICON CHIP. This circuit uses a single 555 timer IC and four transistors. Q1 is a high-voltage Darlington tran- This view shows the assembled Ignition Switcher PC board. It’s virtually identical to the Engine Immobiliser circuit published in the December 1998 issue of SILICON CHIP. sistor designed for ignition systems. It can switch the heavy coil current and can withstand the voltages that are produced across the coil (typi­cally around 250V peak) when the engine is running normally. The four 75V zener diodes between the collector and emitter of Q1 prevent voltages over 300V from damaging the device. Normally, the Ignition Switcher circuit is quiescent (ie, not active) and transistor Q1 is off. The circuit is activated by a low signal from the Rev Limit Controller and this turns transis­tor Q4 off. When this happens, 555 timer (IC1) is able to oscillate, at a frequency determined by the two 100kΩ resistors and capacitor C1, connected to pins 2, 6 & 7. The resultant waveform at pin 3 is a square wave. Each time pin 3 goes high it turns on Q3 and this turns on Q2 and Q1. Each time Q1 turns on, it effectively shorts out the ignition points or the main ignition coil driver transistor (in a transistorised ignition system). And each time this happens, no sparks are delivered to the engine. Spark switching rate C1 must be selected to suit the rev limit for your engine. To do this, you must do a simple calculation, as follows: Spark rate = revs x sparks/rev ÷ 60. Fig.4: based on our previous Engine Immobiliser circuit, the Ignition Switcher shorts out the main switching transistor in the car’s ignition system, effectively removing 8 out of every 16 sparks, once the red-line limit is reached. April 1999  83 Table 1: Choosing C1 Spark Rate up to 250sp/s 250 to 300sp/s 300 to 350sp/s 350 to 420sp/s 420 to 500sp/s 500 to 600sp/s C1 0.47µF 0.39µF 0.33µF 0.27µF 0.22µF 0.18µF The figure for revs is the red-line limit for your car’s engine. The figure for sparks/rev is the number of firing strokes per revolution of your engine. For example, a 4-cylinder (4-stroke) engine has two firing strokes/revolution, a 6-cylinder has three firing strokes/revolution and a V8 has four firing strokes/revolution. You multiply these two figures and divide by 60 to get a result in sparks per second. For example, if you have a 6-cylinder engine with a 6000 RPM red-line limit, multiplying 3 by 6000 and dividing by 60 gives a result of 300 sparks/second. If you have a 4-cylinder with a 8000 RPM limit, the result is 267 sparks/second and for a V8 with a 5000 RPM limit, the result is 333 sparks per second. This should give you the picture. The value for C1 can then be chosen from Table 1. Note that C1 does not set the rev limit. This is done by setting trimpots VR3 & VR4 on the Rev Limit Controller. C1 merely sets the number of sparks which are blocked out during the limiting action at the specified RPM. Fig.5: use this component layout for the Rev Limit Controller circuit if your car has a reluctor distributor. Check your etched PC board carefully for defects before installing any of the parts and make sure that all polarised parts are correctly oriented (transistors, diodes, ICs, electrolytic capacitors, etc). Table 2: Resistor Colour Codes  No.   2   1   2   2   2   1   8   3   6   5   2 84  Silicon Chip Value 4.7MΩ 470kΩ 220kΩ 100kΩ 47kΩ 22kΩ 10kΩ 4.7kΩ 2.2kΩ 1kΩ 10Ω 4-Band Code (1%) yellow violet green brown yellow violet yellow brown red red yellow brown brown black yellow brown yellow violet orange brown red red orange brown brown black orange brown yellow violet red brown red red red brown brown black red brown brown black black brown 5-Band Code (1%) yellow violet black yellow brown yellow violet black orange brown red red black orange brown brown black black orange brown yellow violet 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 brown black black gold brown Table 3: Capacitor Codes        Value IEC Code EIA Code 1µF   1u  105 0.1µF 100n   104 .056µF   56n   563 .033µF   33n   333 470pF   471   470 100pF   101   100 Power for the Ignition Switcher is taken from switch S1 in the Rev Limiter circuit. Diode D2 isolates the circuit and a 0.1µF capacitor decouples the supply to transistors Q2 & Q3. IC1 is protected from voltage transients by the 10Ω resistor in series with the supply and the 16V zener diode ZD1. The 100µF capacitor decouples the supply rails. Construction The Rev Limit Controller is built on a PC board measuring 117 x 102 mm and coded 05304991. This board fits into a plastic case measuring 140 x 111 x 35mm and we have designed a label measuring 133 x 27mm for the front panel. The Ignition Switcher is built onto a PC board measuring 106 x 60mm and coded 05412981. This board can be fitted into a small plastic case measuring 82 x 53 x 30mm or merely fitted with a sleeve of heatshrink tubing. Fig.5 shows how the Rev Limit Controller board is wired for a distributor with reluctor pickup. Fig.6 shows how it should be wired if you have Hall Effect, points input or low voltage signal from an engine management computer. Make sure you use the correct overlay diagram when assembling this PC board. Fig.7 shows the component overlay for the Ignition Switcher board and remember that you need to consult Table 1 to pick the value for C1. You can begin construction by checking the PC boards for shorts between tracks and possible breaks and undrilled holes. Fix any problems before inserting any components. Then insert and solder all the links as shown on the overlay diagrams. Insert and solder in the resistors, using Table 2 as guide to the resistor colour codes. You can also use a digital multimeter to measure each one. Fig.6: if your car has does not have a reluctor distributor (ie, uses points, Hall Effect pickup, etc) use this layout to wire up the Rev Limit Controller. Lamps 1 & 2 should be green, while Lamp 3 is red. Fig.7: this is the layout for the Ignition Switcher board. Note that the zener diodes (ZD1-ZD5) must all be oriented correctly, otherwise the circuit won’t work. In particular, note that ZD5 faces in the opposite direction to ZD4. The assembled board should be enclosed in a plastic case or heatshrink tubing and mounted under the dashboard. April 1999  85 The Rev Limit Controller board is mounted inside a standard plastic case (140 x 111 x 35mm). Use automotive hookup wire for all external connections. Take care with the orientation of the ICs when you are installing them. Next, solder in all the diodes, including the zeners, and take care with their orientation. The transistors can t Shop soiled bu ! HALF PRICE be installed next and be sure to place the correct type in each position. Then insert the capacitors and note that the electrolytic capacitors must have the correct polarity. Table 3 shows the codes which will be shown on the MKT types. REG1 is mounted horizontally, with its metal face towards the PC board. Bend the leads to insert them into the holes allo­ cated before securing the regulator with a screw and nut. Simi­larly, transistor Q1 on the Igni- 14 Model Railway Projects THE PROJECTS: LED Flasher; Railpower Walkaround Throttle; SteamSound Simulator; Diesel Sound Generator; Fluorescent Light Simulator; IR Remote Controlled Throttle; Track Tester; Single Chip Sound Recorder; Three Simple Projects (Train Controller, Traffic Lights Simulator & Points Controller); Level Crossing Detector; Sound & Lights For Level Crossings; Diesel Sound Simulator. Our stocks of this book are now limited. All we have left are newsagents’ returns which means that they may be slightly shop-soiled or have minor cover blemishes. SPECIAL CLEARANCE PRICE: $3.95 + $3 P&P (Aust. & NZ) Order by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail your order with cheque or credit card details to Silicon Chip Publications, PO Box 139, Collaroy, NSW 2097. 86  Silicon Chip tion Switcher PC board is mounted horizontally and with a heatsink sandwiched between the device and the PC board. Use a screw and nut to secure this assembly in place. Finally, insert the PC stakes and trimpots. Case The front panel of the case requires holes for trimpot access, power switch S1 and the indicator lamps. Use the front panel label as a guide to the positioning of the holes. You will also need to drill the holes in the rear panel for the grommets. Fit the front panel label in position and cut the holes out with a sharp hobby knife. The Rev Limit Controller board and front panel can be placed in the case and secured with four self-tapping screws into the integral standoffs in the base of the case. Attach S1 and the lamp bezels in position and connect hookup wire from the +12V and GND terminals on the main PC board and pass these through the grommet. Similarly connect up wires to the reluctor or coil or low voltage input which are required to connect to the ignition trigger output on the engine and to the input of the Ignition Switcher. Complete the wiring to switch S1 and to the lamp terminals. Testing Starting with the Rev Limit Controller, apply 12V between the +12V and GND terminals on the main PC board. This done, check that the output of regulator REG1 is at +5V. You will now need to apply some voltage to pins 3 of IC1 using a 10kΩ resistor between this pin and the 5V supply. Now adjust VR1 and check that the light comes on. Similarly, check Lamp 2 with VR2 and Lamp 3 with VR3. Check that pin 7 of IC2b goes low when Lamp 3 is lit. Note that you can check operation using a signal generator. Apply signal to the points input or reluctor terminal and adjust the output frequency to monitor operation of the lamps. Installation The Rev Limit Controller can be installed into the vehicle using automotive connectors to make the connection to the +12V ignition supply. Use automotive wire for this connection. The ground connection Parts List Rev Limit Controller 1 PC board, code 05304991, 117 x 102mm 1 plastic case, 140 x 111 x 35mm 1 front panel label, 133 x 27mm 1 SPDT toggle switch (S1) 2 green 12V indicator lamps and bezels (Lamp 1, Lamp 2) 1 red 12V indicator lamp and bezel (Lamp 3) 1 M3 screw and nut 4 self-tapping screws 2 small rubber grommets 1 100mm length of 0.8mm tinned copper wire 1 1m length of twin figure-8 medium duty wire 1 1m length of red medium duty hookup wire 1 1m length of black medium duty hookup wire 1 1m length of green medium duty hookup wire 1 1m length of yellow medium duty hookup wire 14 PC stakes 3 10kΩ vertical trimpots (VR1VR3) 1 200kΩ vertical trimpot (VR4) Semiconductors 1 LM2917 frequency-to-voltage converter (IC1) 1 LM358 dual op amp (IC2) 1 7805 5V 3-terminal regulator (REG1) 3 BC327 PNP transistors (Q1-Q3) 1 BC337 NPN transistor (Q4) 1 16V 1W zener diode (ZD1) 1 4.7V 1W zener diode (ZD2) Capacitors 1 100µF 16VW PC electrolytic 3 10µF 16VW PC electrolytic 1 2.2µF 16VW PC electrolytic 1 1µF MKT polyester 1 0.1µF MKT polyester 2 .056µF MKT polyester can be made to the chassis with an eyelet and self-tapping screw. Attach the case in a position convenient to the driver and secure it with suitable brackets. This done, connect up the signal input from either the points, Hall effect, low voltage or reluctor outputs. Now start the engine 1 .033µF MKT polyester 1 470pF ceramic 1 100pF ceramic Resistors (0.25W, 1%) 2 4.7MΩ 6 10kΩ 1 470kΩ 6 2.2kΩ 2 220kΩ 3 1kΩ 2 47kΩ 1 10Ω 1 22kΩ 0.5W Miscellaneous Hookup wire, solder, etc. Ignition Switcher 1 PC board, code 05412981, 106 x 60mm 4 PC stakes 1 mini heatsink 19 x 19 x 9.5mm 1 M3 x 9mm screw 1 M3 nut Semiconductors 1 555 timer (IC1) 1 MJH10012, BU941P power Darlington transistor (Q1) 1 BC327 PNP transistor (Q2) 2 BC337 NPN transistors (Q3, Q4) 1 16V 1W zener diode (ZD1) 4 75V 3W zener diodes (ZD2ZD5) 1 1N4148, 1N914 signal diode (D1) 1 1N4004 1A diode (D2) Capacitors 1 100µF 16VW PC electrolytic 1 0.1µF MKT polyester 1 C1 (see text) Resistors (0.25W, 1%) 2 100kΩ 2 1kΩ 2 10kΩ 1 82Ω 5W 3 4.7kΩ 1 10Ω Miscellaneous Automotive wire, automotive connectors, solder, etc. and set VR4 to its mid-setting. To adjust the three trimpots (ie, VR1, VR2 and VR3), the engine should be under load. In practice, this means you need to drive the car along a quiet (no traffic) street in low gear while a passenger does the adjustments. Adjust VR1 so that Lamp 1 lights April 1999  87 WARNING! The external leads from the Rev limit Controller pass through two rubber grommets on the rear panel of the case. This engine rev limiter blocks out ignition sparks and should only be used as a final protection against engine damage. It should not be used to limit engine RPM each time it is wound out at every gear change. The reason for this is that at limiting there is the risk of backfire as the exhaust will contain an explosive mixture of unburnt fuel. In addition, the unburnt fuel adds to air pollution. about 1000 RPM below the red line. This done, adjust VR2 so that Lamp 2 lights about 600 RPM below the red line. Finally, adjust VR3 so that Lamp 3 lights at the red line. If you have to wind trimpots VR1VR3 fully clockwise in order to turn on their respective lamps, wind VR4 slightly clockwise. Alternatively, if these adjustments are too sensitive, wind VR4 slightly anticlock­wise. Note: this adjustment procedure is no longer recommended. See page 107 of the October 2007 issue for in­ formation on how to adjust the unit using a signal generator. Connecting the boards Fig.8: actual size artwork for the Rev Limit Controller PC board. Fig.9: actual size artwork for the Ignition Switcher board. 88  Silicon Chip You can now attach the Ignition Switcher board to the Rev Limit Controller circuit to test for correct limiting action. The boards can be wired up using automotive wire, following the diagrams of Fig.7 and Fig.5 or Fig.6. We used light duty wires for all wiring except for the wires to the ignition coil and ground. Be sure to ground the Ignition Switcher to a suitable chassis point using an eyelet and self-tapping screw. This is to allow the heavy current flow through Q1, when it is disabling the ignition. The Ignition Switcher board must be insulated from the chassis by enclosing it in a plastic case or sleeving it with heatshrink tubing. Now test the operation of the limiting action on the en­gine. The engine should lose power when limiting is taking place but we must caution against driving in this condition for any­thing more than a few seconds, SC because of the risk of backfire.