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Is your car turbocharged? Lucky you! But wait – literally: you really should hang about at the end of each trip 'til your turbo cools down enough to let you safely turn off the engine. Don’t have the time? You can solve this problem with our new T urbo imer for turbo engines By John Clarke Turbocharged engines have become very popular in recent years. Most car manufacturers have at least one turbo engine model in their range. Turbos aren’t just for performance cars or four-wheel-drives, either: many a manufacturer has found that a turbo does wonders to tiny cars with tiny engines, turning them into the socalled “pocket rockets”. One big advantage of the turbo is that it can give a considerable performance advantage over the standard unboosted types without adding too much extra complexity. Turbochargers work by directing the exhaust gas flow from the engine through a small turbine. This turbine in turn drives a compressor which boosts the atmospheric air pressure before it is applied to the inlet manifold of the engine. The resultant higher air density allows more fuel to be added prior to ignition, producing more engine power or more efficiency for the same power. Turbochargers are often mistaken for superchargers (and vice versa) because they have the same basic effect. The difference is that a supercharger 24  Silicon Chip compressor is driven by a belt directly from the engine. Turbos are usually lower in cost but can give the best of both worlds; heaps of power when you want it and fuel economy at other times. But the turbo must be allowed to cool down properly at the end of a trip before you switch the motor off. Features •  Automatic operation •  90 second timeout •  Under temperature disable •  Reset switch •  Ignition signal output for alarm •  Facility to override alarm systems which disable the ignition •  No battery power drain after timeout period What happens is that the turbine in the turbo spins very fast in the hot exhaust gases (100,000 rpm is not unusual). An immense amount of heat is involved (it’s not unusual for the inside of a turbo to glow bright cherry red!) and that heat can do a lot of damage if the turbo doesn't cool down before the oil flow to the bearings stops. Needless to say, when you turn off the engine the oil pump stops pumping and the oil stops flowing! If this happens, the oil remaining in the bearings can be cooked. It carbonises, leaving gritty coke-like residue: just what you don’t need in a high speed, high performance bearing! What you do need is a way to keep the oil flowing after you’ve stopped the vehicle – and the only simple way to do that is to keep the engine idling for a minute or so. This Turbo Timer does that job for you. You can lock up your car and walk away, safe in the knowledge that the Turbo Timer will run the engine for just long enough to safely cool down the turbo and then switch the engine off. And if you’re only nipping down to the shops for a litre of milk, the The Turbo Timer, housed in a small plastic box. Immediately below is the relay which bypasses the ignition key switch, keeping the engine running for 90 seconds after switch off. Below that again are the thermistor and reset switch. Turbo Timer won’t cut in: it will only operate after the engine has reached normal temperature. The circuitry is dead simple: a 555 timer and one or two automotive relays do the job. Automatic operation Sitting in your car for a minute or two after stopping might sound easy but it’s easy to forget; it’s also easy to think “once won’t hurt”. The S ILICON C HIP Turbo Timer won’t let you forget or miss out because if the engine is hot enough it will automatically keep running for 90 seconds or so after you switch off. You can even alight from your car and switch on the alarm during this period in readiness to leave. Of course there are going to be times that you do not want or need the Turbo Timer facility and we have catered for this. Firstly, the Turbo Timer does not operate until the engine has reached normal operating temperature. This is a fully automatic feature which requires no action on your part. It is useful if the car is just driven up the road and the engine has not had suf- ficient time to warm up. In this case the engine need not be run at idle to cool down the turbo bearings. Another scenario happens where the engine has reached normal operating temperature but has not been working hard and where the turbo has not been in operation during your drive; for example during the slow trip to work in the morning where the engine hardly revs past idle. In this case we have provided a reset feature. You simply press the reset switch immediately after switching off the ignition and the engine will stop. We have catered for alarm systems which may be triggered by the ignition being on during the time-out period. The Turbo Timer incorporates an ignition output which is disabled during the time-out period. Also, some alarms disable the engine from operating by breaking the ignition circuit at some point or by shorting the coil. In either case there is the option to add a relay which counteracts these effects during the time-out period. The disabling feature of the alarm is regained after the timeout period. The Turbo Timer electronics are housed in a small case, controlling a relay mounted outside the case. The relay contacts connect across the ignition switch to maintain ignition power during the time-out period. The relay is a heavy duty type specifically for automotive use: the contacts can handle up to 25A. Circuit The circuit for the Turbo Timer uses a single 555 timer, IC1, a transistor or two and a sprinkling of other components. The circuit is powered from the switched side of the ignition switch via diode D1 and a 33Ω resistor, giving an 11.4V supply rail from a nominal 12V car battery. The 16V zener protects against voltage surges while the 100µF capacitor smooths the supply and also maintains it for a short time when the ignition switch is turned off. This is important for correct circuit operation. When IC1 is powered via the ignition and assuming the thermostat switch TH1 is open (pin 4 at 11.4V), the output at pin 3 is low and both transistors Q1 and Q2 are off. The November 1998  25 Fig. 1: the circuit of the Turbo Timer is based on a 555 timer IC. It is shown with the optional components (RLY2, D4, Q2 and the 2.2kΩ resistor) if an alarm bypass circuit is required. 220µF capacitor at pin 6 is held discharged via the 1kΩ resistor and the low state of pin 7. The trigger input (pin 2) is held high via the 100kΩ pullup resistor while the negative side of the 2.2µF capacitor is also pulled high via the normally closed contacts of RLY1 and the 1.8kΩ resistor. When the ignition switch is opened (ie, the ignition key turned off), the negative side of the 2.2µF capacitor is pulled low via the 10kΩ resistor. The positive side of the capacitor follows this voltage down, triggering IC1 via pin 2. This releases the low on pin 7 to allow the 220µF capacitor at pin 6 to charge via the 390kΩ resistor. The now high pin 3 drives the base of Q1 and Q2 via 2.2kΩ resistors. Transistor Q1 drives relay RLY1's coil and Q2 drives the optional RLY2, if fitted. RLY1's common and normally open contacts close, shorting the ignition switch. The normally closed contact is open and held low via the 10kΩ pull-down resistor. This contact can be used for an alarm system ignition input since it is low during this timeout period but follows the ignition supply at other times. The 2.2µF capacitor at pin 2 now begins to charge up to the full supply voltage via the 100kΩ resistor and the 10kΩ resistor to ground. 26  Silicon Chip Diode D2 protects the 555 timer by preventing the voltage at pin 2 from being elevated to unsafe levels, as could happen if the 2.2µF capacitor was fully charged and the relay contacts opened. This would lift the negative side of the 2.2µF capacitor to +10.2V, pushing the positive side to more than 21V, in all likelihood blowing the IC input. D2 also protects the IC from overvoltage caused by any spikes from the ignition system when the RLY1 normally closed and common contacts are connected. The output at pin 3 1/remains high and the relay is held on via Q1 until the 220µF capacitor at pin 6 charges up to 2/3 of the supply. When this threshold is reached, pin 3 goes low, switching off transistor Q1 (and Q2 if fitted). The relay contacts of RLY1 revert to their normal position, cutting off the supply to the ignition circuit, stopping the engine. At the same time, the 220µF capacitor is discharged via the 1kΩ resistor at pin 7. If the reset switch S1 is pressed during the charging period, the 220µF capacitor charges immediately via the 1kΩ resistor and the timing period ceases. You may be concerned about any delay between turning the ignition key off and the relay contacts pulling in. Of course there is a small lag but in practice it doesn't matter – it's much quicker than turning your ignition key off and on again very quickly. The engine doesn't have time to stop. Temperature detection The thermostat (TH1) at pin 4 disables operation of the timer whenever it is closed. It is installed on the radiator of the vehicle so that it can monitor the engine temperature. When the radiator is cold, the thermostat contacts are closed and they hold the reset pin low. This stops pin 3 going high so the timeout does not operate. When the radiator temperature reaches about 80°C, the thermostat opens and so pin 4 is pulled to 2.4V via the 10kΩ and 2.7kΩ voltage divider between the supply rail and ground. The circuit can now operate normally when triggered, with pin 3 going high for 90 seconds. The 100µF capacitor at pin 4 is included to prevent IC1 from being triggered when the ignition is switched on and TH1 is open. The 2.7kΩ resistor ensures that the 100µF capacitor is fully discharged. Relay 2 Relay 2 is included to enable the Turbo Timer to operate even if you have an alarm system which disables the ignition system. The relay contacts are changeover types so that you can make the required connection if the alarm system breaks the circuit or break the connection if the alarm applies a short to part of the ignition. This relay (along with Q2 and its 2.2kΩ base resistor) is optional and need not be used even if you have an alarm of this type. It means, though, that the alarm cannot be set until after the turbo timeout period. Standing around waiting for the turbo timer to time out is about as convenient as sitting around waiting for a minute or so to turn the engine off – hence its inclusion for those vehicles fitted with ignition-disabling alarms! Construction Most parts for the Turbo Timer are mounted on a PC board coded 05411981 and measuring 104 x 58mm. The PC board can be housed in a suitable case measuring 130 x 68 x 41mm. Alternatively, you can house the entire circuit in heatshrink tubing. The photographs show a plastic case but there are some security arguments for a metal (diecast) case; more on this subject shortly. The PC board was sized to clip into the plastic catches on the side of the box. Make sure that it is of the correct width to fit snugly in position. Use a file to narrow down the PC board if it is too wide. Begin construction by checking the PC board for shorts between tracks and possible breaks, then insert and solder in all the PC stakes. The resistors can be installed next using the accompanying colour code table as a guide to selecting each value. Alternatively, you can use a digital multimeter to measure each value. If in any doubt as to a resistor’s value, check it anyway. Diodes can be mounted next, taking care with the polarity of each. Make sure that you use a 1N914 or 1N4148 type in the D2 position. ZD1 must be a zener diode; it may be marked 1N4745. Take care of any polarity-conscious components: transistors Q1 & Q2 (which are positioned as shown with the curved side toward the edge of the PC board); IC1; the diodes (don’t mix up the zener with the others) and the electroyltic capacitors Fig. 2: all components except the reset switch, thermistor and relay(s) mount on a small PC board. Take care with D2 and ZD1 – they sometimes look almost identical. Fig. 3: this full-size PC board pattern can be used to etch your own board or used as a reference when checking a commercial or kit board. We mounted our relay on the end of the case, but there is no necessity for it to be so mounted. If you mount yours on the end of the case, drill the case to accept the mounting bolt for the relay and about a 10mm hole for the wiring loom to exit. Wiring We used light duty wire for all wiring except for the wires to the ignition switch and wires connecting to termi- nals 87 and 30 of the relay. These must be wired with heavy duty automotive wiring. Use insulated crimp female spade connectors to connect to the relay terminals. You can test the Turbo Timer using a 12V supply with a rating of at least 200mA. First connect the positive supply to “+12V from battery” and the negative to the “chassis earth” terminals on the PC board. Do not connect the TH1 thermostat at this stage but November 1998  27 Parts List 1 PC board, code 05411981, 104 x 58mm 1 case, 130 x 68 x 41mm 1 20A 12V horn relay with change over contacts (RLY1) 1 80°C thermostat with normally closed contacts 1 normally open pushbutton switch (S1) 7 insulated 6mm female spade connectors 2 crimp eyelets 8 PC stakes Use this photograph in conjunction with the PC board overlay when assembling your Turbo Timer. you can connect the reset switch to its terminals. Now short the “+12V from battery” and “+12V from ignition switch” PC stakes. Nothing should happen but when you disconnect this wire (which simulates the opening of the ignition switch) , the relay should be activated. The relay should be de-energised after about 90 seconds. Try the operation again and check that the relay drops out when the reset switch is pressed. Temporarily short the thermistor terminal to the chassis earth terminal and ensure that the relay does not operate. This simulates operation on a cold engine. If the circuit operates properly you are now ready to install the Turbo Timer into your vehicle. Security One important consideration for a Turbo Timer is vehicle security – not, as you might imagine, the problem of the vehicle being driven away during the timeout period. You are usually close enough to the vehicle during that period for it not to be a problem. Vehicle manufacturers these days go to considerable lengths to hide, or camouflage, ignition wiring to make it just that much harder for thieves. You will have to identify which wires are which to install the Turbo Timer – and it will probably take some time (a commodity most thieves don’t have). Having connected the Turbo Timer successfully, you will have bypassed a lot of that security and identified the two most sought-after connections (for a thief) – the ignition terminals. For this reason, your installation needs to be carefully thought out. It is almost certainly best NOT to install the Turbo Timer under the dashboard where it can be easily got at or where a thief can spend time without drawing too much attention. The best spot is probably inside the engine bay, hidden if at all possible. If you can make it look like part of the wiring loom (eg, with heatshrink tubing) so much the better. If you mount the Turbo Timer in a case, remember that underneath the bonnet of a turbocharged vehicle is a very hot, hostile environment. Ensure Semiconductors 1 555 timer (IC1) 2 BC337 NPN transistors (Q1,Q2)* 3 IN4004 1A diodes (D1,D3,D4) 1 1N914, 1N4148 diode (D2) 1 16V 1W zener diode (ZD1) Capacitors 1 220µF 16VW PC electrolytic 2 100µF 16VW PC electrolytic 1 2.2µF 16VW PC electrolytic 2 0.1µF MKT polyester Resistors (0.25W, 1%) 1 390kΩ   1 2.7kΩ 2 1kΩ 1 100kΩ   2 2.2kΩ∗ 1 33Ω 2 10kΩ   1 1.8kΩ Option 1 20A 12V horn relay with change-over contacts (RLY2) Miscellaneous Automotive connectors, Automotive wire, solder, etc. * Q2 and one 2.2k resistor are also optional if RLY2 is not fitted that the case is mounted well away from the “hot” side of the engine and that a suitable case is used to prevent water ingress. In fact, we would prefer to see a metal diecast case used. Sure, Resistor Colour Codes         No. 1 1 2 1 2 1 2 1 28  Silicon Chip Value 390kΩ 100kΩ 10kΩ 2.7kΩ 2.2kΩ 1.8kΩ 1kΩ 33Ω 4-Band Code (1%) orange white yellow brown brown black yellow brown brown black orange brown red violet red brown red red red brown brown grey red brown brown black red brown orange orange black brown 5-Band Code (1%) orange white black orange brown brown black black orange brown brown black black red brown red violet black brown brown red red black brown brown brown grey black brown brown brown black black brown brown orange orange black gold brown radiator hose using tie wire, cable ties or similar. One contact for the thermal switch connects to the Turbo Timer PC board while the second terminal connects to chassis near the radiator using a self-tapping screw to secure the eyelet. Incidentally, if you ever need to disable the Turbo Timer (eg, for vehicle service), shorting the two thermistor terminals together is the easiest way to do it. You may wish to connect up the optional second relay (along with Q2 and its 2.2kΩ base resistor) if you have an alarm system installed. Fig. 4: full-size artwork for the label on the plastic box used for the prototype. This Note that you should use the igmay need slight enlargement or reduction (eg by a photocopier) on a metal box. nition output on the Turbo Timer they cost a few bob more – but what’s connectors. if the alarm monitors the ignition. your car worth? You will also need a chassis point Details of how to wire up the second to connect the ground supply of the relay are shown in Fig.5. There are two Installation circuit to the battery negative termi- alternatives catered for in this wiring. nal. This can be an existing screw in Taking into account the above Firstly in Fig.5a, the wiring diagram comments, find a suitable position for the metalwork or a separate self-tap- shows how to wire the relay to counping screw which secures the eyelet mounting the unit. teract the alarm unit from shorting out Locate the fused side of the ignition terminal for the ground lead in place. the ignition. If mounting the relay externally you circuit and the fused side of the battery Fig.5b shows how to counteract supply – you may need a workshop will also need to find a suitable screw the alarm unit when it open circuits manual to help you with this because – the mounting plate usually needs to the ignition system. The normal be grounded. it is not normally something you’ll alarm ignition disable feature will be If you are the only driver of the maintained once the turbo timeout find in an owner’s manual. In most vehicles, the fusebox is vehicle, it is better to mount the reset period has expired. Use heavy duty switch in a not-too-obvious position. automotive wire for the connections. mounted under the dash. It's easy to check which connectors are fused If you have a dashboard with any spare If the 90 second delay is too short using either a 12V test lamp or a mul- switch mounting plates, you could or too long, you can adjust it by vartimeter – pull out the fuse and see if use one of these. If others drive your ying the 390kΩ resistor between the vehicle, or if putting it in for service, positive supply and pin 6. Increasing you still have voltage! The ignition circuit is usually most let them know it has a Turbo Timer the resistor will increase the time, defitted and where the reset switch is! easily accessed under the bonnet creasing it will (surprise!) decrease the The thermal cutout switch can be timeout period. The 220uF capacitor rather than under the dash. Most vehicles have the ignition fuse separately mounted on the radiator body but could also be increased to 470uF for not on the cooling core. You may be a longer period. mounted under the bonnet. The wiring to all points should be able to secure it using some steel wire Using It made using good quality automotive which wraps around one of the side chambers by passing it through Operation is virtually foolproof: the cooling fins. Make sure that when you park the vehicle, ensure the surfaces of both thermal it is in "park" (auto transmission) switch and radiator are mating or "neutral" (manual transmission), squarely. engage the handbrake fully, turn the A smear of heatsink com- ignition key off, remove it, get out pound between mating surfac- and lock up. es will ensure better thermal Note the comments above about Fig. 5: if your alarm system disables the contact. setting your alarm, if you have one. ignition by shorting it out, use option (a). As an alternative, on a copFinally, a warning: if leaving the car If it open circuits the ignition system, use (b). per or brass radiator you could in an enclosed area (such as a home solder a couple of M3 nuts to garage) beware of the carbon monoxide the radiator body and secure fumes which will be given off during the thermal switch to these the engine run-on period. Park the using the integral mounting vehicle so that its exhaust is directed plate. outside or leave the garage door open. Otherwise, you may need to Carbon monoxide is colourless, odourattach the thermal cutout to the SC less . . . and poisonous. November 1998  29