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Chapter 13 Using the temperature switch, it’s easy to rig warning lights or alarms for over-high engine or gearbox oil temperatures. In fact, anything’s that hot in the car (with the exception of the exhaust gas and cylinder head) can be monitored. [Ford photo] Temperature Switch A cheap general-purpose adjustable design that can work all the way up to 245°C! T HERE ARE MANY automotive performance applications where you want to turn something on or off on the basis of measured temperature. Radiator cooling fans, overtemperature warning lights or alarms, intercooler or amplifier fans – they all need a cheap and easily-adjusted temperature switch. Temperature switches are available commercially but this build-it-yourself Main Features •  Adjustable temperature switching from 0°C to 245°C •  Double-pole changeover 5A relay contacts •  Selectable rising or falling temperature switching •  Adjustable hysteresis •  Easy to build siliconchip.com.au design has some major advantages over normal thermostats and temperature switches. First, it can be adjusted very finely – you can literally set (to the degree) the temperature at which the switch triggers. Second, the hysteresis (ie, the difference between on and off temperatures) is adjustable. That lets you set the system up so that the device you’re switching isn’t constantly cycling at the trigger point. You can set a wide hysteresis to switch something on and off at two widely spaced temperatures, or a low hysteresis to keep tighter control – the choice is yours! Third, the sensor used in this design is good for temperatures up to 245°C. This means you can monitor engine oil or auto transmission oil temperature, or site the sensor near the brakes to trigger cooling sprays. Basically, apart from exhaust gas and cylinder head temperature, you can trigger the switch with anything on the car that’s hot or cold! Finally, you can configure the sensor so that it reacts very quickly to temperature changes. Construction The Temperature Switch is a simple kit to build but you should make one decision before starting construction. Will you be using it to detect a temperature that is rising to the trip point or falling to the trip point? The Temperature Switch can be configured to work either way but if you know which way you’re going, you won’t have to make changes later on. The detection of a rising temperature will be the more common application – for example, turning on a warning light or fans when the temperature gets too high. But if you want something switched on as the temperature falls – for example, activating a warning light when the outside temperature drops below 3°C to warn of the possibility of black ice on the road – then the Temperature PERFORMANCE ELECTRONICS FOR CARS 77 Fig.1: the temperature is monitored using a thermistor, while either op amp IC1a or IC1b drives transistor Q1 and the relay. Trimpot VR1 sets the temperature trigger point. How It Works The temperature is monitored using a thermistor which exhibits a variable resistance with temperature. At high temperatures, the resistance of the thermistor is low, while at lower temperatures its resistance increases. A 1kΩ resistor from the 8V supply feeds current through the thermistor which then produces a voltage which is inversely proportional to temperature. This voltage is filtered using a 100nF capacitor and fed via a 1kΩ resistor to the inverting input (pin 2) of op amp IC1a which is connected as a comparator. The voltage on IC1a’s non-inverting input (pin 3) is by set-point trimpot VR1 via a 10kΩ resistor. When the thermistor voltage at pin 2 is above the voltage set by VR1 at pin 3, IC1a’s output is low. Conversely, when the thermistor voltage is below the voltage on pin 3, IC1a’s output is high (around +8V). Hysteresis has been added to prevent the output of IC1a from oscillating when the inverting input is close to the switching threshold. This hysteresis is provided by trimpot VR2 and diode D3 in series between pins 1 and 3. Trimpot VR2 enables the amount of hysteresis (actually positive feedback) to be adjusted. With low hysteresis, the temperature only has to drop by a small amount for IC1a’s output to switch low again after it has switched high. If VR2 is set for high hysteresis, the temperature must fall by a much larger amount before IC1a’s output switches low again. Diode D3 sets the direction of the hysteresis action. As shown, it provides hysteresis when pin 1 of IC1a goes high. Alternatively, if oriented in the opposite direction, it will provide hysteresis when RESISTOR COLOUR CODES Value 22kΩ 10kΩ 1.8kΩ 1kΩ 10Ω 78 4-Band Code (1%) red red orange brown brown black orange brown brown grey red brown brown black red brown brown black black brown PERFORMANCE ELECTRONICS FOR CARS 5-Band Code (1%) red red black red brown brown black black red brown brown grey black brown brown brown black black brown brown brown black black gold brown IC1a’s output goes low. Where the circuit is intended to provide a switched output when the temperature goes above a certain value, the diode is oriented as shown on the circuit and parts overlay. If you want the switching to occur when the temperature falls below a certain value, diode D3 is reversed. Op amp IC1b is an inverter which provides a signal opposite in polarity to IC1a’s output. When IC1a’s output goes high, IC1b’s output goes low and vice versa. Link LK1 provides the option for driving the relay with a rising temperature (L/H) or a falling temperature (H/L). It selects the output of IC1a or IC1b to drive transistor Q1 which, in turn, drives the relay. Diode D2 is there to quench the reverse voltage that is generated by the collapsing magnetic field of the relay coil each time it is switched off. Power is obtained from the car’s +12V ignition supply via D1 which gives reverse connection protection. The 10Ω resistor, 100µF capacitor and zener diode ZD1 provide transient protection at the input of regulator REG1. All the circuitry is powered from the 7808 regulator with the exception of the relay, Q1 and LED1 which are driven from the 11.4V supply following D1. siliconchip.com.au The device turns other devices on or off on the basis of sensed temperature. Its sensor can work over the range of 0°C - 245°C, making it useful for monitoring engine oil, engine coolant and transmission oil temperatures, as well as intercooler and inlet air temperatures. Note that link LK1 (to the left of the relay) must be moved to the H/L position and diode D3 (circled) reversed in orientation if the switch is to trigger on a falling (rather than rising) temperature. Use It To Do This . . . •  Operate electric radiator fans •  Over-temperature warning light or alarm •  Operate amplifier cooling fans •  Operate an intercooler water spray or fan •  Operate a brake cooling water spray •  Reduce turbo boost when intake air temperature is high siliconchip.com.au to be connected to a length of shielded single core cable, with the shield (the braid) connecting to the 0V terminal on the PC board. The thermistor isn’t polarised – it can be connected either way around. Insulate the leads of the thermistor using heatshrink tubing so that they cannot short out to each other or to ground. In many cases, 10 µF H/L CT N L/H 100 µF 10k CN + NC CN D2 Q1 RELAY 1 1 0 1ra c 5 0 C ON 1.8k ON NO H/L LK1L/ H K 10k 1k 100nF COM C COM + 10 µF 1k NO NC 22k 1 10k A K + 100 µF K 1M IC1 LM358 ZD1 + TO THERMISTOR *D3 10k A K LED1 VR2 REG1 7808 V21+ +12V DNG GND A VR1 1k 10Ω H CTI WS ERUTAREP MET A D1 K 1N 4148 Switch needs to be configured for a falling temperature. So what are the changes made for the differing configurations? They’re simple: for rising temperature detection, link LK1 is placed in its “L/H” position (ie, to the left when the board is orientated as shown in Fig.2) and diode D3 is orientated so that its band is closest to the bottom of the board. Conversely, to detect a falling temperature, link LK1 is moved to its alternative “H/L” position and diode D3’s orientation is reversed. Easy, huh? When assembling the PC board, be sure to insert the polarised components the correct way around. These parts include the diodes, IC, LED, transistor, voltage regulator and electrolytic capacitors. During construction, follow Fig.2 closely to avoid making mistakes. The thermistor is of the “bare” design – ie, it’s not potted in epoxy or mounted inside a brass fitting. If you want temperature detection to occur very quickly (ie, if you want the thermistor to react quickly, even to small temperature variations), the thermistor should be left exposed. However, if the reaction speed isn’t so important but durability is, you can pot the thermistor in high-temperature epoxy and mount it in the end of a threaded brass fitting. Either way, the thermistor will need A *REVERSE D3 IF LINK LK1 IS IN 'H/L' POSITION Fig.2: this layout diagram shows where each of the parts is placed on the PC board. Use this diagram, the photos of the completed board and the parts list to help you assemble it correctly. Don’t forget to reverse D3 if link LK1 is in the H/L position. PERFORMANCE ELECTRONICS FOR CARS 79 The thermistor’s leads should be insulated and then completely covered in heatshrink tubing so that short circuits can’t occur. If durability in extremes is required (and the sensor doesn’t need to react quickly), it can be potted in hightemperature epoxy and mounted in the end of a threaded brass fitting. Fig.3: here is a typical connection set-up, where the Temperature Switch might be monitoring the temperature of an audio amplifier. The relay’s Normally Open (NO) connection is made to ignition-switched +12V, while the adjacent Common terminal is connected to a fan. The other side of the fan is earthed. When the temperature rises to the set-point, the fan is triggered. Adjustment of the hysteresis pot will determine how low the temperature then has to fall before the fan switches off. the whole thermistor itself can then be covered in heatshrink without slowing its reaction time too much. Testing Once the assembly is complete, you should bench-test the module to make sure it is working correctly. To do this, you’ll need to connect the thermistor to the input terminals (remember, braided side of the shielded cable to 0V) and supply power and earth. First, turn VR2 (just above IC1) fully anti-clockwise. Then turn setpoint pot VR1 anti-clockwise until the relay clicks and the LED comes on. Because VR1 is a multi-turn pot, you may need to rotate it a number of times before the LED lights. Once the switch has tripped, you can then turn the set-point pot back clockwise just enough to turn off the LED and disengage the relay. Now when you heat the thermistor, the LED should immediately come on and the relay click over; cooling the thermistor should cause the LED and relay to turn off again fairly quickly. Finally, turn VR2 (hysteresis) clockwise a little and you should find that the switch takes longer to turn back off when it is being cooled down after being tripped. Fitting Fitting the Temperature Switch to a car is easy. You need only provide an ignition-switched power supply and earth, and then install the thermistor where you want to sense the temperature. For example, if you are controlling a radiator cooling fan, you could place an electrically-insulated temperature sensor on the top tank of the radiator. Or if you want the Temperature Switch to illuminate a warning light when engine or transmission oil gets excessively hot, you could attach the Table 1: Setting The Trip Point Temperature °C Rt Vt °C Rt Vt °C Rt Vt 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 33,944.034 26,120.042 20,286.407 15,894.535 12,557.604 10,000.00 8023.382 6483.660 5275.206 4319.920 3559.575 2950.420 2459.334 2061.059 1736.202 1469.774 1250.116 7.771 7.705 7.624 7.526 7.410 7.273 7.113 6.931 6.725 6.496 6.245 5.975 5.687 5.387 5.076 4.761 4.445 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 1068.105 916.558 789.791 683.278 593.399 517.244 452.462 397.143 349.731 308.953 273.760 243.287 216.818 193.755 173.601 155.938 140.416 4.132 3.826 3.530 3.247 2.979 2.727 2.492 2.274 2.073 1.888 1.719 1.565 1.425 1.298 1.183 1.079 0.985 170 175 180 185 190 195 200 205 210 215 220 225 230 235 240 245 126.739 114.656 103.954 94.454 85.999 78.458 71.718 65.679 60.259 55.384 50.991 47.023 43.435 40.183 37.231 34.547 0.900 0.823 0.753 0.690 0.634 0.582 0.535 0.493 0.455 0.420 0.388 0.359 0.333 0.309 0.287 0.267 You can use this table to set the trip point for the temperature switch, where the temperature is shown in the lefthand column and the voltage required to be on the middle pin of the set-point pot (VR1) is shown on the righthand side. For example, if you want the switch to trip at 145°C, the pot will need to be turned until the measured voltage is 1.425V. 80 PERFORMANCE ELECTRONICS FOR CARS siliconchip.com.au Parts List The temperature switch can be used to operate electric radiator fans – and you can even adjust the difference between switch-on and switch-off temperatures. With high loads like these fans, you should run an extra heavy-duty automotive relay. [Bosch] sensor to the appropriate sump. If you want just the warning light function, you can remotely mount a high-intensity LED within your line of sight – just extend the wires that connect the LED to the PC board, making sure that you keep the LED polarity correct. If you want to turn a device on and off with the temperature switch, you can use the relay’s Normally Open (NO) and Common contacts. Fig.3 shows these connections. The relay’s contacts are rated to 5A – for currents higher than this, use the on-board relay to switch another heavy duty automotive relay. Note that because a double-pole, double-throw (DPDT) relay has been used, another completely independent circuit can also be switched simultaneously. This other circuit can even turn off the second device as the first is switched on. Setting-Up There are two ways of setting the action of the Temperature Switch. First, if you have another means of monitoring the temperature (eg, via an engine-coolant temperature gauge or a temporary temperature probe and display), wait until the measured temperature reaches the desired trigger level, then turn set-point pot VR1 until the Temperature Switch just turns on. The turn-off value will be set by the hysteresis pot (VR2). If you want siliconchip.com.au the turn-off value to be close to the turn-on temperature, set VR2 fully anti-clockwise. If you want the turn-off temperature to be much lower, adjust VR2 further clockwise. The other way of setting the trip point is to make some measurements on the bench. Table 1 shows typical NTC thermistor resistance values for a range of temperatures. The thermistor is 10kΩ at 25°C and falls to 34.5Ω at 245°C. Table 1 also shows the expected voltage across the thermistor at each temperature value, assuming the regulator output is at 8V. If the regulator voltage is slightly different to this, the value will need to be scaled accordingly. For example, if the regulator output is 7.8V, then the output voltage will be the value shown multiplied by 7.8V, all divided by 8V. You can measure the regulator’s output voltage by using a multimeter to probe the right-hand terminal of the regulator with the PC board orientated as in the overlay and photos. Connect the other probe of the multimeter to ground. Make sure that you don’t slip with the multimeter probe and short-circuit the regulator! When VR1 is adjusted so that a particular voltage shown in Table 1 can be measured on its wiper terminal, the switch will trip at the corresponding temperature. For example, if you want the relay to close at 120°C, set VR1 so that its wiper voltage is 2.274V. The 1 PC board coded 05car101, 105 x 60mm 1 NTC thermistor, SOD27 leaded package, -40°C to 300°C (BC components 2322 633 83103) 5 PC-mount 2-way screw terminals with 5mm pin spacing 1 12V PC mount DPDT 5A relay (Relay1) 1 3-way header with 2.54mm spacing 1 jumper shunt with 2.54mm spacing 1 3m length of single core shielded cable 1 50mm length of 4mm diameter heatshrink tubing 1 1kΩ multi-turn top adjust trimpot (VR1) 1 1MΩ horizontal trimpot (VR2) Semiconductors 1 LM358 dual op amp (IC1) 1 7808 3-terminal regulator (REG1) 1 BC337 NPN transistor (Q1) 1 5mm red LED (LED1) 1 16V 1W zener diode (ZD1) 2 1N4004 1A diodes (D1,D2) 1 1N4148 diode (D3) Capacitors 2 100µF 16V PC electrolytic 2 10µF 16V PC electrolytic 1 100nF MKT polyester (code 104 or 100n) Resistors (0.25W, 1%) 1 22kΩ 4 10kΩ 1 1.8kΩ 2 1kΩ 1 10Ω accuracy will be within about 2%. Remember, if you wish the relay to close when the temperature goes above a particular value, install link LK1 in position “L/H” and install diode D3 as shown on the overlay. For the relay to close when the temperature goes below a certain value, install link LK1 in position “H/L” and install D3 the other way around. In most applications, once the Temperature Switch is set, it won’t need to be altered. The PC board fits into a 130 x 68 x 42mm jiffy box, so when the system is working correctly, it can be inserted into the box and tucked  out of sight. PERFORMANCE ELECTRONICS FOR CARS 81