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Chapter 12 Any sensor that outputs a varying voltage can be used by the Simple Voltage Switch to turn things on and off . . . intercooler sprays, boost control solenoids, warning lights, fans, water injection – you name it! Simple Voltage Switch Switch devices on and off using the sensors already under the bonnet! T he Simple Voltage Switch is cheap, easy to build – and very useful. It operates a relay when the monitored voltage reaches a preset level, then switches the relay off when the voltage drops by (another) preset amount. Many engine sensors work at varying voltages and any of these can be tapped into. For example, take a device that you want switched on the basis of load. If your car has a voltage-output air-flow meter (and that’s by far the majority of air-flow meters), then the Simple Voltage Switch (SVS) can use that engine load signal to switch things on and off. Alternatively, the oxygen sensor (in nearly all cars) outputs a voltage that varies with air/fuel ratio, so the SVS could be used to operate You Can Use It To Do This . . . •  Intercooler water spray control (from air-flow meter, throttle position sensor or oxygen sensor signals) •  Anti-lag turbo wastegate control (operating a wastegate disconnect solenoid triggered from the air-flow meter signal) •  Nitrous oxide switching (from throttle position sensor signal) •  Intercooler fan control (from air-flow meter signal) •  Dashboard monitoring LED (eg, oxygen sensor output signal) •  Switching in and out engine management and auto transmission control modifications (from air-flow meter, throttle position sensor or oxygen sensor signals) •  Low battery voltage warning and/or disconnect 72 PERFORMANCE ELECTRONICS FOR CARS devices on the basis of rich or lean air/ fuel ratios. Want yet another example? Well, take the throttle position sensor. Yet again it’s a sensor that outputs a varying voltage, so you can use the SVS to turn things on and off on the basis of throttle position. Using the sensors that are already there is a lot easier than trying to rig up switches or add extra sensors! The SVS opens up a range of possibilities. On the guinea-pig car (an import Maxima V6 Turbo), the SVS was used to trigger a solenoid. The voltage being monitored by the SVS was the standard air-flow meter output and the solenoid closed off the turbo waste-gate from the boost pressure source whenever engine loads were low. This meant that during turbo spoolup, the waste-gate hose was effectively blocked, resulting in a boost increase that occurred as fast as possible. Then, when the mass air flow into the engine reached the preset threshold, the waste-gate was again connected and so the selected maximum boost siliconchip.com.au Parts List 1 PC board coded 05car061, 106 x 61mm 5 PC-mount 2-way screw terminals with 5mm pin spacing 1 12V PC-mount DPDT 5A relay (Relay1) 1 3-way header, 2.54mm spacing 1 jumper shunt, 2.54mm spacing 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) 2 16V 1W zener diodes (ZD1,ZD2) 2 1N4004 1A diodes (D1,D2) 1 1N4148 small signal diode (D3) Fig.1: this shows where each of the components is placed on the main 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 LK1 is in the H/L position. Capacitors 2 100µF 16V PC electrolytic 2 10µF 16V PC electrolytic 1 100nF MKT polyester (code 104 or 100n) Resistors (0.25W, 1%) 2 1MΩ 1 22kΩ 4 10kΩ 1 1.8kΩ 1 1kΩ 1 10Ω pressure was then maintained. Even trickier, the SVS could be set so that a slight initial over-boost occurred, giving even better boost response. In short, the SVS allowed a variable wastegate anti-creep function to be easily implemented – which had the benefit of giving very strong part throttle boost response! The SVS is a brilliant building block that’s easy to set up and very effective. Fig.2: here is a typical connection set-up. The Simple Voltage Switch is fed ignitionswitched power and earth (chassis) connections. The signal input is wired to the airflow meter output signal. One of the relay’s Normally Open (NO) connections is also made to ignition-switched +12V while the adjacent Common is connected to an intercooler water spray pump. The other side of the pump is earthed. When the engine load exceeds a preset level, the water spray will be triggered into action. When constructed, your circuit board should look like this. Make sure that you install the polarised components the correct way around. Construction The SVS is a simple kit to build, however you should make one decision before you lay a soldering iron on it. Will you be using it to detect a voltage that is rising to the trip point or falling to the trip point? The SVS can be configured to work with either type of signal but if you know which way you’re going, you won’t have to make changes later on. The detection of a rising voltage will siliconchip.com.au PERFORMANCE ELECTRONICS FOR CARS 73 Fig.3: the circuit is based on comparators IC1a & IC1b. IC1a compares the input voltage (VIN) to a reference voltage as set by trimpot VR1 and switches its output (pin 1) high or low accordingly. IC1b acts as an inverter, while link LK1 allows the circuit to be set to trigger on either a rising voltage or a falling voltage. The selected comparator output drives transistor Q1 & the relay. How It Works The Simple Voltage Switch relies on comparator IC1a, which compares the input to a reference level. The input voltage (VIN) is divided via two 1MΩ resistors in series which effectively apply one half of the voltage to the inverting input, pin 2, of IC1a. Zener diode ZD2 and the 100nF capacitor are there to protect against transient voltages on the input signal. IC1a’s non-inverting input, pin 3, is connected to reference trimpot VR1, via a 10kΩ resistor. When pin 2 is above pin 3, IC1a’s output at pin 1 is low (ie, close to 0V). When pin 2 is below pin 3, pin 1 is high (at around +10V). Hysteresis (positive feedback from pin 1 to pin 3) has been added to prevent the output from oscillating at the trigger voltage. This is provided via trimpot VR2 and diode D3. This feedback causes the output to “pull” the voltage at pin 3 either higher or lower, depending on whether the output at pin 1 is high or low and the orientation of diode D3. If D3 is 74 PERFORMANCE ELECTRONICS FOR CARS installed as shown (ie, anode to pin 3), the voltage on pin 3 will be pulled lower than the reference voltage set by VR1 when IC1a’s output (pin 1) goes low. However, if pin 1 is high, D3 will be reverse biased and the reference voltage is unaffected. Conversely, if D3 is installed the other way around (cathode to pin 3), pin 3 will be pulled higher than the reference voltage if IC1a’s output goes high. In practice, this means that diode D3 is inserted with its anode towards pin 3 if you want the Voltage Switch to trigger on a low to high (L\H) transition and with its cathode towards pin 3 if you want it to trigger on a high to low (H\L) transition. Basically, the hysteresis is the difference between the switch-on and switch-off voltages and this is set using VR2. IC1b is an inverter and it provides a signal which is the opposite to IC1a’s output. It compares IC1a’s output with the +5.5V set on its non-inverting input. When IC1a’s output goes high, IC1b’s output goes low. And when IC1a’s output goes low, IC2a’s output goes high. Link LK1 provides the option of driving the relay with a falling (H/L) input voltage or a rising (L/H) input voltage, respectively. The output selected (either from IC1a or IC1b) drives transistor Q1 which in turn drives the relay. The diode across the relay coil (D2) is there to quench the reverse voltage that is generated by the collapsing magnetic field of the relay coil. Power for the circuit is obtained from the switched +12V ignition supply. Diode D1 gives reverse connection protection, while the 10Ω resistor, 100µF capacitor and zener diode ZD1 provide transient protection at the input of regulator REG1. The reference circuitry is powered from the output of REG1 (+8V), while the remainder of the circuit is powered from the +11.4V rails which are derived before the regulator. siliconchip.com.au The placement of the link and the orientation of diode D3 (both circled here) will depend on whether you want to activate the switch on a rising voltage or a falling voltage. As shown here, the SVS is configured to trigger on a rising voltage, which is the most common requirement. Reverse the diode and change the position of the link to trigger on a falling voltage. be the more common application – for example, triggering an intercooler spray on the basis of throttle position or air-flow meter voltage – eg, when the air-flow meter output voltage rises to (say) 3.2V, the water spray comes into operation. Below 3.2V, the spray is off; above 3.2V, the spray is on. However, if you want something switched on only at low loads – for example, an intercooler fan when the car is idling – then you’d configure the SVS to detect a falling voltage. In this case, the intercooler fan might come into action when the airflow meter drops below (say) 1.9V. So what are the changes made for the differing configurations? They’re simple: for a rising voltage detection, the moveable link LK1 is placed in its “L/H” position (that is, to the right of the PC board when the board is orientated as shown in the overlay diagram) and diode D3 is orientated so that its band is closest to the top of the board. For detection of a falling voltage, the link is moved to its “H/L” position and the diode’s orientation is reversed. Easy, huh? When assembling the PC board The Simple Voltage Switch can also use the oxygen sensor signal, allowing devices to be turned on when the mixtures are rich or lean. The Voltage Switch won’t load down the signal, so it can still be used by the ECU. make sure that you insert the polarised components (the diodes, IC, LED, transistor, voltage regulator and electrolytic capacitors) the correct way around. During construction, look at the photos and overlay diagram closely to avoid making mistakes. Testing You should always bench test the kit to make sure that it is working as it should. In addition to power and ground connections, you’ll also need to supply the kit with a variable voltage, replicating the sensor output that the SVS will be monitoring. The easiest way to do this is as is shown in the photo on page 76 – it’s just a matter of connecting a pot (eg, 10kΩ) across the power supply, to give a 0-12V variable voltage on the wiper terminal. Apply power and earth and connect the variable voltage signal to the input terminal. Now vary the voltage going to the input and at some stage the relay should click and LED1 should come on (or go off). Using a multimeter, measure the voltage at the signal input (ie, connect the positive probe of the multimeter to RESISTOR COLOUR CODES Value 1MΩ 22kΩ 10kΩ 1.8kΩ 1kΩ 10Ω siliconchip.com.au 4-Band Code (1%) brown black green brown red red orange brown brown black orange brown brown grey red brown brown black red brown brown black black brown 5-Band Code (1%) brown black black yellow brown 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 the signal wire and the negative probe to earth) and measure the voltage at which the switch is activating. For example, with the SVS configured to read rising voltages, as you gradually lift the input voltage the SVS might turn on at 5.00V. Now very slowly reduce the voltage and see at what voltage the SVS turns off. You might find that the latter voltage is 4.80V, meaning that the hysteresis (the difference between the switch-on and switch-off voltages) is 0.2V. Turn the hysteresis pot (VR2 – the single turn pot) and make sure that the hysteresis changes. For example, with a switch-on voltage of 5.00V the switch off voltage might now be only 4.97V – just 0.03V hysteresis! As you turn the hysteresis pot clockwise, hysteresis will increase. Note that one of the tricky aspects of the design is that changing the hysteresis will not change the setpoint, allowing the two to be set up individually (we’ll come back to this below). Next, you can test the action of the setpoint pot (VR1). As you turn the setpoint pot clockwise, the trip voltage will increase. A multi-turn trimpot has been used for VR1 so that the trip point can be adjusted very precisely. If you’re not used to this type of trimpot, be aware that you can keep on turning it endlessly and never reach a clear “stop”! As the specifications show, it’s possible to have the switch tripping at very low voltages indeed, allowing it to work off the output of the oxygen sensor (0-1V in most cars). However, to allow the switch to work at very PERFORMANCE ELECTRONICS FOR CARS 75 An easy way to bench test the Simple Voltage Switch is to temporarily wire a pot across the power supply to provide a variable signal voltage. An adjustable 0-12V will be available on the centre terminal of the pot. Here, the yellow wire connects this variable voltage to the signal input of the Simple Voltage Switch. Connect 12V and earth to the red and black wires respectively and you can easily test the operation of the device. low voltages, the hysteresis also needs to be set very low – that is, fully anticlockwise as your starting point. Note that the switch will not load down the oxygen sensor – it can be used without the signal to the ECU being degraded. Fitting Fitting the SVS to a car is easy. You will need to provide an ignitionswitched +12V supply, earth and the connection to the sensor signal. For an example of the latter, if you are triggering the SVS from the air-flow meter output voltage, you’ll need to first use the workshop manual and/ or your multimeter to find this wire, confirming that it has a voltage on it that rises with engine load. The device that is to be triggered by the relay will normally be switched via the Normally Open and Common relay contacts. Fig.2 shows these connections. 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. If you want to simply monitor a voltage (for example, the oxygen sensor signal voltage), you can delete the relay, instead mounting the LED on the dashboard. In this way, it’s possible to have a LED that stays on when the mixtures are rich, flashes when the mixtures are oscillating in 76 PERFORMANCE ELECTRONICS FOR CARS closed loop mode, and stays off when the mixtures are lean. Set-up There are two ways of going about the set-up: (1).  Measure the on-car sensor voltage and then set up the SVS on the bench to operate at this voltage, so only fine tuning will be needed in the car. (2). Do the complete set-up on the car itself. If you are using an oxygen sensor voltage output to trip the SVS, then the first way is better. For example, if you want the SVS to trip when the oxygen sensor signal rises above 0.6V, then set it up on the bench to do this. When you subsequently fit the device to the car, you’ll only need to make a small adjustment to the setpoint pot – which is much better than trying to find where the 0.6V trip-point is over the whole pot range! However, if you want to turn on Main Features •  Adjustable switching level between 0V and 16V at input •  DPDT 5A relay •  Configurable to switch on rising or falling voltage •  Adjustable hysteresis •  High input impedance – won’t load down sensors a device on the basis of engine load (ie, on the basis of the air-flow meter signal), it’s best to do it on the car. That’s because the air-flow meter signal varies across a much wider range and it’s unlikely that you’ll have a good feel for the precise voltage where you want it to trip until you do some on-car testing. When setting up, always set the hysteresis pot to its minimum setting (ie, fully anticlockwise) and then adjust the trip-point until the SVS triggers when you want it to. If the relay tends to chatter around the trippoint, increase the hysteresis. When it is tripping at the correct voltage for the application, assess how long the device continues to operate as the voltage again drops (assuming the SVS is set to trip on rising voltages!). For example, if you are using the SVS to trip an intercooler water spray on the basis of air-flow meter voltage, does the spray go off fairly quickly as the load again drops? In some applications, the hysteresis setting will be critical (the variable anti-wastegate creep system mentioned at the beginning of the story is a good example), while in other applications it won’t matter much at all. In most cases, once the SVS has been set, it won’t need to be altered. The PC board fits straight into a 130 x 68 x 42mm jiffy box, so when the system is working correctly the board can be inserted into the box and  tucked out of sight. siliconchip.com.au