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Featurescontrol of external/recirculate air • Automatic le) here applicab vent in car (w aler t: n O concentratio • Two-level C D LE l flashes Warning leve and triggers lly lights LED fu l ve Alarm le tput DPDT relay ou y operation or toggle rela ry ta en om M • justments and alarm ad ng ni ar w e at steresis • Separ alarm level hy d an ng ni ar w • Adjustable justment calibration ad ut tp ou or ns • CO se anual reset • Optional m ion • 12V operat mode • Circuit test Carbon Monoxide ALERT By John Clarke Carbon monoxide is an insidious poison which can – and does – kill. It’s colourless, odourless and sneaks up on you! The first time victims become aware of carbon monoxide poisoning could be too late. C arbon monoxide (CO) is a pretty simple gas, consisting of one carbon and one oxygen atom. It will eventually mate with a spare oxygen atom and reform into the more natural – and relatively harmless – carbon dioxide, or CO2. But it remains as CO more than long enough to cause real harm to us mere mortals! Moreover, carbon monoxide poisoning can be very difficult to treat. While carbon dioxide (CO2) is not poisonous as such (in fact the air we breathe naturally contains around 0.03% carbon dioxide), in sufficient quantity it can displace oxygen in the air and therefore cause suffocation. However, carbon monoxide (CO) in any significant quantity is poisonous because it affects the carrying of oxygen in our blood. One of the main problems is that CO has 200 times greater affinity with the haemoglobin in your blood siliconchip.com.au than does oxygen. It literally stops the blood supply from carrying oxygen around the body and, most importantly, to the brain. When the brain suffers from oxygen starvation, permanent damage and even death can result in an amazingly short time – just a few minutes. Therefore, even if there is a relatively good supply of fresh air, a relatively small concentration of CO in that air places you in significant danger. Where vehicles are concerned, CO is almost always accompanied by exhaust fumes which do have an odour, so preventative/remedial action can hopefully be taken. But if you’re a smoker (with already elevated CO levels in your blood) or suffering from an allergy such as hay fever, there is a real chance that your sense of smell might be compromised enough not to notice. August 2005  61 So you really need to avoid breathing in CO. The effects of this dangerous gas. You don’t have to spend $$$$$$ on also tend to be cumulative – the body can get rid of it over a luxury car – just a few $ on this ingenious kit. time but prolonged exposure can be deadly – literally. Our Carbon Monoxide Alert monitors the level of CO You may have seen those signs in tunnels and deep, gas and warns if it reaches danger levels. It has an earlylong cuttings asking you to turn off your car engine during warning flashing LED for low CO concentrations and an delays. There’s a good reason for that – unfortunately, the alarm output that is set for a higher CO concentration level. signs are ignored by many. A continuously lit LED indicates the alarm and a relay is If you have ever been caught in heavy traffic, you may switched on. have noticed that you are becoming tired and developing a The relay contacts can be used to power a siren or it can headache. While the frustration of sitting in traffic may be be used to switch the vehicle’s cabin air vent switch from affecting you, this is probably not the reason for your indispoexternal to recirculate so that outside air does not enter sition. In fact, these are classic symptoms of exposure to CO. the cabin of the car. In this last application, the CO sensor The table above shows needs to be mounted the effects of exposure outside the cabin so to CO gas against the Effects of CO gas poisoning and symptoms that it monitors outconcentrations in the side air rather than 50ppm Exposure for a few hours normally does not result in any air. It is likely that the cabin air. symptoms CO concentration will When used to con100ppm Exposure for a few hours results in a slight headache (usually rise to greater than trol the air vent, the in the forehead) 100ppm in heavy trafCO Alert can operate 500ppm Exposure for one hour results in a headache with increasing fic and perhaps even one of two types of severity over time more if the number of external/recircula1000ppm Exposure for 20-30 minutes results in a headache, dizziness cars ahead of you do tion controls found and nausea, with possible death within 2 hours not to have a catalytic in cars. These are the 4000ppm Exposure results in possible death within 30 minutes converter in their exelectrically-activated haust system. types that typically Modern cars with catalytic converters will produce concontrol a solenoid, allowing pneumatic actuators to open siderably less CO gas than their older counterparts and this or close the vent. The CO Alert is not suitable for controlis because one of the functions of the catalytic converter ling manually operated sliders that physically move the is to convert the CO gas into CO2. The conversion is done vent to open or closed positions. using a platinum catalyst that aids in converting the exhaust There are two types of electrically activated vent conproducts of hydrocarbons and CO into CO2 and water. trols. One is a push-on/push-off switch and the other is While overall CO levels may be on the decrease, there the momentary switch that toggles between external and are still relatively high levels in the air along main roads; in recirculation for each pressing of the switch. This last type fact anywhere there is a concentration of vehicles with their is the typical switch used with climate control systems. engines running. Areas where the exhaust gases are not well For use with a push-on/push-off switch, the CO Alert will ventilated can also be a problem – such as underground and switch on its relay whenever there is an alarm condition. high-rise car parks, tunnels, garages and sheds. External/recirculation control is then possible by wiring There are two ways CO can build up inside your car. The first is where the car PURGE PURGE PURGE exhaust system has holes or leaks that al5V low the exhaust gases to leak into the car CO cabin via poorly fitting seals around the SENSOR door or via open windows. To prevent this, HEATER 60s 90s 60s 90s 60s 90s VOLTAGE it is recommended the exhaust system be periodically checked for leaks. 1.4V The second way is from other vehicles’ exhausts. Think about it: you are sitting in TIME a line of traffic (whether stopped or moving) LOW and the exhaust pipe of the vehicle ahead of you is directing the flow of exhaust gases (including CO) away from that vehicle – CO straight towards yours. SENSOR OUTPUT If your air intake is set to “fresh” you aren’t RESISTANCE taking in fresh air at all: you’re sucking in the fumes of the cars in front! In slow or stopped traffic, the only safe option is to switch your HIGH air intake to “recirculate” or to turn the fan/ TIME air conditioner off completely. FRESH AIR (0ppm CO) (100ppm CO) In some of today’s luxury cars, a sensor Fig.1: the CO sensor goes through a 150-second cycle, purging itself automatically detects carbon monoxide levfor one minute then looking for carbon monoxide during the next 90 els and switches the air input from outside seconds. When CO is detected, its output resistance drops significantly. to recirculate, thereby avoiding a build-up 62  Silicon Chip siliconchip.com.au It’s larger than life but this photo shows the completed PC board, ready for installation inside a case or mounting under the dash, as explained in the text. The carbon monoxide sensor is the cylindrical silver device, top right of picture. the relay contacts to this switch. Note that the car’s external recirculation switch should be kept set for the external position so the relay can take over when recirculated air is required. For use with the momentary switch, the CO Alert relay momentarily switches the relay both when the alarm switches on and also when it switches off. This will therefore duplicate the momentary switch action to select recirculate when the alarm is on and external air when the alarm goes off. The action should remain correctly in synchronisation, selecting recirculation for the alarm condition unless the car’s external/recirculation switch is pressed to change the settings. Synchronisation is also ensured because we have in- OPTIONS 12V IN RESET LK1 LK2 REG1 S1 5V WARNING/ALARM λ PURGE CONTROL (Q1) LED1 60/90 SECONDS TIMER (IC1) RB5 SENSITIVITY CO SENSOR COMPARISONS (IC1) AN2 RELAY DRIVER (Q2, RLY1, LED2) VR1 AN4 VR2 AN3 VR3 SET HYSTERESIS ALARM AN0 VR4 siliconchip.com.au AN1 VR5 SET HYSTERESIS WARNING LEVEL CONTROLS RELAY CONTACTS Fig.2: operation of the CO Alert is easiest to understand by referring to each of the functions in this block diagram. Most of these occur via microcontroller IC1. August 2005  63 REG1 7805 +5V OUT 10Ω IN 10 µF 16V 100nF 4 14 Vdd 75Ω E C 10Ω 1k B 11 AN4 MQ-7 CO SENSOR B H VR1 50k AN0 AN2 IC1 PIC16F88 100nF 4.7k 1k RESET S1 2005 RB7 17 LEDS K VR4 WARNING 10k SET RELAY1 K D2 1N4004 RELAY CONTACTS 1 NC COM NO A 2.2k TP5 AN1 RA6 18 RELAY CONTACTS 2 NC COM NO VR5 WARNING 10k HYSTERESIS 1k 15 A B C Q2 BC337 BC327, BC337 E 7 MOMENTARY OR ON/OFF LK1 SC  13 0V K λ LED2 TP4 1 – 100 µF A 16V VR3 ALARM 10k HYST. 2 TP1 1k +12V ZD1 16V 1W +12V TP3 AN3 A VR2 ALARM 10k SET 3 Q1 BC327 H A TP2 MCLR RB5 100 µF 16V K 1W + GND 100 µF 16V D1 1N4004 6 RB1 RB3 9 TEST LK2 E B C 470Ω RB0 Vss 5 A λ LED1 K CARBON MONOXIDE ALERT/ALARM 7805 MQ-7 CO SENSOR H B B A A H ZENER – + 1N4004 A K IN OUT GND Fig.3: for a complex circuit, it’s pretty simple! The PIC microcontroller takes care of most functions. cluded an alarm memory. This remembers the current alarm status so that if the engine is switched off while there is an alarm condition, the alert will momentarily switch the relay on when the car is restarted. This will switch the vent back to external air. This memory is then cleared, so if the engine is switched off and on again, the relay will not momentarily switch again. There are some cars that automatically select external air when the car is started. For this case, we have provided an option that disables the alarm memory feature. So regardless whether or not there was an alarm condition when the car is switched off, the external/recirculation settings will not be altered when the car is started. In any event, you can press the external/recirculation switch on the car at any time to change the setting if it somehow gets out of synchronisation. A reset switch on the CO Alert can be used to switch off the alarm and return the air vent selection to external. Other (non-vehicle) CO alarms We have concentrated on automotive uses for this project because that’s the primary “market”. But there is nothing to stop you using this alarm to warn of dangerous CO buildups, anywhere they are likely to be generated – in fact, it’s entirely practical! 64  Silicon Chip One application which springs to mind would be in caravans and mobile homes: every year there are reports of people being poisoned through the use of unflued heaters (or even gas stoves). Other suggestions would be around stationary engines such as power generators (especially as these are likely to be in confined spaces). Here the relay output could be used to turn on an exhaust fan. The CO sensor Detection of the CO gas in the air is possible by using a sensor that is specifically designed to be sensitive to this gas. The CO sensor is made up of an aluminium oxide ceramic tube that is coated with tin dioxide to detect the CO gas. Two electrodes are attached to the tin oxide layer to provide for measurement of the change in resistance with CO concentration. Tthe ceramic tube is heated with a coil to bring the sensitive layer up to the required temperature for CO sensing. In addition, the temperature is periodically raised beyond the normal measurement value in order to burn off impurities that may affect the readings. This purging is performed over a 60-second period, followed by a 90- second measurement period. The actual CO concentration measurement is made at the end of the 90-second measurement period. Fig.1 shows the cycle of events when making a CO gas siliconchip.com.au Fig.4: the component overlay (above) with a matching samesize photo (right). Between the two of these, you shouldn’t have any problems with construction. Watch polarity on the electrolytic capacitors and semiconductors. measurement. Initially the sensor has the full 5V applied to its heating coil for 60 seconds. This performs the purge. The resistance across the measurement terminals falls to a relatively low value during the purge. After purging, the heater coil is reduced in temperature by reducing the voltage applied to the coil to 1.4V. The measurement resistance becomes high at the end of 90 seconds when exposed to fresh air. If the sensor is exposed to air containing concentrations of CO gas, the measurement resistance will be lower than the fresh air concentration reading after the 90-second measurement period. Block diagram The block diagram for the CO Alert is shown in Fig.2. The sequence for sensor purging and measurement plus warning and alarm point detection is under control of a microcontroller, IC1. IC1’s RB5 output drives the purge control block to adjust power to the heater coil so that the CO sensor is supplied with the full 5V to the heater during the 60-second purge and with 1.4V during the 90-second measurement period. The sensor output is monitored at IC1’s AN2 input, while sensitivity to the CO concentration is adjusted using VR1. In operation, the sensor voltage at the AN2 input is compared with the settings at the warning AN0 input and the alarm AN4 input. If the sensor voltage goes above the warning setting but below the alarm setting, then the RB3 output will flash the warning/alarm LED on and off. Should the sensor voltage go above the alarm setting, the LED will stay lit. siliconchip.com.au When the alarm LED becomes lit, the alarm relay will also switch on if the option at RB1 input is set to on/off. If the RB1 option is set to momentary, then the relay will switch on for about one second before switching off again. When the sensor voltage goes below the alarm threshold, the relay will switch off for the on/off setting and momentarily switch on and off for the momentary setting option. Note that hysteresis has been included for both the warning and alarm settings. Hysteresis prevents the alarm switching on and off repeatedly or “hunting” when the sensor voltage equals the alarm setting. The levels on the AN1 and AN3 inputs respectively set this. Hysteresis alters the switching point for the warning and alarm settings so that the CO concentration must fall more than “just below” the warning and alarm settings before switching off these conditions. For example, with the alarm setting, the alarm condition will be set when the sensor voltage reaches the value set by the alarm input but will not be switched off until the voltage drops below the “alarm hysteresis” setting. Finally, input RB0 provides a test function. This cycles the purge/measurement cycle much faster than the normal 60s and 90s rate, enabling the circuit to be tested without having to wait for more than two minutes to see the effects of changing the settings. The circuit The circuit is made relatively simple because of the use of a microcontroller instead of discrete ICs. Without the microcontroller, we would have required at least three ICs August 2005  65 Parts List – Carbon Monoxide Alert 1 PC board coded 05108051, 120 x 60mm 1 plastic utility box 130 x 68 x 43mm 1 MQ-7 CO gas sensor (Jaycar RS-5615) 1 panel-mount momentary pushbutton switch (NO contacts) (S1) 5 2-way PC-mount screw terminals, 2.54mm pitch 1 12V DPDT 10A relay (RLY1) 1 cigarette lighter plug 1 18-pin DIL IC socket 1 mini heatsink, 19 x 19 x 10mm 1 M3 x 10mm screw and nut 12 PC stakes 2 2-way pin headers, 2.54mm pitch 2 jumper shorting plugs 1 300mm length of 10A dual figure-8 wire with polarity stripe 1 200mm length of hookup wire Semiconductors 1 PIC16F88 micro controller programmed with cosensor.hex (IC1) 1 7805 5V regulator (REG1) 1 BC327 PNP transistor (Q1) 1 BC337 NPN transistor (Q2) 1 16V 1W zener diode (ZD1) 2 1N4004 1A diodes (D1,D2) 2 5mm red LEDs (LED1, LED2) Capacitors 3 100mF 16V PC electrolytic 1 10mF 16V PC electrolytic 2 100nF MKT polyester (0.1mF – code 104 or 100n) Resistors (0.25W, 1%) 1 47kW* 1 4.7kW 1 2.2kW 1 470W 1 75W 1 10W 1 50kW horizontal trimpot (VR1) 4 10kW horizontal trimpot (VR2-VR5) (* – for testing/setup only) 4 1kW 1 10W 1W For external sensor mounting, add 2m 4-core cable For remote reset switch and LED add 1m 4-core cable plus 1 momentary push button switch with LED indicator plus considerably more resistors, capacitors, diodes and transistors. The extra features such at the momentary or on/off action of the relay, the alarm memory function and the test option would all have added more complexity to the circuit. Fortunately the microcontroller has allowed us to incorporate the complexity without adding to the circuit. Also the microcontroller can use an internal oscillator, further reducing the parts count when the extra accuracy of a crystal is not required. Output RB5 is used to control the purge/measurement cycle for the sensor. It does this by going low (to 0V), switching on transistor Q1 due to the base current now flowing through the 1kW resistor. The transistor allows the full 5V to be applied to the heater inside the CO sensor. 66  Silicon Chip This purge cycle lasts 60 seconds. After RB5 goes high (to +5V), so Q1 switches off. The heater is now driven from 5V via a series resistance of 85W. In conjunction with the 33W resistance of the CO sensor’s heating element, this sets the heater current to the recommended 42.4mA, with 1.4V across the sensor. The sensor’s measurement terminal A connects to the 5V supply, while the B terminal connects to ground via a 50kW trimpot and 4.7kW resistor. This resistance forms a voltage divider with the sensor terminal resistance and a voltage appears at terminal B that is proportional to the measured CO concentration. The 1kW resistor and 100nF capacitor filter any noise which may develop on this high impedance circuit, particularly if the sensor is mounted remotely. The alarm, warning and hysteresis settings are provided by trimpots VR2-VR5 respectively. These are each connected across the 5V supply so each can be adjusted from 0-5V. Typically, the settings trimpots would be set to around 1.5V and 3V respectively, while the hysteresis settings would be no more then 0.5V. Test points provide measurement connection points for a digital multimeter. The warning/alarm LED is driven via the 470W resistor and microcontroller output RB3. This LED flashes at a nominally 1-second rate during the warning phase. The LED stays lit during the alarm condition. RA6 drives the relay via Q2. When RA6 is at 5V, transistor Q2 switches on and powers the relay, which closes the common and normally-open contacts. LED2 lights to show whenever the relay is on. Diode D2 quenches the reverse voltage spike when the relay switches off after RA6 goes low. The reset switch ties input RB7 to ground when pressed. It is normally held high via an internal pull-up resistor when the switch is left open. LK1 and LK2 tie the RB1 and RB0 inputs low when these links are installed. If a link is left out, its associationed input is pulled high via an internal pull-up resistor. Power is 12V, supplied from the car battery – normally via a cigarette lighter plug. Diode D1 protects against reverse polarity connection and zener diode ZD1 limits transient voltages to 16V. The 10W series resistor prevents excessive current flow during the clamping action. REG1 provides a regulated 5V output for the circuit. The 100mF and 10mF capacitors decouple the supply. As mentioned, the CO Alert can be built either as simply a monitor that gives visual indication of the warning and alarm CO concentration levels. This version has the unit built into a box with the sensor, reset switch and indicating LEDs mounted in the box. Alternatively, it can be built to actively control the cabin air vents in a car where such control is possible. In this case, the sensor is mounted external to the car’s cabin and the warning/alarm indicating LED and reset switch are mounted on the car’s dashboard. The LED and switch can be separate items or you can use a combined reset switch and LED assembly for a neater job. The unit would be most easily powered by connecting to an ignitionswitched terminal in the fusebox. Construction Begin construction by checking the PC board. If you are installing it in a box, the corners will need to be shaped so it will clear the internal pillars in the box. siliconchip.com.au Install the resistors, making sure the correct value is placed in the shown position. You can use the accompanying resistor colour code table to check values or use a multimeter to measure the value. When installing the regulator and IC socket, make sure they are oriented correctly. You will note that we haven’t yet placed the microcontroller IC in its socket. Similarly, the electrolytic capacitors must be oriented with the polarity as shown on the overlay diagram. One trimpot, VR1, is a different value to the rest – check that it is correctly placed. Use PC stakes for the test points and for the sensor connection points. These stakes support the sensor above the PC board when used in the box and for connecting the wires in the remotely located sensor version. When installing Q1 and Q2, make sure that the correct type is placed in each position. The LED (LED1) can mount directly on the PC board (actually 23mm above the PC board) when used in the box. If used on the dashboard, either separately or within an indicator switch, use PC stakes to make wiring easier. Position and solder the terminals and Here’s one mounting option for the CO sensor – inside a case with the relay. LK1 and LK2 are made from 2-way warning LEDs clearly visible through the lid. Alternatively, you could header strip and jumper shunts. Attach the mount the PC board under the dash with the alarm outputs switching the heatsink to the regulator using a screw and air intake. nut. The heatsink is oriented with the fins as shown. be on and indicated by LED2. When mounting in the box, drill holes in the box lid for Wind VR1 back so TP1 reads 2.8V and the alarm LED the sensor, the reset switch and two LEDs. Also drill holes will go from always lit to flashing and the relay will in the side of the box suitable for the wiring entry to the switch off. screw connectors. Retest the operation in momentary mode with the Before moving on to the testing stage, give your finished jumper plug inserted at LK1. The relay should switch project a thorough check for soldering, component placeon for a second or as the alarm goes on and also as the ment and where appropriate, component polarity. alarm goes off. You can also test the hysteresis operation by setting voltTesting. age at TP3 and TP5 to a small voltage using trimpots VR5 Initially, solder a 47kW resistor across the A and B sensor and VR3 respectively. If you set the hysteresis too large, PC stakes. Apply power and check for 5V between pins 5 & 14 on the IC socket. If this is OK, disconnect power and Fig.5: here’s how to wire insert IC1. Set VR1, VR3 and VR5 fully anti-clockwise; VR2 the relay to to about three quarters of its travel (in a clockwise direcNC the vehicle’s tion) and VR4 at mid setting. Jumper plug LK2 is inserted COM air intake for the test mode and the LK1 is left open. NO switch, with Check voltages at the test points by connecting your both NO and multimeter common lead to TP GND and the positive NC types NORMALLY OPEN EXTERNAL/RECIRCULATION SWITCH lead to each of the other test points in turn. There should (CLOSE TO RECIRCULATE) covered. be about 3V at TP2, 2.5V at TP4, 0V at TP3 and TP5 and 0.5V at TP1. Check the trimpots settings if the values are not similar to these. Rotate VR1 until the voltage at TP1 is higher than the NC 2.5V set at TP4. LED 1 should now flash after a second or COM so. Wind VR1 back so that TP1 reads 2V and the LED will NO stop flashing. Note that there may be a one or two second delay in the switching. NORMALLY CLOSED EXTERNAL/RECIRCULATION SWITCH Rotate VR1 until the voltage at TP1 is higher than the 3V (OPEN TO RECIRCULATE) set at TP2. Now LED1 should stay lit and the relay should siliconchip.com.au August 2005  67 NC CARBON MONOXIDE ALARM NO C chamber is usually a good position for the sensor, as this is the “gathering point” for the air which enters +12V the vehicle. You will require four wires to make the A and B connections and NO GND the two heater connections on the C sensor. Insulate the terminals with heatshrink tubing to prevent any of NC the wires shorting to the chassis. The PC board can either be placed TP GND in a box or attached under the RESET dashboard as a free PC board and protected against shorting to the 05108051 chassis by wrapping it up in some bubble wrap or heatshrink tubing. Fig.6: same-size PC board pattern, as seen from the copper side, for etching your However, you will initially need own or checking commercial boards. access to adjust the trimpots. The diagrams show how to wire the alarm or warning may never switch off. This will be up to the air vent switch. For the push-on push-off switch, the case if the hysteresis is set to a larger value than the you need to find out whether the switch is open or closed warning or alarm trip points of VR4 and VR2. Typically, when the air vent is set for external air (some vents close hysteresis needs only be 0.5V or less. to act as a bypass to external air, others open to allow exYou can test the alarm memory by switching off power ternal air in). You can do this by setting external air and when the alarm is on. When the power is reapplied, the then switching off the ignition. Check for a closed or open relay should switch on for a second. If you do not want this switch using your multimeter set to Ohms. A low reading feature, then switch off power and press and hold down of less than one ohm will indicate that the switch is closed. the reset switch. Now turn on power while still pressing If the switch is open for external air, then connect the the reset switch. Release the switch after a second and common and normally open relay contacts across this the feature should be disabled. Check this by switching switch. The switch is kept in the open position. If the off power when the alarm is on. The relay should not switch is closed for external air, then the common and operate when power is reapplied. Note that pressing the normally-closed relay terminals connect across the switch reset switch during power up as before can reactivate the and the switch is pressed so that it is in its normally open memory feature. position for recirculate. Remove the 47kW resistor across the CO sensor stakes and For the momentary switch operation, external and rein their place, connect the CO sensor itself. Now connect circulation air control is invariably made with a normally your multimeter (set to measure low DC volts) between TP open switch. The common and normally open relay conGND and the top heater terminal. Check that this voltage tacts should be connected across the switch. goes from 5V to 1.4V to 5V etc every second or so. The test Calibration link (LK2) can now be removed. The manufacturer recommends that the sensor be purged There is no easy way to calibrate the sensor to specific for 48 hours. This is required to ensure accuracy of readings. CO concentrations. In general, you can set trimpot VR1 However, for our purposes, the CO Alert should be able to to mid setting and set VR4 to 1V and VR2 to 2V. Set the be used immediately, noting that readings may change after hysteresis trimpots (VR3 and VR5) to 0.25V each. Check several hours use, once the sensor has purged of impurities that the warning LED flashes when cruising in relatively introduced during manufacture or during storage. busy traffic and that the alarm LED stays lit in heavy stopstart traffic. Installation You can increase sensitivity to CO emissions by turnAs mentioned before, if the CO Alert is used to coning VR1 higher (ie, more clockwise). If the sensing is too trol the air vents, the unit needs to be installed with the sensitive, set the warning and alarm trimpots (VR4 and sensor mounted externally to the cabin. The plenum VR2) more clockwise. SC Resistor Colour Codes 1 1 1 1 1 1 1 No. 1 1 1 4 1 1 1+1 68  Silicon Chip Value 47kW 4.7kW 2.2kW 1kW 470W 75W 10W 4-band code (1%) yellow violet orange brown yellow violet red brown red red red brown brown black red gold yellow violet brown brown violet green black brown brown black black brown 5-band code (1%) yellow violet black red brown yellow violet black brown brown red red black brown brown brown black black brown brown yellow violet black black brown violet green black gold brown brown black black gold brown siliconchip.com.au