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CARBON MONOXIDE ALARM By JOHN CLARKE Exposure to carbon monoxide (CO) gas produces an insidi­ous form of poisoning which at best can give the victim a head­ache and at worst can result in death. This CO Gas Monitor warns you of rising CO levels and emits a loud tone when the concentra­tion reaches a preset threshold. Features • • • • • • • • • Sensitive detection of CO gas (<200ppm) Uses rugged and reliable semiconductor sensor Sensitivity adjustment Precautionary CO level visual alarm Main higher CO level visual and auditory alarm Main alarm reset on continuous tone alarm Automatic purging of sensor 7-minute CO sensing time every ten minutes 2-minute sensor heat purging time MAY 1999  61 D ON’T BE TOO COMPLACENT here is that people often associate diz- on the dashboard of your vehicle or mounted towards the rear of a van or about the risks of CO poison- ziness and nausea with “car sickness” ing in your car, particularly if or “motion sickness”. However, it’s station-wagon. you drive an old “bomb”. Admittedly, quite possible that what is described While presented as a standalone there’s not much risk in a modern car as “car sickness” is really a good dose unit and perfectly practical as deof carbon monoxide. or if the vehicle is well maintained. scribed, even this small case might On older cars, however, there’s a Table 1 shows the effects of various look out of place in a today’s modern very real danger if the seal around the levels of CO concen­tration. As you cars – that’s if you can find a suitable rear door or bootlid has deteriorated can see, even small concentrations mounting position at all. suffi­ciently to allow exhaust gases to spell real danger. That’s because A more practical arrangement seep into the cabin. carbon monoxide has over 200 times might be to mount the PC board If the bootlid or rear door no longer more affinity with the haemoglobin without the case under the dashboard, in your blood than oxygen. It literally seal properly, ex­haust gases can be with the CO sensor, front panel LEDs stops the blood supply from carrying and switches mounted somewhere sucked into the rear of the vehicle oxygen and if enough haemoglobin is more suitable. as you drive along. Rust holes are affected, the brain suffers from oxygen another problem. Another idea would be to mount the starvation. Opening a window doesn’t ease the project, complete in its case in, say, In severe cases, a blood transfusion the boot, again with the CO sensor, situation; in fact it can often turn a bad situation deadly by altering LEDs and switches brought airflow within the vehicle. to a small plate mounted Table 1: Effects of CO Gas Poisoning and Symptoms You could be driving along on or under the dashboard. seemingly unaffected while Regardless of where it’s CO Gas      Symptoms your rear seat passengers cop installed, this unit is very Concentration a bad dose of carbon monsensitive to the presence of 50ppm Exposure for a few hours normally oxide (CO) poisoning with CO gas. Just driving along results in no symptoms possible fatal consequences. in heavy bumper-to-bumpThe other big danger is er traffic with the window 100ppm Exposure for a few hours results in from a poorly main­tained exopen or the airconditioner a slight headache in the forehead haust system. If there’s a hole set to “fresh-air” is enough 500ppm Exposure for one hour results in a anywhere in the system, it’s to trigger the unit, for exheadache with increas­ing severerity possible that exhaust gases ample. over time could seep past any defecSo far, we’ve only talked tive seals and into the cabin. about using this alarm in 1000ppm Exposure for 20-30 minutes results The answer here is obvious a motor vehicle. With a in a headache, dizzi­ness and nausea; – make sure that the exhaust suitable 12V supply, the possible death within 2 hours system is regularly inspected unit could also find use in 4000ppm Exposure results in possible death and correctly main­tained. a service workshop or anywithin 30 minutes Driving in bumpwhere else where exposure er-to-bumper traffic can also to CO gas is a risk. is the only way to save the victim expose you to excessive concentraThe unit is very easy to use and has from death. tions of CO gas. Because there are just two switches and four indicator By the way, it’s estimated that a LEDs on the front panel. The functions so many cars so close together, it’s heavy cigarette smoker will have of the Power and Reset switches are inevitable that there will be some about 5% of his/her available hae- self-explanatory, as is the function exposure to exhaust gases. moglobin tied up by CO at any given of the Reset switch. The other three This applies particularly if you drive with the window open or with time. The symptoms for CO poisoning LEDs are designated “Heat”, “Alarm” begin to occur when this percentage and “Warning”. your interior air fan or airconditionreaches about 10%, while the onset ing set to “fresh air”. Setting the fan The Heat LED indicates a heat or airconditioner to “recirculate” is of death occurs at about 20%. purging cycle for the gas sensor, while These figures suggest that a smoker the Alarm LED lights (and an internal the answer here, especially if you are is far more susceptible to CO poi- piezo alarm sounds) when a critical stopped at traffic lights. soning than a non-smoker, simply CO level is reached. Finally, the WarnHow do you know when you’re be­cause they are starting out from a ing LED flashes to give a preliminary being exposed to excessive CO levels? Well, you might not know until it’s 5% higher base. warning that the CO concentration is too late. That’s because CO is utterly on the rise. CO monitor colourless and odorless but nonetheThe CO sensor itself protrudes from Because CO gas is impossible for the rear panel of the case. This is a less quite deadly. First symptoms, from quite small the individual to detect, we set out to low-cost semiconductor unit made design an effective yet easy-to-build concentrations of CO gas, are headby Nemoto. It contains a heating aches, nausea and dizziness, while CO monitor. The end result is the element and a semiconductor senexposure to higher levels quickly self-contained unit described here. sor surrounded by a catalytic layer. It is housed in a small plastic in- These parts are contained in a 19mm causes unconsciousness and death. er cylindrical case with six An interesting point to consider strument case which can be placed diamet­ 62  Silicon Chip Self-contained in a plastic case and plugging into a car cigarette lighter socket, the CO Alarm can be moved from vehicle to vehicle. Alternatively, it could be “built in”, with or without the case. ° pins protruding from the base and with a double gauze wire mesh over the element. The double layer of wire mesh is there to prevent an explo­sion if the sensor is exposed to dangerous concentrations of inflammable gas. In operation, the sensor is heated to a temperature of 130-170°C and when CO gas becomes trapped on the catalytic layer, electrons are transferred to the semiconductor element. This markedly reduces the effective resistance of the semiconductor element to reveal the presence of the gas. When the gas dissi­ pates, the resistance of the semiconductor layer returns to normal. Over time, other gases such as hydrogen, petrol vapours and alcohol vapours are ab- sorbed onto the catalytic surface and cause contamination. To prevent false readings, these are periodically burnt off by raising the temperature of the heating element to around 450°C. By the way, in case you’re wondering, this unit is really only suitable for detecting carbon monoxide. It is fairly insen­sitive to hydrocarbon vapours (although it can detect very high concentrations), which means that it is unsuitable for detecting petrol fumes. Block diagram Fig.1 shows the block diagram of the CO Alarm. The circuit is powered from a 12V supply (eg, via the cigarette lighter socket) and this is regulated to give a +5.5V rail using REG1. This rail then supplies the heater and the semiconductor element in the CO sensor, along with the rest of the circuit. The sensor heater element must be driven correctly to suc­ cessfully purge any contaminating gases on the semiconductor element. The specifications state that the CO sensor be heated to 450°C for 1-3 minutes, while the CO sensing time should be 6-10 minutes. These times are set by the Timer circuit, with LED2 switching on during the heat purging period. In this case, the Timer circuit heats the sensor element for two minutes when power is first applied and repeats this 2-minute heating (purge) cycle every 10 minutes after that. Fig. 1: operation of the CO Alarm is easily understood when you break it down into circuit elements, as shown in this block diagram. MAY 1999  63 Fig.2: the circuit might seem a little complicated at first glance but it’s quite simple. It uses just three ICs, six transistors, a 3-terminal regulator and a handful of other parts plus, of course, the carbon monoxide sensor. During each 2-minute heating period and for one minute afterwards, the signal output from the CO sensor is grounded via Q3 in the Output Control section of the circuit. This is done to prevent false readings. At the end of each 3-minute period, Q3 turns off and so the signal from the CO sensor is fed to the following comparator stages. There are two comparator stages here – a “latching com­ parator” based on IC1a and a “warning comparator” based on IC1c. Basically, the warning comparator monitors the CO sensor output during the 7-minute sensing period. If the CO 64  Silicon Chip level reach­es a moderate level during this time, it enables a Flasher cir­cuit (IC1d). This in turn drives LED4, which flashes on and off to provide a preliminary warning. If the CO level subsequently rises past a critical point, the latching comparator (IC1a) lights LED3 via transistor Q4. It also activates a tone generator circuit based on IC1b and this then drives the piezo alarm via Q5 and Q6. Note that the piezo driver is modulated by the flasher so that the sound occurs in short bursts rather than continuously. The latching comparator now remains in this state until it is reset. This takes place automatically at the end of the first minute of the sensing period. If the CO level is still high after the reset, the comparator immediately returns to the latched-on state. Conversely, if the output from the CO sensor is below the comparator threshold at the time of reset (ie, the CO level has dropped), the comparator output switches low and turns off the alarm. Alternatively, the circuit can be reset manually at any time, so that the CO level can be retested. If CO is present, the output from the CO sensor will nor­mally only go low when heat purging starts again at the end of the 10-minute cycle. Provided it had already been triggered during the latter part of the sensing period, the piezo alarm will continue to sound into the purging period but the tone will change from pulsed to continuous. This continuous tone indicates that the manual reset can be used to silence the alarm. The circuit OK, so much for the basic theory of operation. To find out how it all works in practice, take a look now at the full circuit diagram (Fig.2). It might seem a little complicated at first glance but it’s really quite simple. It uses just three ICs, six transistors, a 3-terminal regulator and a handful of other parts – plus, of course, the CO sensor. The +12V rail from the car’s battery comes in via switch S1 and is applied directly to the input of REG1, an LM317T adjust­able regulator. Zener diode ZD1 protects the regulator from vol­tage transients, while the 100µF capacitor provides supply decou­pling. In operation, REG1 produces 1.25V between the adjust (ADJ) and output (OUT) terminals. The 120Ω resistor between these terminals sets the current between them to 10.4mA and this cur­rent flows Fig.3: use this component layout in conjunction with the photo overleaf to help with construction. to ground through trimpot VR1. Setting VR1 to 408Ω gives 4.25V between ADJ and ground, which means that the applied, its output at pin 3 is high and period is 0.693 x 150kΩ x 220µF. This output of REG1 will be at 4.25 + 1.25V the 220µF capacitor charges towards gives figures of 38.12s and 22.87s = 5.5V. In practice, VR1 is simply ad- the positive supply rail (Vcc) via the respec­tively, for a total period of just justed for the correct output voltage. 100kΩ and 150kΩ resistors. When the over one minute (60.99s). A second 100µF capacitor decou- voltage at pin 6 subsequently reaches In turn, pin 3 of IC2 clocks IC3, a ples the regulator output, while LED1 2/3Vcc, pin 7 switches low, as does 4017 decade (divide-by-10) counter. provides power indication. The 470Ω pin 3, and the capacitor discharges This counter has 10 independent resistor in series with LED1 limits the via the 150kΩ resistor until it reaches outputs which se­quentially go high 1/ Vcc. At this point, pin 7 goes open current through it to about 7mA. on receipt of a clock signal from IC1. 3 IC2, a 555 timer wired in astable circuit again, pin 3 goes high and the When power is first applied, IC3 is capacitor charges once more to 2/3Vcc. mode, forms the heart of the clock reset via the 10µF capacitor on pin circuit. Its timing components are This cycle repeats indefinitely while 15 (this capacitor briefly pulls pin ever power is applied. connected to pins 2 & 6 and consist 15 high) and so its “0” output at pin The charging period for the 220µF 3 is high. As a result, transistor Q1 is of a 220µF capacitor and the 150kΩ capacitor is simply 0.693 x (100kΩ + turned on via D6 and the associated and 100kΩ resistors. It operates as 150kΩ) x 220µF, while the discharge 4.7kΩ base resistor. Q1, in turn, drives follows: initially, when power is first MAY 1999  65 This photograph of the completed project, looking from front to back, gives you a good idea of how large the project is. It will also help with component placement during assembly. the base of Q2 which also turns on and con­nects pin 6 of the CO sensor to ground to apply the full 5.5V rail across the heating coil element. Q2 also turns on LED2 to indicate that the heater is operating. At the same time, transistor Q3 turns on via diodes D6 and D8. This transistor shunts the output of the CO sensor to ground via a 10kΩ resistor, to prevent the following comparator stages from detecting any false signals. When IC2 subsequently clocks the “1” output (pin 2) of IC3 high (after one minute), transistors Q1-Q3 all remain on due to the forward bias now provided via diode D7. At the end of the second minute, the “2” output (pin 4) of IC3 switches high and forward bias to Q3 is supplied via D9. Conversely, D8 is reverse biased and so Q1 & Q2 switch off to end the heating (purge) cycle after two minutes. Note, however, that a residual current still flows through the heater coil to ground via the parallel 180Ω and 3.9kΩ resis­tors on pin 6 of the sensor. The effective voltage across the heating coil is now only 0.8V and so the temperature quickly drops towards the desired 130-170°C operating range for CO sens­ing. 66  Silicon Chip Q3 remains on during this time, to short the sensor output to prevent false readings while the temperature stabilises. At the end of the third minute, IC3’s “2” output goes low, transis­ tor Q3 turns off and the signal from the sensor is now fed to comparators IC1a & IC1c via a 10kΩ resistor. Diode D1 isolates the comparator inputs. Warning comparator IC1c, part of an LM324 quad op amp, is the warning com­parator. Its pin 13 inverting input is biased to 1.28V by a voltage divider consisting of 33kΩ and 10kΩ resistors and this sets the comparator threshold. The output from the CO sensor appears at pins 5 & 7, while trimpot VR2 sets the sensitivity. Normally, when CO concentrations are low, the output from the sensor is less than the comparator theshold voltage (1.28V). As a result, pin 14 of IC1c is low and D5 pulls pin 9 of IC1d low to prevent this flasher oscillator from operating. Conversely, if the CO sensor output rises above 1.28V (ie, if excessive CO is detected), the voltage on pin 12 of IC1c will be greater than the voltage on pin 13. When this happens, pin 14 of IC1c switches high and reverse biases D5, thereby allowing the flasher oscillator based on IC1d to operate. IC1d is also part of the LM324 quad op amp package and is wired as a 0.5Hz oscillator. Its period of oscillation is set by the 100kΩ feedback resistor between pins 8 & 9 and by the asso­ ciated 10µF timing capacitor. The two 10kΩ resistors on pin 10 nominally bias the non-inverting input to half supply (1/2 Vcc), while the 10kΩ feedback resistor between pin 8 and pin 10 provides hysteresis. This feedback resistor provides upper and lower threshold voltages of +3.67V and +1.83V respectively. The circuit works as follows. When no CO gas is present, pin 9 of IC1d is held low and so the output at pin 8 is high and PNP transistor Q7 is off. However, if CO gas is detected, D5 becomes reverse biased as described previously and so the 10µF capacitor on pin 9 of IC1d charges via the 100kΩ feedback resis­tor until it reaches the upper threshold voltage (ie, 3.67V). At this point, pin 8 switches low and so Q7 turns on and lights LED 4 via a 470Ω resistor. The 10µF capacitor now discharges via the 100kΩ feedback resistor into pin 8 until it reaches the lower threshold voltage (1.83V). When it reaches this point, pin 8 goes high again, Q7 turns off and the 10µF capacitor again starts charging towards the upper threshold voltage. This cycle continues indefinitely and so LED4 flashes at a 0.5Hz rate while ever CO gas is present. Latching comparator IC1a is the latching comparator. Its pin 1 output switches high when the sensor output reaches half supply (ie, 2.25V), as set by the two 10kΩ bias resistors on pin 2. This high output in turn pulls pin 3 high via D2 and a series 10kΩ resistor and so the comparator output is latched high, even if the sensor output immediately drops below 2.25V. This turns on Q4 which in turn lights LED3 (alarm). Note that when pin 3 of IC1a is latched high, D1 is reverse biased. This ensures that the high on pin 3 has no affect on the sensor output. As soon as pin 1 of IC1a switches high, D3 is also reverse biased and so IC1b starts oscillating. This “tone generator” stage works in exactly the same way as the oscillator based on IC1d, except that the timing components on its pin 6 input are much smaller in value. As a result, IC1b oscillates at about 3kHz. IC1b drives Q5 & Q6 which together function as a push-pull output stage. In turn, these drive the piezo alarm to provide the audible alarm. Note, however, that the 3kHz alarm tone is not continuous but is modulated by IC1d. That’s because each time the output of IC1d switches low, it also pulls pin 6 of IC1b low via D4 and a series 4.7kΩ resistor and thus disables the tone genera­tor. IC1d oscillates at a 0.5Hz rate, which means that the tone generator stage (IC1b) operates in 1-second bursts. The latching comparator can be reset at any time by press­ing the Reset switch S2. This momentarily pulls pin 3 of IC1a low via a 1µF capacitor. If the voltage from the CO sensor is below the latching comparator threshold, then the comparator output stays low and the circuit reverts to the monitoring status. If not, it will go high again immediately after the reset and re­trigger the alarm. Alternatively, if the Reset button isn’t pressed, the latching comparator is automatically reset when the “4” output (pin 10) of IC3 goes high and switches on transistor Q8. This occurs one minute into the sensing period (ie, at the end of the fourth minute). As for a manual reset, the alarm is immediately retriggered if the sensor output is still above IC1a’s threshold voltage; otherwise it resumes its monitoring role. If the alarm immediately retriggers after the automatic reset, it will con- tinue to sound until IC3’s “4” output switches high again 10 minutes later. This means that the alarm will even continue to sound during the next heat purging period (unless, of course, the reset button is pressed). When the heat purging process starts, however, Q3 turns on and so pin 14 of IC14 goes low. As a result, oscillator IC1d is disabled which means that it no longer drives LED 4 (via Q7) or modulates the audible alarm. Therefore, the audible alarm switches from pulsed to continuous tone when the heat purging cycle begins. Pressing the Reset switch will now turn the alarm off, since the sensor output is effectively grounded by Q3 and can no longer retrigger the latching comparator. Note that the heat purging process does not start until six minutes after the automatic reset has taken place. That’s because IC3 is a decade counter and it takes a further six minutes for outputs “5-9” (not shown on the circuit) and then “0” to go high in turn. Construction Building the CO Alarm is easy since virtually all the parts are mounted on a single PC board coded 05303991 (117 x 102mm). Fig.3 shows the assembly details. Before installing any of the parts, carefully check your PC board for etching defects by comparing it with the published pattern. In particular, check for shorted or broken tracks and undrilled holes. Begin the assembly by installing the three wire links, then install PC stakes at the external wiring points. You will need 10 PC stakes in all – four for the CO sensor leads, two for the power supply connections, two for switch S2 and two for the piezo alarm. Once the PC stakes are in, you can install all the resis­tors. Table 2 shows the resistor colour codes but it’s also a good idea to check them using a digital multimeter, just to make sure. The three ICs can then be installed, followed by the diodes and the zener diode. Make sure that these semiconductor parts are correctly oriented. Now for the transistors. Be careful here, because there are three different types used and they all look the same. In partic­ular, be careful not to confuse the BC327 and BC337 types (one is a PNP transistor, the other an Parts List 1 Nemoto NAP-11A semiconductor type CO gas detector 1 PC board, code 05303991, 117 x 102mm 1 front panel label, 133 x 27mm 1 small instrument case, 110 x 140 x 35mm (see text) 1 automotive lighter plug 1 piezo transducer 1 SPDT toggle switch (S1) 1 momentary contact switch (S2) 1 1m length red/black figure-8 wire 1 60mm length 0.8mm tinned copper wire 1 80mm length yellow hookup wire 1 80mm length blue hookup wire 1 160mm length red hookup wire 1 cordgrip grommet 10 PC stakes 4 5mm LED bezels Table 3: Capacitor 2 3mm screws and nuts Codes 4 small self-tapping screws to [sb]Value IEC EIA secure PC board [sb]0.1uF 104 100n [sb].015 153 15n Semiconductors 1 LM324 quad op amp (IC1) 1 555 timer (IC2) 1 4017 divide-by-ten decoder (IC3) 1 LM317T adjustable regulator (REG1) 2 BC547 NPN transistors (Q1,Q8) 4 BC337 NPN transistors (Q2-Q5) 2 BC327 PNP transistors (Q6,Q7) 1 16V 1W zener diode (ZD1) 9 1N914, 1N4148 switching diodes (D1-D9) 4 5mm red LEDs (LED1-LED4) Capacitors 1 220µF 16VW PC electrolytic 2 100µF 16VW PC electrolytic 4 10µF 16VW PC electrolytic 1 1µF 16VW PC electrolytic 2 0.1µF MKT polyester 1 .015µF MKT polyester Resistors (0.25W, 1%) 1 330kΩ 1 220kΩ 1 150kΩ 3 100kΩ 1 33kΩ 19 10kΩ 2 4.7kΩ 1 3.9kΩ 1 2.2kΩ 2 1kΩ 5 470Ω 1 330Ω 1 180Ω 1 120Ω 1 10Ω 1 500Ω horizontal trimpot (VR1) 1 5kΩ horizontal trimpot (VR2) Miscellaneous Solder, etc MAY 1999  67 Table 2: Resistor Colour Codes                   No. 1 1 1 3 1 18 2 1 1 2 5 1 1 1 1 Value 330kΩ 220kΩ 150kΩ 100kΩ 33kΩ 10kΩ 4.7kΩ 3.9kΩ 2.2kΩ 1kΩ 470Ω 330Ω 180Ω 120Ω 10Ω 4-Band Code (1%) orange orange yellow brown red red yellow brown brown green yellow brown brown black yellow brown orange orange orange brown brown black orange brown yellow violet red brown orange white red brown red red red brown brown black red brown yellow violet brown brown orange orange brown brown brown grey brown brown brown red brown brown brown black black brown 5-Band Code (1%) orange orange black orange brown red red black orange brown brown green black orange brown brown black black orange brown orange orange black red brown brown black black red brown yellow violet black brown brown orange white black brown brown red red black brown brown brown black black brown brown yellow violet black black brown orange orange black black brown brown grey black black brown brown red black black brown brown black black gold brown NPN type). Note that Q8 needs to be bent over flat on the PC board to allow room for the Reset switch. Next, install the capacitors and note that the electrolytic types must be oriented correctly. Table 3 shows the value codes for the MKT polyester types. Regulator REG1 has its leads bent at right angles so that it can be mounted with its metal face flat against the PC board. Bend the leads as shown in the photo, so that they mate with the board mounting holes, then secure the regulator to the board using a screw and nut before soldering the leads. A separate heatsink isn’t required for REG1 – its metal tab allows sufficient cooling. The board assembly can now be completed by installing VR1, VR2 and the four LEDs. Mount the LEDs with a 20mm lead length so that they can later be bent over and pushed through the bezels fitted to the front panel. Make sure that the LEDs are correctly oriented – the anode lead is the longer of the two. In particular, note that LED4 is oriented differently to LEDs 1-3. Case And here is the final assembly, looking from the rear. The round grey object on the bottom left is the CO sensor, which could be mounted external to the case. 68  Silicon Chip As previously discussed, the prototype CO Alarm was housed in a low-profile instrument case measuring 110 x 140 x 35mm. Whether or not you use this case is up to you and your particular method of mounting. If you do, you will Table 3: Capacitor Codes   Value 0.1µF .015µF IEC 104 153 EIA 100n 15n have to drill four holes in the front panel to take the switches, plus four more to accept LED mounting bezels. Another two holes are drilled in the rear panel for the cordgrip grommet and CO sensor. First, the front panel. The best way to go about this job is to attach the label and then use this as a guide for drilling the holes. Alternatively, you can use the full-size artwork published with this article as a drilling template. Take care with the holes in the rear panel – both the sensor and the cordgrip grommet (for the 12V supply leads) should be a tight fit. The best way to make the sensor hole is to first drill a small pilot hole and then carefully enlarge it to size using a tapered reamer. The other hole should be carefully pro­filed to suit the shape of the cordgrip grommet. The PC board can now be installed in the case and secured using four self-tapping screws. These go into the integral stand­offs in the base of the case. This done, mount the switches on the front panel, then slide the panel into its slot at the front of the case and push the indicator LEDs through their matching bevels. All that remains now is to complete the wiring as shown in Fig.3. Use automotive cable for the supply leads and make sure these are firmly secured to the rear panel using the cordgrip grommet. The CO sensor can be wired using light-duty hookup wire, while switch S2’s contacts solder directly to the PC stakes adjacent to Q8. We mounted the piezo transducer on the lid of the case using hook and Fig. 4: use this same-size PC board pattern to make your own board or to check a commercial board for etching/drilling defects before commencing assembly. loop fasteners but a dab of super glue would also work. Finally, attach a cigarette lighter plug to the 12V supply lead. Testing You’re now ready for the smoke test. Rotate VR1 fully anti­clockwise, apply power to the circuit and measure the voltage on the output (centre) lead of REG1. Adjust VR1 for a reading of 5.5VDC and check that both LED1 and LED2 are now alight, indicat­ing that power is present and that the heating cycle has begun. If LED2 fails to light, try switching the power off and then on again, to activate the power-on reset for IC3. If that fails, check the 5.5V rail on pin 4 of IC1, pin 8 of IC2 and pin 16 of IC3. Assuming that all is well, wait for two minutes and check that the Heat LED (LED2) extinguishes. If you want to check operation of the sensor, place it near the exhaust pipe outlet of a running engine. Both the CO warning LED and the main alarm should be activated after a short time. Switch the power off and on again if you want to initiate the heat purging sequence immediately. This will also stop the main alarm if it has latched on. Installation The CO alarm is installed inside the vehicle and can be placed on the dashboard. Note that if you are already using the lighter socket for some other purpose, you can obtain a double lighter socket from automotive retailers or from Jaycar. VR2, the sensitivity control, should initially be set to mid-position and this should suit most applications. If you want greater sensitivity, adjust VR2 anticlockwise. Conversely, to decrease the sensitivity (eg, if the unit generates lots of nuisance alarms), adjust This same-size front panel artwork can be copied and used directly and/or used as a VR2 clockwise from its drilling template for the front panel. Artworks for panels and PC boards are also mid-position. SC available on the SILICON CHIP website, www.siliconchip.com.au MAY 1999  69