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Exhaust gas monitor for cars & vans Are you worried about exhaust gases seeping into your car with possible tragic consequences? If so, you'll be interested in this project. It's an exhaust gas monitor that can be fitted to any van or car and sounds an alarm in the presence of exhaust gases, particularly carbon monoxide. By BOB FLYNN & LEO SIMPSON Do you get a headache during or after a trip in your car or van? Is it a slight frontal headache or a real head-splitter? Either way, you could be suffering from the effects of exposure to carbon monoxide. There are two easy ways that you can be exposed to this utterly colourless, odorless but very deadly gas. First, you may be driving in slow "bumper to bumper" traffic. Because so many cars are close together with their engines running 14 SILICON CHIP at idle, it is inevitable that you will be breathing exhaust gases, either from the cars in front, at the sides or even from your own car. And if you have your airconditioner on the "fresh air" setting, the effects may be worse than if you drive with your window open. Second, if the rear door of your van or the boot of your car does not seal properly, exhaust fumes will be sucked in at the rear of your vehicle as you drive along. And con- trary to what you may think, opening the windows may not make very much improvement. So while you are driving along seemingly unaffected, your rear seat passengers could be getting a very bad dose with possible fatal consequences. Don't think that just because your exhaust system is not faulty that you are safe. Normally operating vehicles can produce lots of carbon monoxide, sufficient to put you and your family in great danger. Table 1 shows the effect on humans of various concentrations of carbon monoxide in air. Note that these are mostly short term effects (apart from death which is a long term effect). An interesting point to consider here is that people often associate dizziness and nausea with "car sickness" or "motion sickness". However, if you look at the modest concentrations of CO in Table 1, it is quite possible that what is often blamed on "car sickness" could be a good dose of carbon monoxide. Why is carbon monoxide so dangerous? The reason is that it combines with haemoglobin in the blood and stops it carrying oxygen. If enough haemoglobin in your blood is affected, your brain will suffer from oxygen starvation. As Table 1 shows, quite modest concentrations of carbon monoxide can spell real danger. This is because carbon monoxide has over 200 times more affinity with haemoglobin than oxygen. In severe cases of carbon monoxide poisoning, a blood transfusion is the only way to save the victim from death. And what of the long term effects of frequent exposure to modest levels of carbon monoxide? At the time of writing this article, we had no information on this aspect but we would not be surprised if there were adverse effects. The risk of exposure On a number of quite new vans and cars SILICON CHIP has examined, the rear door or boot lid did not seal properly because the rubber TABLE 1 Concentration of CO In Air Toxic Symptoms Developed 0 .02% (200ppm) Slight headache within 2 to 3 hours. 0.04% (400ppm) Frontal headache within 1 to 2 hours, becoming severe within 2.5 to 3 .5 hours. 0.08% (800ppm) Dizziness, nausea and convulsions within 45 minutes; insensible within 2 hours. 0.16% (1600ppm) Headache, dizziness and nausea within 20 minutes; death within 2 hours. 0 .32% (3200ppm) Headache, dizziness and nausea within 5 to 10 minutes; death within 30 minutes. 0.64% (6400ppm) Headache, dizziness in 1 to 2 minutes; death in 1 0 to 1 5 minutes. 1.28% (12,800ppm) Death in 1 to 3 minutes. gasket was defective or nonexistent (quite common on courier vehicles) or the catch was out of adjustment. In addition, many Japanese vans have a battery compartment in the floor of the rear compartment. Often, this is inadequately sealed and lets exhaust fumes in. As these vehicles are driven +12V TGS812 GAS DETECTOR 10k 2.2k .. SENSITIVITY ADJUST VR1 20k 100k .,. 4.7M 4700 .,. OSCILLATOR 3.9k TURN ON DELAY +5V +12V 120k 39011 +12VC>-<) 100 1.. + .. + 16VWJ .. 120k CAPACITANCE MULTIPLIER 10 0.11 0.1l 16VWJ POWER LED2 GREEN .,. B 100 + 16VWI LL - ,. EOc VIEWED FROM BELDW -~- ~K GND EXHAUST MONITOR Fig.1: when gas is detected, the resistance of the TGS 812 sensor drops and pin 7 of IC1a goes high. This allows oscillator IC1d to run and thus pulse the alarm on and off via transistor Q2. IC1h & IC1c mute the alarm for 2 minutes when it is first turned on, to give the heater time to purge the sensor. ]UL Y 1989 15 Fig.2: here's how to install the parts on the PCB. The TGS 612 sensor can be installed either way around but note carefully the orientation of transistors Qt and Q2. The 7605 3-terminal regulator is mounted on the copper side of the board. 12V METALLISED POLYESTER CAPACITORS + 5.6 0 □ COPPER SIDE OF BOARD) Value 0.1µF IEC EIA 1 OOn 104Z RESISTORS □ □ □ □ □ □ □ □ □ □ □ □ No. 1 2 3 2 1 1 1 2 1 1 1 Value 4 .7MO 120k0 100k0 10kn 4.7k0 3 .9k0 2 .2k0 1 kO 4700 3900 1800 5.6n along the road they will quite naturally suck in their own exhaust fumes and, as our tests showed, you can't guard against this simply by opening the windows at the front. The only remedy in this case is to stop the vehicle and fix the leak. But what if your boot lid or rear door is sealing perfectly? Are you safe from exposure to the carbon monoxide in exhaust gases? No you are not. If you are driving in traffic or stopped at traffic lights with other vehicles, you are bound to be exposed. This applies particularly if you drive with a window open or with your air-conditioning set to "fresh air". In these circumstances you should set the air-conditioner to "re-circulate". So whether or not your vehicle has defective seals, you still need an exhaust gas monitor to tell you that you are exposed. Exhaust monitor With these problems in mind, we 16 SILICON CHIP 4-Band Code yellow violet green gold brown red yellow gold brown black yellow gold brown black orange gold yellow violet red gold orange white red gold red red red gold brown black red gold yellow violet brown gold orange white brown gold brown grey brown gold green blue silver gold 5-Band Code yellow violet black yellow brown brown red black orange brown brown black black orange 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 white black black brown brown grey black black brown not applicable set out to produce a simple, effective yet easy to build exhaust gas monitor. The end result is a unit housed in a small plastic case and using a solid state TGS 812 gas sensor made by Figaro of Japan. The TGS 812 gas sensor is a semiconductor device based on sintered tin oxide (SnO 2). When combustible or reducing gases are absorbed on the semiconductor surface, its resistance is markedly reduced. When the gas dissipates, the resistance returns to normal. Examples of combustible gases are hydrocarbon vapours (from petrol, methylated spirits etc), natural gas, methane, hydrogen and so on. And the prime example of a reducing gas is carbon monoxide. It is also combustible and burns with a clear blue flame. The TGS sensor is housed in a 17mm diameter case with 6 pins protruding through the base. At the top it has an opening 10mm in diameter which is covered by two layers of very fine mesh. There is also a small hole on the underside, again covered with mesh, so that gases can pass through the unit. Inside is a heater element and the semiconductor element. The heater consumes about 600 milliwatts and is there to purge the semiconductor element of gases which are absorbed onto its surface. The double layer of wire mesh is there to prevent the risk of explosion when the sensor is exposed to dangerous concentrations of gas. The circuit Now let us have a look at the circuit (Fig.1). Besides the TGS 812 gas sensor, it uses a quad op amp (ICl), a 3-terminal regulator, two small transistors, a piezoelectric siren and a few other parts. The 7805 3-terminal regulator provides a constant + 5V to the circuit from a nominal 12V supply such as a car battery. The + 5V feeds the heater circuit (pins 2 and 5) of the TGS 812 as well as the rest of the circuit, except for the piezo The three electrolytic capacitors are mounted horizontally on the PCB so that they clear the front panel. Note that the TGS 812 must be installed so that the top of the sensor is level with the piezo alarm. siren which is powered from the + 12V supply. Basically what happens is that the circuit monitors the resistance of the TGS 812 sensor, between pins 1,3 (joined together) and pins 4,6. When the resistance drops, the circuit sounds an alarm. Op amp ICla is the core of the circuit; it monitors the resistance of the TGS 812. The TGS 812 is connected in series with a l0k0 resistor to form a voltage divider across the + 5V supply. The voltage at the centre point of this divider is monitored via a 4.7k0 resistor at pin 5 of ICla. This voltage is compared with a reference voltage set by VR_l at pin 6. Normally, the resistance of the TGS sensor is high and so the voltage at pin 5 of ICla will be low. This means that the output of ICla will be low. When carbon monoxide or .other relevant gas comes in contact with the TGS sensor, its resistance goes low and so the voltage at pin 5 of ICla goes high. This causes the output of ICla to go high and this allows oscillator stage ICld to run. It oscillate~ at about lHz and turns PARTS LIST 1 1 1 4 8 2 2 4 2 plastic utility box with plastic lid, 130 x 45 x 68mm, Dick Smith Electronics Cat. H-2851 or equivalent front panel label (optional) PC board , code SC05107891, 106 x 57mm piezo alarm , 32mm diameter, with inbuilt 3 .6kHz oscillator (Dick Smith Electronics Cat. L-7022 or equivalent) cigarette lighter plug 3-metre length of light duty twin flex with coloured trace miniature SPST switch 1 0mm tapped spacers 3mm machine screws, 6mm long 3mm machine screws, 12mm long 3mm nuts 3mm washers PC pins Semi conductors 1 TGS 81 2 gas sensor (Figaro) Q2 on and off once every second to drive the piezo alarm. This alarm is loud enough to be effective but is 1 LM324 quad op amp (IC1) 1 7805 5V 3-terminal regulator 2 BC548 NPN transistors (01, 02) 15V 1 W zener diode (ZD1) 5mm red light emitting diode (LED 1) 5mm green light emitting diode (LED 2) Capacitors 1 1OOµF 16VW PC electrolytic 1 1 OOµF 3VW tantalum or 1 6VW low leakage electrolytic 1 1OµF 1 6VW PC electrolytic 3 0 .1µF monolithic ceramics Res istors (¼W, 5%) 1 4.7MO 1 2.2k0 2 120k0 2 1 kO 3 100k0 1 4700 2 10k0 1 3900 1 4.7k0 1 1800 1 3 .9k0 1 5 .60, ½W 1 20k0 trimpot (vertical adjust) not so loud as to be painful. Since the base current for Q2 flows through LED 1, there is also a JULY 1989 17 The PCB is secured to the case lid using machine screws, washers and tapped 12mm spacers. Note how the 3-terminal regulator is installed. visual indication of the alarm. IClb and IClc provide a 2-minute muting period for the alarm when it is first turned on. This is necessary because when first turned on the TGS sensor will be "poisoned" by gases which have been absorbed on its surface while it has been in the unpowered state. Its resistance is therefore initially quite low but within two minutes the heater will purge the device and its resistance will go high. IClb can be regarded as an RC timer with a very large capacitance connected to pin 2 (ignore IClc for the moment). When power is first applied, the large capacitance will have zero charge and so the voltage at pin 2 will be zero. This causes the voltage at pin 1 to be high and turn on Ql. This pulls the voltage at pin 5 low and stops IC1a from performing its normal monito'r ing function. The capacitance referred to above then charges via the associated lOkO resistor and the voltage at pin 2 rises. After about two minutes, the voltage at pin 2 will exceed that at pin 3 and the voltage at pin 1 will go low. This turns Ql off and then ICla can function as normal. Capacitance multiplier IClc provides the large capacitance referred to earlier. Obtaining a predictable time delay of more than a minute or so where the physical size of the capacitor is limited can be a problem due to the 18 SILICON CHIP By wiring the power leads to a cigarette lighter plug, you can easily move the unit from vehicle to vehicle. input bias current needed by the op amp and the leakage current in the capacitor. Solving this problem took some ingenuity: we had a spare op amp in the package and so we decided to use a capacitance multiplier. It works as follows: IClc is connected as a "voltage follower" by virtue of the 120k0 resistor between pins 13 and 14. This means that any voltage appearing at its pin 12 will be reproduced exactly at its output, pin 14. Now consider what happens when we first apply power to IClc. The lO0µF capacitor will initially have no voltage across it so the voltage at pin 12 will be zero. Hence, the voltage at pin 14 will also be zero. The lOOµF capacitor at pin 12 now starts to charge up via the associated 120k0 resistor. As the voltage at pin 12 starts to rise, the voltage at pin 14 increases by exactly the same amount. What this means is that the voltage drop across the 120k0 resistor is exactly the same as the voltage across the lkO output resistor and this, in turn, means that the current flowing in the lkO resistor will be 120 times larger than the current flowing via the 120k0 resistor into the lO0µF capacitor. Therefore, as far as the external circuit is concerned, the junction of the lkO and 120k0 resistors behaves like a capacitor which is 120 times as large as the lO0µF capacitor connected to pin 12. Ingenious, eh? This effective capacitance of 12,000µF combines with the 10k0 resistor at pin 2 of ICl b to provide an initial turn on delay of 2 minutes. Power for the circuit comes from the vehicle's 12V battery and feeds via a 5.60 resistor to the input of the 7805 3-terminal regulator. A 15V 1W zener diode provides protection against reverse polarity connection or spike voltages superimposed on the battery line. On the output side of the regulator, a green LED in series with a 3900 resistor gives an indication· that the unit is turned on. Construction The Exhaust Monitor is housed in a standard plastic utility box with a plastic lid. It measures 45 x 68 x 130mm (Dick Smith Cat. H-2851 or equivalent). All the circuit components are fitted onto a printed circuit board (PCB) measuring 57 x 106mm and coded SC05107891. This board is mounted onto the lid of the case so that the TGS gas sensor and piezo alarm protrude slightly from the front panel. The first step in construction is to carefully examine the PCB for any faults in etching; eg, shorts between tracks or pads and open circuits (breaks) in tracks. This done, insert two PC pins for the + 12V and OV supply connections. Now insert and solder the resistors and the three O. lµF monolithic capacitors. The latter will most likely be blue in colour and labelled '104' according to the EIA code. The three electrolytic capacitors are mounted so that they can be Problems? ... and you don't have our .120page catalogue . . . "' N <at> -$- ♦ 26.5 12.5 10 0 13 46.75 46.75 DIMENSIONS IN MILLIMETRES ~ HOLES : A : B: C: 0: 32.5 DIA. 17.5 DIA. 5 DIA. 3.5 DIA. Fig.3: dr~ng temp!ate for the case lid. Holes are required for the alarm, sensor, trimpot adJustment and board mounting screws. laid over on their sides. You can Checking, checking then install the LM324 IC, the 20k0 Now check your work thoroughtrimpot and the two transistors. ly. Are all components where they Make sure that these components should be and cross-checked to are correctly polarised before you agree with the circuit? They are'? solder them into place. Good. If not, fix 'em. The 7805 3-terminal regulator is Now connect up a length of mounted on the copper side of the polarised twin flex and connect a board so that air can circulate 12V supply. If the 5.60 resistor around it. Poke its leads through the starts smoking, you've connected board holes so that they protrude the supply around the wrong way. about 6mm on the component side. The alarm should beep briefly Solder the leads so that the 7805 is when power is applied and then vertical, as shown in the photo. nothing should happen for about Now mount the piezo alarm on two minutes. the board. It is secured with two Now rotate the trimpot clockwise 3mm x 12mm-long machine scre~s until the alarm turns on. It should and nuts and then its leads are beep on and off continuously at soldered to the board. Note that the about once per second. If it does leads are polarised: red for rotate the trimpot anticlockwise un~ positive, black for negative. · til the alarm turns off. Now use That done, the TGS 812 can be your multimeter to set the trimpot soldered into circuit. Its leads are for a reading of + 2V between pin 6 arranged so that it can go into cirof ICl and OV. This is most convecuit either way around but polarity niently measured at the trimpot is not important. Its leads should be wiper (ie, the centre terminal) and soldered so that the top of the senthe PC pin connection for OV. sor is level with the top of the piezo This setting is purely arbitrary alarm. The two LEDs should be mounted by the way and one which we found so that they stand just slightly gives reasonable sensitivity without higher than the top of the piezo · nuisance triggering of the alarm. If alarm. Watch the polarity of the you want the alarm to be more senLEDs - their longer lead is positive sitive, rotate the trimpot clockwise. You can test the Monitor by blow(ie, the anode). At last . . . a TRADE catalogue for the consumer ARISTA .. . your one-stop problem solver. Video plugs and sockets ... Video extension speakers .. . Video flyleads . .. Video RF interference filters .. . Video splitters .. . Indoor antennas . .. Video switching units . . . Down converters . . . Video speaker controllers . .. Video camera lights . .. Video tape rewinders ... Video cine adaptors .. . Video head cleaners . . . Video splicing kits . . . Video storage cases ... Video dust covers .. . Video leads ... Scart plug leads . .. Video dubbing kits ... Video headphones .. . Video shotgun and wireless microphone systems .. . Pre-amplifiers with video inputs .. . Video camera stands .. . Just about anything you want. ... Try us .. . NOW! Get your catalogue FREE from your local ARISTA dealer or send $2.50 P&H and your return address to: ARIST~ ELECTRONICS PTY LTD PO BOX 191, LIDCOMBE, NSW 2141 JULY 1989 19 from the PC pins and the switch wired up as shown in the wiring diagram (Fig.2). Put a knot in the twin flex as a strain relief where it enters through the hole in the end of the case. Finally, secure the board assembly to the lid with four 3mm machine screws. Connecting it up Fig.4: here is a full size artwork for the PC board. ing cigarette smoke into the sensor. It will immediately sound off, proving that you are poisoning yourself. Yes, cigarette smoke contains carbon monoxide, apart from other toxins. If you don't smoke, you can test the Monitor by letting it sniff the cap off a bottle of methylated spirits. This completes the board assembly and checking. It remains for it to be assembled into case. Case assembly Before the PCB can be mounted, the lid of the case must be drilled. Fig.3 is a drilling template, showing the holes for the alarm, sensor, trimpot adjustment and board mounting screws. We also drilled three 6mm diameter holes in each side of the case to allow ventilation of the sensor and the 7805 regulator. Additional holes must also be drilled for the SPST switch and for the supply leads (twin flex). Two holes should also be drilled in the base of the case to allow it to be mounted permanently, if desired. Now affix the adhesive front panel, if one has been supplied. We assume that kits of this project will come with the case ready punched and screen-printed, by the way. In the first instance, kits of this project will be available from Dick Smith Electronics. With all holes drilled in the lid of the case, the printed board can be mounted. Fit a tapped spacer at each corner of the board, with a washer under each spacer. The twin flex should then be removed You have two options for connecting the Exhaust Monitor in your vehicle. First, you can use a cigarette lighter plug (DSE Cat. P-1675 , Jaycar Cat. PP-2000 or Altronics Cat. P-0160). This allows you to move the unit from vehicle to vehicle. Second, you can wire the unit to the + 12V accessory supply at the vehicle's fuse panel. This way, the Monitor will be powered up only when the ignition is on. On other gases To conclude, we must make one final point. The Exhaust Monitor is sensitive to gases other than those occurring in exhaust. The one that comes particularly to mind is methane. That means that if you or one of your passengers suffers from flatulence the alarm will sound. Footnote: while the exhaust monitor will detect petrol fumes, it is not suitable for use on boats in its present form as salt water can damage the sensor. We hope to present a version for boats in a future issue. l§;l EXHAUST MONITOR ALARM ADJUST SENSITIVITY POWER Fig.4: use this artwork to make your own front panel label. 20 SILICON CHIP