Circuit knowledge

A gas sensor circuit discussed in the previous article can be also implemented for testing whether a vehicle driver is drunk to dangerous levels or not. Here we learn the construction of this circuit and some other important hints regarding the circuit. A comprehensive information regarding the assembly of the proposed alcohol tester circuit can be seen in this article. The sensing tube is made from a piece of polythene tubing, about 25 cm long with an inside diameter of about 15 mm, into which the gas sensor fits tightly (take the sensor with you when you go to buy the tubing). The tubing should be securely glued to the inside and outside of the case wall with two-component epoxy resin.

Fit the gas sensor in the foremost part of the tube: if the diameter of the tube is slightly too large, wind some tin or aluminium foil around the sensor until the whole fits tightly. Under no circumstances should you use any glue near the sensor as this could give rise to spurious indications for a long time to come. The sensor should therefore be secured with a suitable aluminium or plastic hood as shown in the figure. The openings behind the sensor in the tube ensure that the sensor is well immersed in exhaust gas so that it works fairly rapidly. Furthermore, these openings prevent a build-up of gas behind the sensor (which is pervious to gases) that could give rise to spurious readings. The LEDs should be mounted in their sockets in the top moulding of the case. Spray a liberal quantity of lacquer over their terminals, or, better still, brush some two- component epoxy resin over them, which will make the whole sturdier in use.

Calibration: Operate the tester in the open air and, using P1, set the voltage on pin 8 of IC2 to a level between 1 and 3 V. If that is not possible, reduce the value of P1. Next let the gas sensor heat up for at least a couple of hours. The setting of the thresholds of the window discriminator to correspond to 1 per cent and 4.5 per cent CO content requires the use of a reference tester. Many garages, MOT testing stations, as well as the test centres of the AA and RAC have such a unit, and you should be able (during a quiet period) to get the use of it for a little while. During the measurement hold the preheated gas sensor in the gas stream about 10 cm from the exhaust outlet. After a short while, the sensor has stabilized and can then be compared with the reference. Adjust the choke (engine running!) so that the reference meter indicates a CO content of 4.5 per cent.

On the CO tester set P2 such that the red LED just does not light: if you really love the environment, set P2 so that the red LED just lights. Follow the same procedure with a CO content of 1 per cent (hopefully this is possible), but in this case adjust P3 until the yellow LED just lights. As already stated before, the accuracy of measurement is set between narrow limits; none the less, the CO tester makes it possible to check the cleanliness of your engine regularly and conveniently. It is possible to power the tester from the cigarette lighter via a suitable two-core cable (about 3 m long): a plug for this purpose is obtainable from most good car accessory shops.

the proposed alcohol gas sensor proper is based on a mixture of chemical and physical properties. Basically, when heated, it behaves like a resistance of which the value diminishes when the CO content increases. Perhaps that is putting it a little too simple, because the sensor reacts not only to carbon monoxide, but also to a number of other gases and gaseous substances in the atmosphere, from alcohol, which is blue but indicated by the red LED, through propane and butane to the carbon hydroxides (found, for instance, in stink bombs). This is illustrated in figure below. None the less, the primary harmful constituent of the exhaust gases is carbon monoxide which is detected with great accuracy. Flashing of the LEDs indicates that the tester is not yet ready for use, because the gas sensor has not heated up. Only when the LEDs stop flashing, that is, after about 3. . .5 minutes, can the measurements be commenced.

 Construction hints for the alcohol tester circuit

First, the voltage regulator, IC1. This integrated device provides a stabilized supply of 5 V at a maximum current of 1 A from the pulsating battery voltage. Capacitors C1 and C2 provide further smoothing of the supply voltage. The heating element of the gas sensor is connected in parallel with the regulator. Both IC1 and the sensor get noticeably warm during use and IC1 is therefore fitted onto a heat sink which improves heat conduction in the closed case. The regulator also provides thresholds for comparator lC2 which are preset by P2 and P3.