Thursday, October 2, 2014
Gas Sensor Circuit
The proposed gas tester circuit can be simply tested by breathing into the sensor mouth which will confirm its working, since our breath contains CO2 will be immediately detected by this gas sensor circuit.
Its resistance (which in clean air is of the order of 60 k) diminishes as soon as it detects impurities in the air. The CO concentrations normally found in exhaust gases will reduce the resistance to under 3 k. This value will vary somewhat from one sensor to another and, additionally, it is affected by air temperature, humidity, and level of heating voltage.
You will by now understand where the technical limits on measurements with semiconductor gas sensors lie. Before reading further, it may be wise to reread the description of the TCA965 window discriminator. The input voltage to lC2 (pin 8) which is set by P1 rises with increasing air pollution. The thresholds of the discriminator, corresponding to 1 per cent and 4.5 per cent CO contents respectively, are set with P2 and P3. With clean air, the input on pin 8 lies below the lower threshold (pin 7): LED D1 (green) lights because pin 2 of IC2 is logic ’0’in the range of 1.. .4.5 per cent CO, the yellow LED (D2) lights, because the input to pin 8 now lies within the window. When the CO content exceeds 4.5 per cent, red LED D3 lights, because the voltage on pin 8 now lies above the upper threshold (pin 6). So far, so good. All that remains is the setting of the thresholds, but this will be detailed under ’Assembly and calibration.
Preheating indicator
The gas sensor must be preheated every time before it can be used. To save you having to keep looking at the clock, a small auxiliary circuit serves as flashing indicator. After the tester has been switched on, IC3, in conjunction with N3 and N4, generates a low logic level for about three minutes. Gate N2 inverts this signal so that the rectanguIar pulse generator based on Nl drives transistor T1 in rhythm with the pulse rate. After the pre—heating period has lapsed, the output of N3 goes high and the rectangular-pulse generator is inhibited. As the base of Tl is then permanently at high logic level, the LEDs light continuously instead of flash. The pre-heating period is determined by R7 and C4, while the flashing frequency depends on the values of R6 and C3.
Its resistance (which in clean air is of the order of 60 k) diminishes as soon as it detects impurities in the air. The CO concentrations normally found in exhaust gases will reduce the resistance to under 3 k. This value will vary somewhat from one sensor to another and, additionally, it is affected by air temperature, humidity, and level of heating voltage.
You will by now understand where the technical limits on measurements with semiconductor gas sensors lie. Before reading further, it may be wise to reread the description of the TCA965 window discriminator. The input voltage to lC2 (pin 8) which is set by P1 rises with increasing air pollution. The thresholds of the discriminator, corresponding to 1 per cent and 4.5 per cent CO contents respectively, are set with P2 and P3. With clean air, the input on pin 8 lies below the lower threshold (pin 7): LED D1 (green) lights because pin 2 of IC2 is logic ’0’in the range of 1.. .4.5 per cent CO, the yellow LED (D2) lights, because the input to pin 8 now lies within the window. When the CO content exceeds 4.5 per cent, red LED D3 lights, because the voltage on pin 8 now lies above the upper threshold (pin 6). So far, so good. All that remains is the setting of the thresholds, but this will be detailed under ’Assembly and calibration.
Preheating indicator
The gas sensor must be preheated every time before it can be used. To save you having to keep looking at the clock, a small auxiliary circuit serves as flashing indicator. After the tester has been switched on, IC3, in conjunction with N3 and N4, generates a low logic level for about three minutes. Gate N2 inverts this signal so that the rectanguIar pulse generator based on Nl drives transistor T1 in rhythm with the pulse rate. After the pre—heating period has lapsed, the output of N3 goes high and the rectangular-pulse generator is inhibited. As the base of Tl is then permanently at high logic level, the LEDs light continuously instead of flash. The pre-heating period is determined by R7 and C4, while the flashing frequency depends on the values of R6 and C3.
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