A danger always exists when combustible gases such as propane or gasoline are confined in a small area. The toxic-gas alarm shown in Fig. 1-45 uses a tin-oxide-semiconductor sensor. A thin filament coil is heated from a 12-V battery via IC1 and IC2, which pulses the voltage to the sensor coil, thus saving a considerable amount of energy. Zener diode Dl provides a constant filament supply voltage to the sensor coil. The sensor's resistance lowers as the sensor is exposed to toxic gases, such as hydrogen, carbon
monoxide, and propane. As the sensor's resistance decreases, the gate voltage to the SCR increases. When the threshold voltage on the gate is reached, the SCR fires and an alarm buzzer is activated. Once triggered, the buzzer sounds and switch S1 must be used to reset the alarm. Because the sensor has a fair amount of thermal inertia, S1 should be in the OFF or OPEN position for about three or four minutes upon initial power-up, allowing the sensor to stabilize, thus preventing false alarms. The sensitivity control R7 is adjusted to the desired value prior to triggering the SCR.

The toxic-gas alarm is sensitive to less than 100 ppm of carbon monoxide. This simple gas alarm is useful for boats, basements, and cabins. It could save a life. Some of the companies listed in the Appendix offer plans and kits for various toxic-gas sensors.

Part List

NOs    Part      Description

1         C1        0.01-MFD, 25-V capacitor (disk)
1         D1        9-V zener diode (NTE139A)
1         D2        4-V zener diode (NTE5068A)
1         D3        SCR (NTE5408)
1         Q1       2N2222 transistor
1         U1       CD4013 CMOS flip-flop
1         U2       CD4049 CMOS hex inverter
1        BUZ      6-9V piezo buzzer
1        S1        Toxic-gas sensor TGS203(Figaro or equivalent)

   Schematic