Simple Purpose Alarm
The alarm may be used for a variety of applications, such as frost monitor, room temperature monitor, and so on. In the quiescent state, the circuit draws a current of only a few microamperes, so that, in theory at least, a 9 V dry battery (PP3, 6AM6, MN1604, 6LR61) should last for up to ten years. Such a tiny current is not possible when ICs are used, and the circuit is therefore a discrete design. Every four seconds a measuring bridge, which actuates a Schmitt trigger, is switched on for 150 ms by a clock generator. In that period of 150 ms, the resistance of an NTC thermistor, R11, is compared with that of a fixed resistor. If the former is less than the latter, the alarm is set off. When the circuit is switched on, capacitor C1 is not charged and transistors T1–T3 are off. After switch-on, C1 is charged gradually via R1, R7, and R8, until the base voltage of T1 exceeds the threshold bias. Transistor T1 then comes on and causes T2 and T3 to conduct also. Thereupon, C1 is charged via...