At the heart of this circuit is a precision integrated temperature sensor type LM35 (IC1), which provides an accurately linear and directly proportional output in mV, over the zero to +155 degrees C tempera-ture range. The LM35 develops an output voltage of 10 mV/K change in measured temperature. Designed to draw a minimal current of its own, the LM35 has very low self heating in still air. Here the output of the LM35 is applied to the non-inverting input of a compara-tor wired around a CA3130 opamp (10).
A voltage divider network R3-P1 sets the threshold voltage, at the inverting input of the opamp. The threshold voltage deter-mines the adjustable temperature trip level at which the drcuit is activated. When the measured temperature exceeds the user-defined level, the comparator pulls its output High to approx. 2.2 V causing transistor T1 to be forward biased instantly. T2 is also switched on, supplying the oscillator circuit around IC3 with suf-ficient vohage to start working. The 555 set up in astable mode directly drives active piezoelectric buzzer Bil to raise a bud alert. Components R7, R8 and C4 deter-mine the on/off rhythm of the sounder. A transistor based relay driver may be driven off the emitter of Ti (TIM Similarly, replacing the piezo sounder with a suit-able relay allows switching of high-power flashers, sirens or horns working on the AC mains supply.