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 temperature 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 comparator wired around a CA3130 opamp (IC2). A
voltage divider network R3-P1 sets the threshold voltage, at the
inverting input of the opamp. The threshold voltage determines the
adjustable temperature trip level at which the circuit 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 sufficient voltage to start working.
The 555 set up in astable mode directly drives active piezoelectric
buzzer Bz1 to raise a loud alert. Components R7, R8 and C4 determine the
on/off rhythm of the sounder. A transistor based relay driver may be
driven off the emitter of T1 (TP1). Similarly, replacing the piezo
sounder with a suitable relay allows switching of high-power flashers,
sirens or horns working on the AC mains supply.
Author: T. K. Hareendran – Copyright: Elektor Electronics 2007