Description
At the heart of this overhead detector (fire alarm) 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. This can be used as part of
fire smoke detectors but do not use it as a home fire alarm system.
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.
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.
Circuit Diagram
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.
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