Help with Fire alarm circuit

Thread Starter


Joined May 24, 2011
Hi guys, i have tried to develope a Fire alarm circuit, the thermostor senses the heat around it and the resistance increases and the NPN transistor gets biased and sets of the alarm. But, since i am new to this i would like to have ur help. I would like to know the type and the value of the thermoster and the value of the variac. Once i test it out and if it works i will try adding a SPDT relay between the transistor and the buzzer. I would also like to know any suggetions to improve the circuit. Thx in advance.:D



Joined Jun 15, 2011
I don't know if that design will work as you want. Turn-on will be slow and not well-defined.

You don't have a variac, it is a variable resistor (or could be a potentiometer).