In hot summer it is very much significant to supervise your home from fire. Here is a simple circuit of a fire alarm using the light bulb starter as the heat sensor.

This circuit is powered by a 3V battery.


  • Starter
  • 220ohm resistor
  • UM66
  • 8ohm speaker
  • BC548
  • 3v battery
  • Connecting wire
  • PCB (Printed Circuit Board)
  • Multimeter

How to make it:

Firstly check availability of all the components as per the requirements. Then connect all the components as shown in the circuit diagram.

IC UM66 is connected to supply and its output is given to a transistor for amplification.

UM66 is a melody integrated circuit. It has an inbuilt tone and beat generator. It has a 62 notes ROM to play music. When the power is turned on, the melody generator is reset and melody begins from the first note. The speaker is driven by BC548, which is an NPN transistor connected to the output of the UM66.

For limiting the base current we have used 220ohm resistor, so that the transistor doesn’t get hampered.

We are using light bulb starter as the heat sensor. This has a metal plate and a pin with small gaps. When starter gets heated the metal plate starts expanding and gets in contact with the pin, which completes the circuit and a note is generated in the speaker. For fast sensing we can remove the glass body of the starter.

Source: Circuiteasy


About Pavan Shetty

Masters in Embedded systems, Intern Embedded Engineer.

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