Here is a simple circuit that can be used as a fire alarm. 3 volts is sufficient to operate this project. There is not much on the circuit. The UM66 IC is connected to its source and its output is fed to a transistor for amplification.
UM66 is a complete miniature tone generator with a 64-note ROM, an oscillator and a preamplifier. For the amplification we have used an NPN transistor which is BC548. Here BC548 makes a common emitter circuit. To limit the base current we have used a resistance of 220 ohms so that the transistor will not be damaged.
For the heat sensor we have used the light tube starter instead of the manual switch. In a starter there is a metal plate and a pin with a small hole. When the starter heats up then the starter metal plate expands and comes into contact with the pin and the circuit is completed and we get the audio from the speaker. For quick detection, we can use the starter without its glass body, carefully breaking the glass cover.