hi and welcome to the another project video.today i am going to show you a simple fire alarm circuit .this circuit is based on transistor switch like like my other projects.
it is made with only five components that and these are easily available.
here you can see complite circuit diagram that led is optional
Now connect battery with the circuit
you can see when i light a matchstick it emits infra red rays that is detected by ir photo diode and that put alarm on.infra red ray also presents in sunlight.almost all hot object emits ir rays.