Flame, gas and smoke detector for fire alarm. You can combine this circuit with alarm circuit. The output will be the relay which switch on and switch off the alarm.
Schematic diagram:
The list of components needed:
The above scheme diagram is the diagram of flame detector. The circuit diagram for gas detector and smoke detector separated in different circuit design diagram which can be downloaded from the provided link.
All of schematic diagrams (flame detector, gas detector and smoke detector) are quite simple and easy to built. It uses 12V power supply to work, you may use batteries or accumulator or DC power supply for powering the circuit.