A minimum amount of hardware is needed to build a simple flame or fire failure monitor that enables you to monitor the status of the flame in a chamber/furnace. Owing to its integrated alarm generator, the interface can be easily adapted for use with a wide range of light/heat sensors. Here a readily available photo transistor 2N5777 (T1) works as the flame sensor. The 5V regulated dc supply for this circuit can be derived from a 12 V mains adaptor or a 9V rechargeable battery.
In the presence of the flame, T1 conducts (resistance low) and most of the base current of relay driver transistor T2 find an alternative easy path via T1. As a result T2 remains cutoff. If flame is absent, T1 behaves almost an open circuit, and the current through P1 and R1 flows into the transistor’s (T2) base.
Next, the 5V reed relay is energised and 5V positive supply is extended to the alarm generator circuit built around the siren generator UM3561(IC1). Output signals from IC1 is amplified by transistor T2 to drive a standard 64 Ohm 1W mylar speaker.
For optimum perfomance, enclose phototransistor (T1) in a suitable long tube and add an optical assembly (see fig.2). The adjustment of sensitivity set preset pot P1 is also very critical.