This is a simple automatic emergency light which remains OFF when the mains power is available and switches ON when there is a power failure. When mains power is available, it is stepped down by transformer T1, rectified by diodes D1&D2, filtered by the capacitor C2 and regulated by the 9V regulator IC 7809 to produce 9V DC. This 9V DC is used for charging the battery. While the battery is charging the diode D3 is forward biased and this makes the emitter potential of transistor Q1 lower than the base potential and this makes the transistor OFF. The lamp will not glow.
When AC mains is not available, the diode D3 will be reverse biased and this makes the emitter potential of transistor Q1 higher than base potential and Q1 becomes ON. The lamp glows and continues to glow as long as there is no mains supply.The capacitors C1 and C3 acts as decoupling capacitors.
Circuit diagram with Parts list.
- Assemble the circuit on general purpose PCB.
- The transformer T1 can be a 230V AC primary, 12-0-12V seconadary, 500mA transformer.
- Battery B1 can be a 6V lead acid battery or five 1.2V NiCd cells in series.
- The fuse F1 can be a 0.5A one.