EMERGENCY LIGHT

Here is a circuit of an emergency light. The circuit assumes that AC main powers is stepped down and converted to DC. To simulate a blackout, simply use the BreakMainsPower switch. As long as the power supply is present, transistor Q1 conducts. Since the base of the transistor Q2 is connected to the collector of Q1, transistor Q2 and Q3 do not conduct and hence the lamp remains off. LED glows as long as the supply is present. When the power supply fails, the base drive to Q1 disappears. Thus Q1 stops conducting and its collector voltage jumps to battery voltage and starts conducting, switching on the lamp instantly. The load current is supplied by the battery. Whenever the power supply is restored, Q1 starts conducting turning Q2 & Q3 off and the lamp is switched off. Transistor Q2 conducts and provides sufficient base drive to transistor Q3.