The consumer unit (or ‘electricity meter’) or the fuse in some houses is a badly lit place. If a 12 volt transformer is also located in this place, with a circuit it may be used to provide emergency lighting by two high-current LEDs. These diodes are powered via a small circuit that switches over to four NiCd batteries when the main fails. The output voltage of the transformer is rectified by bridge rectifier D1 to D4 and filtered by capacitor C1.
Automatic Emergency LED Light Circuit Diagram
The batteries are charged continuously with a current of about 7.5 mA via diode D5 and resistor R2. The base of transistor T1 is high via R3, so that the transistor is cut off. When the mains voltage fails say by SW2, C1 is discharged via R1; when the potential across it has dropped to a given value, the battery voltage switches on Q1 via R3 and R1, provided switch S1 is closed. When Q1 is on, current of 20mA flows through diodes D7 and D8 LEDs. The light from these LEDs is sufficient to enable the defect fuse or the tripped circuit breaker to be located.
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