The consumer unit (or
‘electricity meter’) cupboard in some older houses is a badly lit place.
If the bell transformer is also located in this cupboard, 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 mains fails. The output voltage of the bell
transformer is rectified by bridge B1 and buffered by capacitor C1. The
batteries are charged continuously with a current of about 7.5 mA via
diode D1 and resistor R2. The base of transistor T1 is high via R3, so
that the transistor is cut off. When the mains voltage fails, C1 is
discharged via R1; when the potential across it has dropped to a given
value, the battery voltage switches on T1 via R3 and R1, provided switch
S1 is closed. When T1 is on, a current of some 20 mA flows through
diodes D4 and D5. The light from these LEDs is sufficient to enable the defect fuse or the tripped circuit breaker to be located.
Author: H. Bonekamp
Copyright: Elektor Electronics