While smoke alarms are quite cheap devices, the cost of 9V batteries
quickly exceeds their purchase price. Added to that is the irritation of
random beeps from the alarm as the battery reaches the end of its
useful life. This circuit allows typical smoke alarms to be powered from
the 12V supply in a burglar alarm while still keeping the standard 9V
batteries in place. It extends the 9V battery life to that of its "shelf
life" as the battery is only required to drive the smoke alarm in the
event the 12V supply is removed or shorted out.
Circuit diagram:
In
normal operation, the LM317 supplies 9.7V and this is fed via diode D2,
resulting in just over 9V at the smoke alarm supply terminals. Q1 is
not biased on, so the 9V battery is disconnected from the circuit. If
the 12V supply is removed, the output of the LM317 will be 0V and Q1
will be biased on via the 4.7kO resistor and thus the smoke alarm will
continue to be powered. The circuit could be assembled on a piece of
Veroboard and fitted inside the smoke alarm. Alternatively, you could
house the circuit and 9V battery within a standard electrical
flush-mount box which the smoke alarm covers when mounted.
Author: Paul Blackler
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