Thanks for the simple solution Russell! I am a newbie compared with
you engineers though so what do you mean by a reversed biased diode -
that the diode is connected in reverse? Forward biased? Thanks again
for the simple solution! By the way - I need both a permanent power
supply for this smoke alarm and a battery supply in case the power
goes out because a fire elsewhere in the house melted the insulation
and caused the circuit breakers to trip, for example. Thanks!

"Russell Powell" wrote in message .net...
If you've a permanent supply available, why run off of the
batteries in the first place? Place a diode between battery
output and permanent supply. Have permanent supply 9.7V
such that the diode is reverse biased while permanent power is applied.
When permanent power goes away, diode becomes forward biased.
Circuit operates off of battery supply.


--
sincerely,

Russell Powell

Artisan Components
SR. FAE - U.S. Central

469-438-6589