"Casey Wilson" wrote in message
...

"S Narayan" wrote in message
...
Got the Radio Shack count up/down timer which can be set easily. It is
big
though (2" or so on a side). I connected an external AAA battery and put
in
a red LED across the speaker. I can now see when the timer expires with
the
flashing LED since I doubt I could hear the speaker go off.


Can you provide a schematic? Please.


I don't have the timer with me now, but IIRC, I experimented by putting
several types of LEDs across the speaker inside, and it worked. In fact, the
speaker works while the LED flashes. I didn't care if the speaker died, so I
used the largest LED I had. I believe, there is some some kind of a
charge-pump type circuit which steps up the voltage to tens of volts to
drive the piezo type speaker. The somewhat high voltage won't hurt the LED,
since the speaker driver cannot provide much current. The whole timer used
to run off a 1.5V button cell. The standby current drain is around 5uA and
putting an external AAA battery will keep it going as long as the battery
manages to stay alive which may be a several years. The self discharge may
be higher than what the timer needs!