"Joy" wrote in message
news
On Tue, 25 Mar 2008 13:40:49 -0400, James Sweet
wrote:

"Joy" wrote in message
news
I am trying to make a beeper to plug into a sonic alert unit for a deaf
person.

I have a 12V DC transformer directly wired to a 3-20V DC beeping siren.
When the transformer is plugged in, the siren goes BEEP...BEEP...BEEP as
it should, but when the power is cut, the beeping fades out instead of
stopping immediately. BEEp...Beep...beee...

Is there a simple way to get the siren to silence as soon as the power
is
cut?

Thanks so much!

Joy


Use a transformer with a smaller filter capacitor, or connect a 12V
incandescent light bulb in parallel with the beeper to drain the charge
when
power is removed. Another option is to switch the 12V with a relay rather
than switching power to the primary of the transformer.


Thanks for the idea! I can try the 12V bulb since that should be easy to
find (she says) and easy to do. And inexpensive. The transformer was just
lying around so I'd rather not have to use (buy) a different one.

I will try this as soon as I can. It sounds like a resistor would also
work. If so, would there be an easy way for me to calculate the
resistance?



Yeah a resistor would be fine too, you can calculate it using ohms law V=I*R
where V is the voltage, I is the current in Amps, and R is the resistance in
ohms. The I that you need depends on the size of the filter capacitor which
depends generally on the capacity of the transformer, some experimentation
may be needed.