On Tue, 25 Mar 2008 14:54:35 -0400, bz wrote:

bz wrote in
98.139:

Joy wrote in newsp.t8k1rkvqug52hc@user-
47b243c432.myhome.westell.com:

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

one last try. Sorry. I missed seeing that you were using a 'DC
transformer'.

They have a capacitor built in that is holding a charge.

All you need to do is disconnect the siren from the power supply.
It should shut off immediately.

You should do this with a 110 vac relay.

Hook the coil of the relay in parallel with your power supply.
When the power comes on, you want the relay to close and hook the siren
to the power supply.
When the AC goes off, you want the relay to open and disconnect the siren
from the power supply.

The power supply will only use power when the beeper is activated. And
you won't be wasting power with a resistor and possibly damaging your
power
supply.


Would a 110 vac relay be activated by a 12-15V DC transformer?
And this would mean both the relay and the siren would be powered by the
same power supply (the dc transformer) except that the relay would also
switch the siren. Is this correct?

I WAS trying to be really simple which is why the light bulb or resistor
idea sounds good.

How much damage could I do by using a resistor box and starting with a
high value and slowly working down until it worked acceptably? I guess I
could damage something in the process, but how likely do you think it
would be?

Joy