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Help needed designing simple circuit


David Nebenzahl

Jan 1, 1970
(famous last words, "simple circuit" ...)

OK, so I'm trying to come up with a simple (maybe even elegant) solution
to a simple problem. Have an idea I want to run by y'all.

Function: person has a motion-detector light installed in their home.
They want a buzzer/bell/annunciator of some kind to go off *momentarily*
whenever the light is activated.

Here's my idea for the circuit:

First of all, please don't laugh at this. I am *not* a double-E or in
any way an electronics expert. Also keep in mind that this is the
farthest thing from a mil-spec application. It's just for fun; no life
support medical devices will depend on it.

I'd like to know the following:

1. Will this circuit even work?
1a. Will it work but end up destroying one or more components?
2. Is there a simpler way of accomplishing this task?
3. If it'll work, what are the right component values?


D1 is a half-wave rectifier. C2 filters the DC to produce more-or-less
ripple-free current. C1 provides the momentary "on" signal, by charging,
then "shutting down" when charged (sized according to RC time constant
to provide the desired "on" time). R2 and R3 form a voltage divider to
supply the appropriate base voltage to Q1. R1 acts as a voltage divider
in series with the load to supply the appropriate output voltage. (I
chose 24 volts DC arbitrarily; it might be less, probably not more.)

Component sizing:

R1 would obviously have to be large enough (in terms of power capacity)
to handle the load. The load would probably have a minimal current draw.
And since it would only be "on" momentarily, R1 could probably be a bit
undersized without worrying about damage.

Q1 would also need to be large enough to handle the load. I'm thinking a
common TO-220 type might work fine.

R2 & R3 could be small 1/8 watters.

OK, have at it. Rip 'er apart!

Made From Pears: Pretty good chance that the product is at least
mostly pears.
Made With Pears: Pretty good chance that pears will be detectable in
the product.
Contains Pears: One pear seed per multiple tons of product.

(with apologies to Dorothy L. Sayers)