"David Nebenzahl" wrote in message
s.com...
On 3/28/2009 12:26 PM David spake thus:

"David Nebenzahl" wrote in
message
s.com...

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:
http://www.geocities.com/bonezphoto/...e-shotBell.gif

Why not take a crack at calculating the values of the
components and post your final circuit here. That way we
will get a good idea of how adept you are at circuit
design. I would still recommend putting R3 across C2
rather than C1 as per my original suggestion. That
insures the drive to the transistor truly goes to zero
after some amount of time.

Hey, thanks for sticking with me. Check latest circuit
incarnation at
http://www.geocities.com/bonezphoto/...e-shotBell.gif.

Component Value
-----------------------------
C1 100 uf/200 V.
C2 1 uf/100 V.
R1, R2 500 Ω, 2 W
R3 220 kΩ, 1/8 W
R4, R5 10 kΩ, 1/8 W
D1 1N4001
Q1 D1266 or equiv.

Rationale for values (assuming ~20 volt, 100 mA load):

C1: large enough to filter bulk of ripple
C2: sized for proper "on" time (WAG)
R1, R2: voltage divider to yield ~20 volts for load
device;
calculations dictate 5 watt load, but since it's
of short
duration, 2 w. should be sufficient
R3: bleeder (slow) for C1
R4, R5: this is also a WAG (wild-ass guess); see below
D1: 1A, 600 PIV
Q1: selected because I have these and have used them
before.

OK, I admit that I do not know how to calculate those last
2 resistances for proper biasing of Q1; so sue me. I do
understanfd the general principle of biasing; just never
had the formal training to learn how to calculate values
to implement it. I'd appreciate your comments here, and
I'd be curious to know how far off my guesses were.

BTW, I misunderstood where you wanted the bleeder
resistor.

So give me a grade on my work.

So far the grade is not very good. The transistor is rated
at 60v. When the transistor is off it has to sustain the
full rectified voltage.of about 150v. The same is true of
C2. R3 is still in the wrong place. R5 should be much
smaller than R4 since Vbe will always be under a volt. R1,
R2 are close, but how sure are you that 20v will not
destroy the load? A better choice would be a smaller R1 and
larger R2 since the circuit as shown introduces a lot of C
to B feedback which will drastically slow down the turnoff
time and keep the transistor in a linear mode for longer
than necessary. Need load on time, and hfe of the transistor
you choose to calculate C2, R4, and R5.

Other prople replying have suggested either a transformer
isolated design or the use of a small relay and a simple
diode RC network to drive it. The relay is a good choice if
you can get the power to run the load elsewhere.

David