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From: Rocky Stevens on 10 Jun 2010 11:51
I've set up a 555 timer to put out a pulse of about 2 seconds, and
have confirmed it works with an led. However, when I replace the led
with a relay, the 555 outputs positive voltage indefinitely, keeping
the relay energized. I have confirmed with a multimeter that the
trigger pin is high, and that the output pin is high.

Does anyone know how this can happen? The only thing I can think of is
that the relay coil acts as an inductor, which somehow prevents the
555 from "shutting off"?

FWIW, when I tried putting an led in series with the relay, it lit up,
but after 2 seconds the relay starting vibrating, as if it were being
turned on and off repeatedly.

From: Rocky Stevens on 10 Jun 2010 13:09
Thanks for the input; I am indeed just driving it straight from the
555. I gotta say, I was going nuts last night trying to figure out
what was going on; I come from a strong software background, where
once you have an output from one system, it cannot be affected by the
system it is outputting to (if that makes any sense). Once I had
established that I had the 555 set up correctly (by lighting an LED),
it had not occurred to me that substituting the LED with another
"output" could affect the 555. I have since read up on inductive
kickback and realize what you mean regarding bringing the output lower
than ground.

Thanks again!
From: Tim Wescott on 10 Jun 2010 13:14
On 06/10/2010 10:09 AM, Rocky Stevens wrote:
> Thanks for the input; I am indeed just driving it straight from the
> 555. I gotta say, I was going nuts last night trying to figure out
> what was going on; I come from a strong software background, where
> once you have an output from one system, it cannot be affected by the
> system it is outputting to (if that makes any sense). Once I had
> established that I had the 555 set up correctly (by lighting an LED),
> it had not occurred to me that substituting the LED with another
> "output" could affect the 555. I have since read up on inductive
> kickback and realize what you mean regarding bringing the output lower
> than ground.
>
> Thanks again!

Question: What could be easier than properly driving a cruddy length of
wire wrapped around an iron stick?

Answer: Lots.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
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