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From: Tim Wescott on 10 Jun 2010 12:23
On 06/10/2010 08:51 AM, Rocky Stevens wrote:
> 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.

Are you driving the relay straight from the 555, or through a transistor
driver? Do you have a catch diode on the relay? Driving a relay
straight from the 555 without a catch diode would bring the 555 output
pin higher than VCC (or lower than ground, depending on how you have the
relay connected), which would make it misbehave. Your "vibrates after
two seconds" symptom sounds a lot like the kind of misbehavior that you
may see, with the flyback from the relay partially resetting the timer,
turning on the relay for a short time, then repeat.

Even if the relay's current draw is within the capability of the 555,
the flyback from it is still an issue that you'll have to deal with --
I'd just use a driver transistor with it, no matter what.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: Jamie on 10 Jun 2010 19:00
Rocky Stevens wrote:

> 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.
>
put a silicone diode across the relay coil to absorb the (-) energy that
is released from the coil when is de energized. This energy could
some how be causing the unit to behave unexpectedly. It also can
damage the 555.




From: Michael A. Terrell on 10 Jun 2010 20:03

Jamie wrote:
>
> Rocky Stevens wrote:
>
> > 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.
> >
> put a silicone diode across the relay coil to absorb the (-) energy that
> is released from the coil when is de energized. This energy could
> some how be causing the unit to behave unexpectedly. It also can
> damage the 555.


Silicone? What a boob!


--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.
From: Herman on 11 Jun 2010 07:54

"Michael A. Terrell" <mike.terrell(a)earthlink.net> wrote in message
news:SMidnfhZQ5P04IzRnZ2dnUVZ_s-dnZ2d(a)earthlink.com...
>
> Jamie wrote:
>>
>> Rocky Stevens wrote:
>>
>> > 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.
>> >
>> put a silicone diode across the relay coil to absorb the (-) energy that
>> is released from the coil when is de energized. This energy could
>> some how be causing the unit to behave unexpectedly. It also can
>> damage the 555.
>
>
> Silicone? What a boob!
>
>
> --
> Anyone wanting to run for any political office in the US should have to
> have a DD214, and a honorable discharge.

See page 7 http://www.doctronics.co.uk/pdf_files/555an.pdf


From: Rocky Stevens on 11 Jun 2010 10:52
I tried putting a rectifier diode across the relay, but still no luck.
I'll try with a different 555 tonight (I may have already damaged the
other 555).
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