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Strange behavior driving a relay from a 555 time in monostable mode

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.

Reply to
Rocky Stevens
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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
Reply to
Tim Wescott

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!

Reply to
Rocky Stevens

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
Reply to
Tim Wescott

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.

Reply to
Jamie

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.
Reply to
Michael A. Terrell

See page 7

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Reply to
Herman

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).

Reply to
Rocky Stevens

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