Strange behavior driving a relay from a 555 time in monostab

[Rocky Stevens]

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.

 

[Tim Wescott]

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

 

[Tim Wescott]

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

 

[Rocky Stevens]

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!

 

[Jamie]

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.

 

[Michael A. Terrell]

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.

 

[Herman]

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:SMidnfhZQ5P04IzRnZ2dnUVZ_s-dnZ2d@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

 

[Michael A. Terrell]

Herman wrote:

"Michael A. Terrell" <mike.terrell@earthlink.net> wrote in message
news:SMidnfhZQ5P04IzRnZ2dnUVZ_s-dnZ2d@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!

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

Silicone is used for bathtub caulk and phony boobs.


Philips is a 'Johnny come lately' 555 OEM. I was using the 555 right
after it hit the market and i think I still have a copy of the '555
Cookbook' I picked up several years later.


--
Anyone wanting to run for any political office in the US should have to
have a DD214, and a honorable discharge.

 

[Rocky Stevens]

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).