one shot timer MC14541BCP retriggers

Hi sed!

When the timer switch for our hallway failed I built a new one. It's
in service for maybe a year now but it has one bug: When the timer
switches off the lights it retriggers sometimes (maybe 40% chance)
and runs for another period. The light just is off only for about
0.1 s so the timer does turn off the relay.

I added a capacitor paralell to the pull down. Don't remember how
bad it was before. What should I do? A bigger cap? I don't know its
current value.

Here's the circuit diagram:
http://bksys.at/bernhard/img/x29/dscn6016-2.jpg

A photo of the timer:
http://bksys.at/bernhard/img/x29/dscn6019.html

The MC14541BCP data sheet:
http://bksys.at/bernhard/img/x29/mc14541b.pdf

Thank you, Bernhard

 

For starters, you should have, say, a 10uF electrolytic by-passed with
a 10nF ceramic mounted close to pin 14 and going to gnd. These values
are not critical so use common sense.

 

Tie pin 5 to gnd

 

That seems to solve the problem. I put the caps in parallel. Is the
ceramic cap to reduce oscillations that would occur with the
electrolytic when the turning off of the relay causes a voltage step?

I left it at a 100k pulldown to be able to turn AR on/off. If the
problem shows up again, I'll tie it to gnd directly.

Thanks, Bernhard

 

The electrolytic provides a low impedance voltage supply/reservoir
directly to the IC and the ceramic by-passes high frequency artefacts
present on the power supply to ground so that they do not affect IC
operation. Noise can be generated by almost any source of switching
or rapid voltage transition either common to the circuit around the IC
or even external to the circuit on the ac mains supply.