Simple timer circuit to open relay after pressing button

Thread Starter


Joined Sep 26, 2012

Everyone was so helpful for my last post that I thought I'd try it again. So here's the objective: I need to build a small timer circuit that closes a relay (so the relay will conduct current) for an adjustable amount of time (0 to 200 seconds) after either of two buttons is pressed momentarily. I've attached a schematic of my first attempt at this.
  • Either button can be pressed (S2 or S3), which charges the RC circuit (C2 and pot R38)
  • The RC circuit is set to have a time constant of 200 seconds (when pot R38 is set at max resistance)
  • This RC circuit is connected to the IN- pin of an LM311 comparator
  • The IN+ pin of the comparitor is set as 3.7V (approx. 1 time constant below 12V)
  • When the RC circuit is discharged, IN- is below IN+ so, according to the LM311 specs, the comparitor output should be high-impedance, so no current passes through relay RY1 and it remains open
  • After S2 or S3 is pressed, the IN- pin voltage remains higher than IN+ for 1 time constant (up to 200 seconds) so the comparitor output should act as a sink (up to 20mA), current passes through relay RY1 (it has a coil current of 11mA), and it remains closed
  • Once the RC circuit has discharged sufficiently, IN- drops below IN+ so the relay opens again

Does this make sense? I've been looking at timer circuits online and they all seem way more complex than this, and I'm not sure why that's necessary. What am I missing?



Joined Mar 14, 2008
Looks basically okay except you don't show the LM311 emitter output pin on the schematic which should go to ground.
You could reduce the value of R31 some to increase the delay beyond one time-constant and reduce the required RC value for a given time.
You should add a small resistor (say 100Ω) in series with the switches to reduce the high surge current into the capacitor, minimizing pitting or welding of the switch contacts.

Please learn to draw your schematics in a more conventional manner with the main circuit components horizontal and signal flow from left to right with power from top to bottom.
Makes them a lot easier to understand.