looking for simple timer


Joined Jun 4, 2014
Doesent that circuit depend on charging a capacitor which can become leaky and make the whole thing inaccurate?
The capacitor is subject to reversing polarity and so would generally be a non-electrolytic type and so pretty low leakage types. The RC oscillator will be subject to the temperature coefficient of the timing components so select low temperature coefficient components here.

If you want accurate, but known in advance, timing then use a crystal oscillator.


Joined Mar 10, 2018
Here is a UP based timer, takes one line of code PWM1_Start( ); typed into
a file at a specific place. Outputs a logic signal.

Accurate +/- 2% over V and T.,
If you want better accuracy +/- .015%, add two caps and a crystal.


Board to use (not shown actual size) -


Only uses small portion of overall chip resources.

Solution is one chip, and 1 bypass cap if you do it yourself. With board just external switch needed.

Note, because logic output you cannot turn on anything that requires power, like a light bulb
or motor or coffee maker.....You have to use a SSR or MOSFET to do that.

It would be easy to add another switch to set timeout period, and LCD to show
status, but that requires some programming.

Regards, Dana.
Last edited:


Joined Sep 24, 2015
Here's a digital timer very similar to the ones I have. Mine are programmable where you can have them on for one minute all the way up to On Monday Morning @ 7:00 AM and Off Friday @ 9:00 PM (or any other time period you may desire).

I've also scrapped a half dozen microwave ovens. Using the control board and finding the relay that controls the magnetron (magnetron not used in the project) I can run an electrical outlet controlled by that relay ANY time period from 1 second all the way up to 99 minutes and 99 seconds (100 minutes 39 seconds) (AKA 1 hour, 40 minutes, 39 seconds). Using other sections of the control board I can run a fan at half speed (low) or full speed, same with the light control circuit. Some MO control boards have several speed levels for the fan and up to three levels for the light. The purpose for mine is to control my soldering iron. I have a habit of turning it on and forgetting about it for hours. With the timer I can run it for 10 minutes (or any other time period up to the max) then have it shut down. If I forget about it I'm not burning it up hours on end.

You can also use the countdown timer and the sonic alert to remind you of the elapse of a set time period (again, up to 99 min; 99 sec.) The light circuit and the fan circuit are not timer controlled.

Oh, and there's also a useful clock on the display.
Last edited: