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Above is a comparator circuit which employs RC time constant to control the ON time of the output(please left-click to enlarge the image). Red plot belongs to the momentary-switch voltage versus time, green plot is the output voltage; and the blue is the current through the LED.

The load connected to the output will be a solid-state-relay such as: https://www.sparkfun.com/products/13015

I'm planning to drive a DC motor with that relay as a next step.

My questions are:

1-) When ON time I want the LED to glow, so I had to use around 270 ohm in the simulation(R6) to obtain a moderate brightness around 11mA. Bu then I had to choose R5 as 470 ohm where I normally choose it 10k in non-loading applications. Is the value of R5 pull up OK?

2-) Regarding C1: Should I use a tantalum capacitor instead of an electrolytic one for stability and long term use?

3-) R5 drives current all the time, so one has to switch off the power. Is there a workaround to prevent that? And LED current never goes to zero. Any ideas about these?

Note: I don't want to use an additional opAmp.


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1 Answer 1


The output is a transistor (NPN) to ground i.e. open collector so if you calculate 270 ohms for the LED, connect the LED directly to the output of the comparator and use a 270 ohm pull up. Having the 470 ohm in series with the 270 ohm will make the LED much dimmer.

If you want to be more economical with power, wire the LED in series with the 270 ohm from output to power rail then, when the comparator output is "open-circuit", there will be no current taken by the output stage. You might have to swap inputs around to give you the correct functionality now.

I'd use a non-polarized capacitor such as ceramic but a tant would be fine.

For more accurate and repeatable timings the 6.2 Mohm discharge resistor is possibly a little too large given that the input bias current could be as high as 0.5 uA (over temperature). I tend not to use resistors greater than 1 Mohm if I can so maybe next time consider a better comparator.

  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$
    – Dave Tweed
    Jun 13, 2016 at 15:09
  • \$\begingroup\$ @user16307 did you get this working in the end? Our comment train has been deleted! Last think I mentioned was using the SSR connected between output and positive supply. \$\endgroup\$
    – Andy aka
    Jun 15, 2016 at 15:05

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