Basically, I need to monitor a circuit's output so I can activate another system. The output voltage of active stage is 140mV.

If I am not wrong (from what I read about transistors) 140mV is not enough to put a BJT transistor into saturation (ON mode).

The end goal is turning on an LED and/or a relay, so I really need transistor in the saturation region.

I don't want to use a micro-controller's ADC to measure the voltage for such simple device (cost wise)

How can solve the problem in a simplest manner possible?

  • 1
    \$\begingroup\$ Is the output AC or DC? \$\endgroup\$ Sep 14, 2014 at 19:11
  • 1
    \$\begingroup\$ it is DC 140mV output \$\endgroup\$
    – dvdmn
    Sep 14, 2014 at 19:19

1 Answer 1


Use a comparator.

Just be sure to get one with an input common-mode range that extends down to the negative rail. TL331, for example, might work for you.

  • \$\begingroup\$ Thanks, it is good to learn something new. just to be clear, lets say I power the comparator with 5v and connect Input- to ground and Input+ to other circuit. Will the output be 5V when Input+ goes to 140mV? \$\endgroup\$
    – dvdmn
    Sep 14, 2014 at 19:37
  • 2
    \$\begingroup\$ The output is open-collector. So you have to provide a pull-up resistor. The benefit is the output high level is whatever you connect the pull-up to, and it doesn't have to be the same as the supply voltage. \$\endgroup\$
    – The Photon
    Sep 14, 2014 at 19:48
  • \$\begingroup\$ Note you should set a threshold for Input- that's slightly above ground (say 100mv), or it'll be 'on' all the time. Also you need to check that your comparator can actually handle inputs that close to the rails. \$\endgroup\$
    – pjc50
    Sep 14, 2014 at 20:14
  • \$\begingroup\$ I found TL331's PSpice library, I will try to run some simulations (Planning to learn spice anyway). Thanks for helping \$\endgroup\$
    – dvdmn
    Sep 14, 2014 at 20:17

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.