Just trying to switch a relay when a comparator goes high with a transistor but the comparator outputs 1.3v when 'low' which is enough to set the transistor.

Using a 741 opamp as the comparator with a zener diode as one input and a pot providing the source for the other input. Positive rail is approximately 13v while the negative is 0v. Obviously the positive side of the relay is connected to power, just isn't shown in the schematic below.

Is there a good way an easy way to fix this or should I be looking to use another chip with lower output?

Thanks! enter image description here

  • \$\begingroup\$ LM741 makes a crappy comparator. LM393 is dirt cheap as well and works much better in this situation. It has open collector output (very easy to attach PNP power transistor). \$\endgroup\$
    – venny
    Commented Oct 18, 2015 at 13:33
  • \$\begingroup\$ Thanks, I'll be sure to look into it next time I need one \$\endgroup\$
    – Eddy
    Commented Oct 19, 2015 at 9:42

1 Answer 1


You could just add a resistor divider:


simulate this circuit – Schematic created using CircuitLab

Also, have you tried it or is this just your calculation? Because, if you try it, it might still work, as the transistor might help pull down the output stage below the limit it has while sinking current.

  • \$\begingroup\$ I did try that actually. Brought the voltage down to about 50mV without the transistor connected, but once it was reconnected it increased to 0.55v. Which didn't make all that much sense to me. \$\endgroup\$
    – Eddy
    Commented Oct 18, 2015 at 11:21
  • 1
    \$\begingroup\$ Then you may have just used a too large division factor? The values I suggested should work. Also, 0.55 V after the resistor might not be incorrect - because, that may be the region where current starts flowing into its base. Have you tried switching the relay that way? If it still does not work, decrease the values of R1 and R2 with the same factor, i. e. set R1 to 1.2 kOhm and R2 to 400 Ohm. \$\endgroup\$
    – J A
    Commented Oct 18, 2015 at 11:23
  • \$\begingroup\$ Yep, ended up being my division factor must've been too large. Tried 10k and 3k and now it works as expected. Thanks for that! \$\endgroup\$
    – Eddy
    Commented Oct 18, 2015 at 11:38

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