I've made this comparator circuit, it has a changing current going through the photodiode and when lighting changes sufficiently it should trigger a 5V signal at the opamp output terminal.

When the photodiode is in low light conditions the non-inverting opamp input terminal has a voltage of 2mV. When I illuminate the photodiode I get a voltage of 20mV. I've adjusted R2 so that the inverting input terminal of the opamp is set to 10mV.

When I illuminate or darken the photo diode nothing changes to the output voltage of this opamp. Also nothing changes when I disconnect or switch the input terminals. It remains high (4.7V) at all times.

(I've measured all voltages directly to the opamp pins)

Is there a design error? Or is the voltage difference too small?


simulate this circuit – Schematic created using CircuitLab


1 Answer 1


There's an error in observing the valid electrical parameter for Vin CM range (common-mode) on the datasheet.

The input transitors need this to bias the NPN differential input amplifier.

You need a ground sensing comparator like the quad-LM339 (PNP inputs) or a negative Vee supply of at least 2V on the 741.

  • \$\begingroup\$ Indeed. Another option if one for some reason has to use the obsolete 741 and only has +5V and GND available is to change the operating condition of the photodiode so the opamp inputs are within its acceptable common-mode range. This can be done with only passives. \$\endgroup\$ Nov 9, 2019 at 8:35
  • \$\begingroup\$ @pericynthion that passive bias solution to Vcc/2 would then demand passive tolerance errors much smaller than 22 Ohms / 22 kOhms or << 0.1% making it much less practical than an LM339 or require another pot making it too sensitive. \$\endgroup\$ Nov 9, 2019 at 18:54
  • \$\begingroup\$ Doesn't need to be exactly vcc/2, you just make sure the reference is also based on the bias. \$\endgroup\$ Nov 9, 2019 at 19:37

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