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I have a weak signal (from a photo-diode) which I need to amplify to drive an LED. I try to accomplish this with an OpAmp TL081 and the circuit given in the image.

What the circuit does: When there is no signal (even if the photo-diode is not even connected), there is a voltage at the LED (about 2.63 V). When the photo-diode is replaced by 0 Ohm resistor, the voltage at the LED increases marginally.

What the circuit should do: When there is no signal, there should be no (or little) voltage at the LED, when there is a signal from the photo-diode, there should be about 3.36 V at the LED.

As possible error sources I identified:

  • mistake in the circuit design
  • mistake when soldering everything together
  • corrupt element (e.g. burned IC)

Question: Is the circuit design ok? Addon: If yes: did I miss any other possible error sources?

circuit

I think the used schematic is like this (if I got the wiring to the OP Amp right, which is not given in this schematic):

schematic

Edit: Orientation of photo-diode was wrong in the diagram, as pointed out by the answer of Andy aka (Thanks!). (At least it was right in the build circuit and therefore not the reason of the problems, but of course this needed to be fixed.)

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  • \$\begingroup\$ Your (nicely drawn) schematic would be so much clearer if you would use Opamp symbols for the opamps instead of the pinout of the chip. For Example: i.stack.imgur.com/O9YXs.png \$\endgroup\$ – Bimpelrekkie Jun 8 '15 at 8:24
  • \$\begingroup\$ If you change the hand drawn circuit diagram following comments made in my answer (i.e. turning the photodiode round) please acknowledge this in your question for reasons of (1) common courtesy and (2) not doing so makes my answer look incorrect from the casual reader's standpoint. \$\endgroup\$ – Andy aka Jun 8 '15 at 8:44
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    \$\begingroup\$ @Andyaka: Sorry, after changing the circuit diagram you were faster with your comment than I was with writing my comment next to your answer. There is (or at least was) another SE site were "edit" notes are/were discouraged. This is my first experience with electronics.se, please excuse but point out any blunders of mine. \$\endgroup\$ – Burgmeister Jun 8 '15 at 12:14
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If you still want to use the TL081, you'll need to use 2 power supplies. You can use a +/-5 combination, but assuming you want the LED to shine with a brightness proportional to the input I suggest +/-12 to +/-15 volts. Assuming +/-12 volts, your circuit should look like

schematic

simulate this circuit – Schematic created using CircuitLab

Note the addition of a couple of capacitors to the op amp power inputs. These will act to prevent the op amp from oscillating due to wiring effects. They may not be necessary, but they should always be used, just in case.

The R3/D4 is provided to limit the bias voltage on the photodiode, and may not be necessary, but you will need to check the data sheet on your photodiode. Look for reverse voltage. Some photodiodes will work with a full 15 volts on them, but some will not.

I have not shown your zero pot (R3 in your schematic), but it should go to pins 1 and 5, with the wiper to the -12.

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Thoughts: -

The TL081 output cannot reach the most negative power supply. It might get to about 2 volts above the negative power supply but this may still be enough voltage to keep a RED LED illuminating.

Another problem is that connecting IN+ to -Ve supply is going to cause problems. Like the output, the input cannot be connected to the negative rail because the input common mode range is about 2 volts inside the power rails (3 volts worst case).

The above are likely to cause your design not to work, but, your circuit is a bit crappy and it's hard to read so maybe there are some other issues.

I will also point out that the direction of the photodiode you have drawn will be forward biased from the positive rail and this will cause problems too. (this has been corrected by the OP).

Power supply shown is 5V in the redrawn circuit but a TL081 requires a minimum supply of about 10V according to the data sheet graphs. This is quite a fundamental flaw and I would recommend using a different op-amp with rail-to-rail capabilities.

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  • \$\begingroup\$ Thanks for pointing out that the direction of the photo diode was wrong in the schematic, I fixed that. Where should I connect IN+ and output to instead of -Ve? I only have +Ve and -Ve, and at least for the output +Ve would not close a circuit, would it? \$\endgroup\$ – Burgmeister Jun 8 '15 at 8:42
  • \$\begingroup\$ You can bias it half way between the rails with a potential divider. \$\endgroup\$ – Andy aka Jun 8 '15 at 8:46
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    \$\begingroup\$ @Burgmeister - your circuit won't run from a supply as low as 5V either. The TL081 needs about 10V to guarantee basic functionality. \$\endgroup\$ – Andy aka Jun 8 '15 at 9:06
  • \$\begingroup\$ The necessary 10V are quite interesting, as the TL081 was sold explicitly as needing not more than 5V ("Every OpAmp can be run with 5V, take this one!"). Its accompanying "manual" (two pages) does not say anything about the required voltage. ti.com/product/tl081 gives "7" as "Total Supply Voltage (Min)" and 36 as Max, therefore changing to 12V is probably a good idea, but "using a different op-amp with rail-to-rail capabilities" would be an alternative. Could you suggest one? (Obliviously I should not ask for guidance in the shop where I got the TL081.) \$\endgroup\$ – Burgmeister Jun 8 '15 at 12:25

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