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I have this schematic:
enter image description here Source: https://drive.google.com/file/d/1CbRIJQbl0sYk1aCY0UPvfPe7hDIebd2M/view?usp=sharing

I use it for a DIY colorimeter using to tricolor RGB LED with a common cathode. I'm using a BPW34 photodiode as sensor. The LEDs R, G and B are turned on individually every 45 ms each, where the LED is turned on for 45 ms then a reading from A1 happens. I have a NEMA 17 connected to the stepper motor driver as shown in the schematics. The stepper motor is used to inject liquid into the cuvette. I have an external 12 V power supply that goes into "VMOT" on the stepper motor driver.

The digital potentiometer goes up to 100 kΩ.

The only issue is that the dark current is about 20-30 when reading on pin A1. Decreasing/increasing the resistance on the digital potentiometer does not alter the reading on pin A1. The LEDs are of course turned off. There is no light at all reaching the photodiode. Is it something about the circuit?

The circuit is working fine - other than the above mentioned problem. The signal I'm reading from pin A1 is with 0.2% variation, which seems satisfactory? Do you have any suggestions for the schematic? Do I need other capacitance values? Is there something missing like resistors, capacitors etc.?

Velvel
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Michael Shaya
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    do not draw lines through components ... turn Q7 rights side up ... the -5 V should be at bottom ... place R1 to left of the vertical line, so that there is no crossing of lines ... place all outputs on one side – jsotola Aug 18 '23 at 21:18
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    _"The circuit is working fine"_ - how can that be when the LM358 op amp output is shorted to GND? – Bruce Abbott Aug 19 '23 at 00:44
  • Jsotola: Noted! – Michael Shaya Aug 19 '23 at 11:01
  • Bruce Abbott: My mistake. The circuit on the breadboard is without the connection to GND. – Michael Shaya Aug 19 '23 at 11:03
  • Inputs to the left, outputs to the right. Positive voltages point up, ground and negative point down. Don't cross over components. Ask someone else who have not seen it before to follow and highlight a signal from input to output. It should be very straight forward for him/her. – winny Aug 21 '23 at 07:37

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