I have a circuit with a TCRT5000 that lets me detect proximity since the reflected IR rays will make the photo sensitive transistor conductive and will pull down the voltage below the 10kΩ resistor, which is pin 1 of the TCRT5000.

enter image description here

This works fine.

Now I would like to read that voltage using an ESP07 which can, according to the datasheet only detect voltages from 0-1V on its ADC pin.

To split down the voltage I would naturally use a voltage divider with 220kΩ and 100kΩ so my new schematic would look like so:

enter image description here.

In a way, the 10kOhm resistor "sort-of-shorts" the path from the ADC to 3V3 which obviously defeats the purpose of the whole operation.

Can I just remove R3 and let the voltage on the ADC "start" at 1V and let it be pulled down further as IR is reflected by an approaching object?

Like so:

enter image description here


1 Answer 1


Your second schematic would work, but the transistor in the sensor has a saturation voltage of 0.4 V at full reflection. So this way the usable ADC range would be 0.4 to 1 V.

If you put the divider in the emitter path you can use the full ADC range.

A small capacitor will suppress RF noise from wiring.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Very nice. I did not think about the saturation voltage. Since that might have worked I'm of course very glad to use the full range of the ADC. Can you elaborate why 20k+10k vs. 200k+100k? \$\endgroup\$
    – Besi
    Feb 12 at 18:46
  • 1
    \$\begingroup\$ Your original test setup uses 10 kohm as load for the transistor and you confirmed working. Using much larger resistors makes the sensor more sensitive, which may be good but it introduces a change. I just didn't want that you measure the dark current. \$\endgroup\$
    – Jens
    Feb 12 at 18:51

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