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I have salvaged an optical sensor board from a HP printer just as the one shown c9101-80190-A to use it other hobby projects.

The problem is, I cannot be sure whether it's working or not, as I get always the same voltages (with and without séparation between the diode and the receiver) as shown in the following schematic.

schematic

simulate this circuit – Schematic created using CircuitLab

enter image description here

How can I find out if a photogate is working or not?

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  • \$\begingroup\$ I'm sorry, I can't really tell what I'm looking at, since this has to be a candidate for the grainiest photo ever. Maybe you could use the website-integrated schematic editor (schematic button in the question editor)? \$\endgroup\$ Commented Jul 24, 2017 at 20:35
  • \$\begingroup\$ How do you know 'D2' is a photo-diode? Did you find a datasheet for the sensor? If it is a photo-diode you have it upside down. It will conduct all the time. \$\endgroup\$ Commented Jul 24, 2017 at 20:50
  • \$\begingroup\$ To be honest, I don't know.I just used the presentation I found somewhere. I am sure there are four nodes (5V, 4.24V, 4.24V, 0V). And these are always having these values with and without obstruction \$\endgroup\$ Commented Jul 24, 2017 at 20:53

1 Answer 1

Reset to default
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. A much more likely arrangement.

Usually these are wired as shown in Figure 1.

  • The LED is permanently on.
  • The photo-transistor is wired in "common-emitter" mode.
  • The collector has a pull-up resistor. This may be on the board you linked or it may be on another board connected to NODE2.
  • With no obstruction the LED turns on Q1 and the voltage at NODE2 is pulled down close to 0 V.
  • With an obstruction Q1 turns off and R2 pulls the voltage up to V+.
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  • \$\begingroup\$ so I think the LED is dead \$\endgroup\$ Commented Jul 24, 2017 at 20:50
  • \$\begingroup\$ According to your diagrams you have wired it incorrectly. The LED will be infra-red. Wire it up as I have shown and see if your smartphone's camera can see it. (The sensors pick up IR.) \$\endgroup\$ Commented Jul 24, 2017 at 20:52
  • \$\begingroup\$ There isn't much to wire, as the circuit is already soldered. I just connected the ground and red to 5V. I use the green to measure the voltage \$\endgroup\$ Commented Jul 24, 2017 at 20:54
  • \$\begingroup\$ Post a decent close-up photo of the front and back of the PCB. Crop them properly. \$\endgroup\$ Commented Jul 24, 2017 at 20:56
  • \$\begingroup\$ I just posted a front picture. I can't get a descent one from the back. If necessary, I can try with another smartphone tomorrow \$\endgroup\$ Commented Jul 24, 2017 at 21:07

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