I have disassembled HP DeskJet 3000 j310a. I've removed paper feeder encoder board (shown on image). The problem is that when I connected it to 3V3 supply (it has builtin resistor on LED side), it always has 1.60-1.63V on output A or B no matter whether the gap is cutted or not. I'm sure sensor is not damaged because the printer was fully working (was a donor for a damaged j310a printer in better condition). What might it be?enter image description here

  • \$\begingroup\$ You need to provide a schematic and part numbers for the sensors on the board. > 90% of your photo is irrelevant. Can you crop it? Explain how the red lines help. \$\endgroup\$
    – Transistor
    Mar 8, 2020 at 21:16
  • \$\begingroup\$ I've cropped it... I added red lines to increase readability. There is no schematic. I just connected the voltage to the VCC and GND cable and took a voltage measurement between A / B and GND. \$\endgroup\$ Mar 8, 2020 at 23:24
  • \$\begingroup\$ Why do you think that's an encoder board? What's on the other side? From what I can see they're through-beam slot sensors of the type that prove that the printhead has reached the home position. If so there will be an LED on one side and a photo-transistor on the other. Add a photo of the other side and explain where it came from and what it does in the printer. \$\endgroup\$
    – Transistor
    Mar 8, 2020 at 23:36
  • \$\begingroup\$ As I said in the post, it's sensor from paper feeder not from carriage. The cables are soldered directly to the encoder. There was round plastic disc with lines between LED and photo-transistor. \$\endgroup\$ Mar 8, 2020 at 23:45
  • \$\begingroup\$ Post of reverse engineering is to draw a schematic of what you've got. \$\endgroup\$
    – Transistor
    Mar 9, 2020 at 0:22

1 Answer 1


I see that this post is very old, but I will still answer it.

There is a difference between a light gate (the one you have on the other side of the board) and an encoder (the one you are trying to test). A light gate simply tells you if there is an object blocking the light, but an encoder is used to track motion in both directions using an encoder strip or disk (clear plastic with black lines).

They are identical on one side as they both have an IR LED emitting light. The main difference is that an encoder has 2 outputs (A and B in your case) which alternate on/off when the black lines of an encoder passes through giving an offset square wave pattern that controllers use to track the motion of a motor or print head.

I advise you watch this video to see the output signal and understand how to use it: https://www.youtube.com/watch?v=0QLZCfqUeg4

I also worked with similar printer parts, and found that they easily burn out even at 3.3v power (these components usually run on around 1.5v) so be careful.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.