Printer component salvage

My colleague's HP Deskjet 3940 died, and he was going to trash it, so I rescued it and (eventually, after a little swearing) took it apart.

The part I'd like to use first seems to be some kind of light-break switch, which conveniently has a plug on the end for 3 pins.

Using Google, and the PCB's part number, I found this page, with an image:

Which doesn't help much.

The left side has E on the top, the right has S. I guess these mean Emitter and Sensor. The traces show the black wire runs to the sensor only, the yellow to the emitter only, and the green to both. Thus Green = Vin, Yellow =- Gnd, Black = Vout.

I tried linking them up using my Arduino as a host, with Vout feeding an LED, but didn't seem to get any change when I put some paper between E and S, and never got light from the LED.

Any hints? I took the machine apart a little too quick to see what was meant to pass between E and S!

Well, if you haven't burned it out...

Green is probably GND, not voltage.

Black should go to a digital input.

Yellow should go to a digital output (likely thru a resistor, try a 1K, then drop to a 470 ohm, then to a 220 ohm).

The LED is likely IR - so unless you have a cheapo IR sensitive CCD camera somewhere (cheap webcams work) won't see anything.

Turn the yellow lead on (HIGH) with your digital write; monitor the pin the black lead is connected to. If you aren't getting any reading, try dropping the resistance down on the emmiter; don't go below 100 ohms (without knowing the specs of the emitter IR led, it is difficult to know what resistor to use).

You may have already burned it out; you probably couldn't see much anyway with even a webcam pointed at it, as the black plastic piece keeps things kinda directional.

I actually picked up a similar sensor yesterday, except mine has the leads going directly into the plastic piece (no PCB); and they are different colors - should be fun to figure out...

Good luck! :slight_smile:

Just tried it like that, heard a little noise and unplugged it again... bit of a whiff, time to start with another sensor ;D

It annoys me when sensors like these don't have their own part numbers on the casing, would be so much easier if I had a datasheet!

edit: Last project I ran on it still works fine, so it's just the sensor which is gone...

Usually you can tell which wires to hook up by looking at the traces on the bottom of the board. Look at the top, it shows you the orientation of the LED. Follow which line also goes to the sensor from the LED and then you can see if they are feeding + voltage or ground through the sensor. You will also see which wire is coming out from the sensor to read from.

Yeah I recently salvaged two similar ones from an old printer (and one from a scanner) but haven't got round to playing with them yet.

Printers can be great for parts (and getting loads of ink on your hands). I also got two stepper motors and a parallel plug, USB plug, power board (with some nice heatsinks) etc etc off it.

The scanner I removed the other sensor from is also my new desk light (with my modified PC power supply providing 12v to its transformer board)

Yeah I take everything apart :wink: It's the best way. And I then recycle all of the metal and any other stuff. Better than the whole thing going in landfill. Lets show everyone that doing electronics can be good for the environment!