Optical switch hack question - Page 1
Forum section: electronics / beginners

author: Mr. Fix
author_rank: Newbie
timestamp: May 08, 2024, 03:05:22 pm
content: I have a laser printer that says the output tray is full when it isn't. It has a optical cut off switch. Can someone tell me what are the typical wires that if I short them, I can trick it to sense there is no paper in the tray?TIA.The switch I have is like this:

author: tooki
author_rank: Super Contributor
timestamp: May 08, 2024, 08:41:58 pm
content: FYI, those are called photointerrupters and are simply an optocoupler with a slot. Google a bit and you can learn about them. We can’t give concrete instructions based on a generic photo alone; we don’t know the pinout of the model in your printer.

author: darkspr1te
author_rank: Frequent Contributor
timestamp: May 09, 2024, 11:45:30 am
content: These optical switches are very very common in laser printers,photo copiers , desktop printers etc and 'generally' dont fail but i have noticed that Sharp photocopiers for example will always breakdown on that part, replacing with complete module fixes the issue for at least a year then it fails again. I ended up swapping out the Sharp unit for one i stol from a HP printer and it's been fine since, so a failure of the led/pick up , i also considered incorrect resistor on the original part owing to it's early failure.You can find new ones at digikey,mouser etcas for pinouts I have found the are mostly the same but the are some "side cases" where the pinout is reversed, reduced or custom (two pin types)darkspr1te

author: Whales
author_rank: Super Contributor
timestamp: May 09, 2024, 10:06:06 pm
content: The photointerrupter should have 4 pins. 2 on one side for the LED (light source) and 2 on the other side for the phototransistor (sensor).If the phototransistor is stuck "open" (ie doesn't pass current when light is shined on it OR the light is dead) then shorting its two pins will simulate it "closing". If you accidentally short the two LED pins instead thenprobablynothing bad will happen because typically the LED is supplied through a resistor which will safely limit current, but I can't be 100% sure without looking at your circuitry.You can also point some (not all) digital or phone cameras at the IR LED observe whether or not it's working. It will appear as purplish-looking light sources (try with something else first like a TV remote to make sure your camera lets you see it).

author: themadhippy
author_rank: Super Contributor
timestamp: May 09, 2024, 10:35:18 pm
content: only issue is often the led is pulsed to avoid false triggering.Having the sensor seeing a permanent on or off signal may cause it to assume somethings wrong

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 10, 2024, 11:37:21 am
content: The sensor in the picture has 5 pin. The 2 pin is the power for the light source. The 3 pin is the receiver. 2 pins are for power and the one pin is output. If you measure the power supplied to those pin you should be able to figure out the pin out. The light source is likely modulated and the receiver is designed to response to only the modulated light but the out put won't be pulsed.

author: SeanB
author_rank: Super Contributor
timestamp: May 10, 2024, 12:52:56 pm
content: Most common failure on those is the LED going low output, and a common quick fix was to increase LED current on it, by lowering the value of the resistor on the main board that supplies it. Typically with the Sharp ones you find the resistor, and place a 470R SMD part in parallel with it, which increases current to the LED so it works again. Common on HP printers, and others that use the Canon engines, where they are run at low current.