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Topic: 3pinned phototransistor from printer encoder (Read 665 times) previous topic - next topic


Jun 01, 2016, 12:58 pm Last Edit: Jun 01, 2016, 01:08 pm by Hendselp Reason: Added pictures

I recently took apart a hp printer an found a few intresting parts. One of which was a dc motor with a encoder disk and pcb attached to it. I took the pcb apart from the dc motor and and took a look at the sensor part. I found a purple flat led and somekind of light transistor (i guess).
I've done a bit of researcs and found only 2 legged phototransistors, but not a single 3 legged one.

Sorry for bad english by the way


found only 2 legged phototransistors
Since a transistor uses a gate to control the flow of voltage, how can a transistor possibly have only two legs?


As far as i know the 3rd pin in 2 legged light transistor is inside the transistor and light just does somekind of voltage diffirence between base and collector to connect base and emitter legs to each other


Jun 01, 2016, 04:30 pm Last Edit: Jun 01, 2016, 04:33 pm by jremington
The one on the left is probably an LED. The one in dark plastic is probably a phototransistor-- the plastic allows IR light through but not visible light, which reduces interference.

Some phototransistors have only two pins, as there is not always a need to make a base connection.

If you look at the resistors on the PCB you can tell the difference. The LED will be connected to a low value resistor, while the phototransistor will be connected to a high value resistor.


Jun 01, 2016, 05:02 pm Last Edit: Jun 01, 2016, 05:04 pm by Cactusface
    Photo-transistors don't use the base connection as the current flow is from collector to emitter, ONLY with light hitting the junction. (there is only a very small change in current between light and dark) But I too have see and used 3 pined devices, just ignore the base. Do the usual transistor test with a DMM to find which pin is which!!

Agree with jremington, ref dark plastic body, filters out daylight, letting IR thro!

Hope it helps, regards

Open your mind! But not too far, your brains might fall out.
Also like Photography, model building and my 300+ Cacti and Succs.


Thank you guys for helping me out with this!!!

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