Visible light Diode

Discussion in 'General Electronics Chat' started by uriahsky, Mar 27, 2013.

  1. uriahsky

    Thread Starter Active Member

    Apr 27, 2009
    I am repairing a coin sensor on a game called Treasure falls. The coin slides down a slot and covers over some type of LED. Usually on games like this there is an IR LED emitter and detector but this one only has a detector of some kind and it is sensing the presence and absence of visible light. I have a known good working unit and I sketched out the schematics on the pcb attached to this device. It is an Op Amp with a transistor and eight resistors and a LED that shows when the sensor is covered or open to light. It compares a set voltage to the voltage in the circuit with the sensor LED. The sensor that the coin covers is a diode of some type. It looks just like a standard 5mm clear LED. I checked on a meter and I get a diode voltage drop of 1.5-2.5V depending on exposure to light. When I measure the resistance with a meter I get 75 ohms when it is exposed to light and up to 300 ohms when covered. What I have learned so far is that it is a light sensing photo detector but I can't seem to find out how to track one down with the resistance range I need. That doesn't show on the data sheets I have seen. Usually data sheets cover almost everything you need but perhaps I don't understand how this resistance is calculated. Is this resistance supposed to be on data sheets or is there another way to figure out the resistance from the data sheet? The manufacture won't provide the info on this unit and won't sell us the part. Thanks, Russ
  2. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Those older sensors that look like clear LEDs are usually photodiodes or phototransistors (with 2 legs). A common value was a MEL12.

    You can make do with many different types of sensors although you might have to adjust the resitances in the circuit to get the output voltages right.