# What is this encoder with micro DC motor in camera auto focus system?

I recently disassembled an old digital point and shoot camera and found the following micro DC motor (as can be seen in the image)

It is probably part of the auto focus system. Also, there is a slotted disk connected to the shaft of the motor and this slotted disk sits in a circular cavity with two sensors (looks like light interrupter sensor).

Now, there are five wires that are coming to these sensors one wire is common between the two (the central wire). I think this encoder works by giving pulses when the disk passes through it, but I am not sure if that really happens. I wish to use this circuit but I do not know what are the 5 wires to the encoder. Is anyone here familiar with this encoder and its working? Kindly help.
Can it be something like this?

## 1 Answer

Your diagnosis is correct. Those are optical slot sensors and use an infrared LED on one side of the slot and, probably, a photo-transistor on the other.

You should be able to identify the LED side using a multimeter diode test function. Expect about 1.4 V forward voltage drop. On power-up you might be able to check LED operation with your phone camera which will be sensitive to infrared.

From the comments:

... why are there 5 wires, and only one wire being shared between the sensors. Shouldn't the Vcc and GND both be shared?

simulate this circuit – Schematic created using CircuitLab

Figure 1. Likely schematic for the flexible PCB showing the off-board LED current-limiting resistors.

Don't forget that you can't parallel LEDs and expect good results. Each will need a series resistor and, since they don't appear on the PCB, they must be in the camera. See if that matches what you have.

• Thank you for your answer. I have one more doubt, why are there 5 wires, and only one wire being shared between the sensors. Shouldn't the Vcc and GND both be shared ? – vvy Nov 15 '17 at 7:18
• See the update. – Transistor Nov 15 '17 at 7:27