Optical sensor

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A simple FFF opto-interrupter module.


Here's a picture of a commonly available optointerrupter, which is also a convenient size for most applications.


It has three wires:

  • Common/Ground
  • LED positive. The other side of this is connected to the ground. No resistor is included, so it should be provided on the controller board.
  • Photo Diode positive. The other side of this is connected to ground, so when the interrupter is open it will pull low. In order for this to work reliably, the controller board should provide a weak pull-up resistor to positive.

The arms extend out 10.5mm with a gap of 5.5mm. The arm width is 4.5mm.

The LED and photo-transistor are mounted inside the arms, facing towards each other through a narrow vertical slot. The slot width is approximately 1mm.

Although it looks fairly solid, it is actually just a thin plastic shell mounted over a small PCB. There are small plastic clips (just little square blocks really) inside the shell that allow the PCB to snap into place. The LED and photo-transistor are soldered into the PCB and extend up the arm approximately half way.

If we use common off-the-shelf parts, they may need to be bent to face each other, which would probably require fatter arms.