IR Photogate

This is a simple photogate using a IR-led and a IR-photodiode. It can sense objects crossing the beam.

  • The photogate is adaptable by changing resistors, IR-led and IR-photodiode.
  • It is a small circuit and has few components.
  • IR uses a specific part of the spectrum, so it has limited interference by other light sources.

In this setup we only change the resistors.

Teacher Notes

Teachers! Did you use this instructable in your classroom?
Add a Teacher Note to share how you incorporated it into your lesson.

Step 1: Schematic

A photodiode can detect light. It can generate a tiny voltage or can act as an resistor. In this setup we use the photodiode as a resistor. When the photodiode is lit by the light-beam from the led, the photodiode allows the current to flow (cathode to anode). When current can flow, the photodiode has a low resistance.


The IR photodiode resistance is measured by a voltage/resistance divider.


To get result we only change the resistors. In this setup I got good results with:

  • The current to IR led is in series with a 680Ω resistor. Lowering the resistance of the led increases the brightness off the led. A resistor value below 270Ω can destroy the Arduino.

  • R1 = 1M: Try different values to get better results, see the formula in the schematic.

Step 2: Program

When the test program is uploaded to the Arduino, you need to open the Serial Monitor to see the results of the photogate. Disturbing the light-beam increases the resistance of the photodiode. A stronger light-beam lowers the resistance, thus a higher voltage at A0. Slide a piece of paper between the led and photodiode and see the new results at the bottom of the screen of the Serial Monitor.

The results are:

  • Left column the current voltage at A0
  • Middle column the average voltage
  • Right column the number of voltages below 2V

If the results are too uniform, experiment with different resistor values.

Be the First to Share


    • Instrument Contest

      Instrument Contest
    • Make it Glow Contest

      Make it Glow Contest
    • STEM Contest

      STEM Contest