Introduction: 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.

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.