#800 IR Photodiodes

Investigating and testing infrared (IR) transmission between IR LEDs and IR photodiodes.

Notes

Infrared transmitters (LEDs) and receivers (photodiodes) are widely available in 3mm and 5mm through-hole tuned for 850nm or 940nm for example from this aliexpress seller.

See also The Art of Electronics 12.6.1 Photodiodes and phototransistors (3rd Edition).

IR LEDs (Infrared Light-Emitting Diodes)

What they do:

• Emit light in the infrared spectrum (typically 850–950 nm).
• Similar to visible LEDs but optimized for IR output.
• Can be clear or tinted; clear lenses allow more efficient IR emission.

Key points:

• Forward-biased → emits IR photons
• Narrow spectral range, often centered near ~940 nm
• Very fast switching (tens of MHz for good emitters)
• Used as transmitters in sensing or communication systems

IR Photodiodes

What they do:

• Detect IR light and convert it into an electrical signal.
• Photodiodes operate in photovoltaic (solar-cell-like) or photoconductive (reverse-biased, fast) mode.
• Phototransistors are more sensitive but slower (because of internal gain).

Key points:

• Reverse-biased → produces current proportional to incident IR light
• Spectral response typically matches IR LED wavelengths (850–950 nm)
• Clear packages maximize sensitivity
• Phototransistor variants provide higher gain at the cost of speed

Summary

Typical applications

• remote controls
• IR data communication (IrDA, legacy systems)
• Proximity sensors
• Break-beam detectors
• IR interlocks
• Night-vision illumination (CCTV, security cameras)
• Optical encoders (quadrature disks, motor feedback)
• Line-following robots
• Gesture sensors
• Optical flame and smoke detectors
• Barcode scanners

Circuit Design

The test circuit is a simple setup between transmitter and receiver:

• a square wave generator drives an IR LED
  • I am using LEAP#791 555 Breadboard Pulse Generator for this (not shown in the schematic)
• the voltage across the IR Photodiode is traced with an oscilloscope

Designed with Fritzing: see ir-photodiodes.fzz.

Testing on a breadboard:

• the clear component is the IR LED transmitter. It is used like other LEDs.
• the black component is the IR photodiode receiver. It is used “reverse-biased”.

Testing with 5mm Components

The following scope traces show test results at varying frequencies:

• CH1 (Yellow) - input square wave
• CH2 (Blue) - IR photodiode voltage response

At 100Hz, we see a perfect reproduction at the receiver:

At 1kHz, we see some slew appearing at the receiver:

At 20kHz,the received signal is significantly distorted:

Testing with 3mm Components

The following scope traces show test results at varying frequencies:

• CH1 (Yellow) - input square wave
• CH2 (Blue) - IR photodiode voltage response

At 100Hz, we see a perfect reproduction at the receiver:

At 1kHz, we see some slew appearing at the receiver:

At 20kHz,the received signal is significantly distorted:

Credits and References

• 100Pcs 3mm 5mm IR LED Diode Transmitter Receiver 850nm 940nm Infrared Photodiode Phototransistor Emitter Lights Emitting Bulb
• The Art of Electronics 12.6.1 Photodiodes and phototransistors (3rd Edition).
• https://en.wikipedia.org/wiki/Photodiode
• https://www.build-electronic-circuits.com/photodiode/
• https://www.electronics-tutorials.ws/diode/photodiode.html