IR Photodiode
An IR photodiode generates current in response to near-infrared photons (typically 750–1100 nm). Combined with bandpass filters and AC-coupled amplifiers, it can pick out modulated IR sources (like a remote control at 38 kHz) from the background daylight noise.
Silicon's bandgap (1.12 eV) allows photons up to ~1100 nm to generate electron-hole pairs. An optical filter blocks visible wavelengths above 650 nm to reduce background interference. In a proximity sensor, the same modulated IR LED illuminates the target and the reflected IR is detected by the photodiode, with synchronous detection rejecting ambient IR.
In plain terms
An eye selected to detect infrared, sometimes with an optical filter like tinted glasses. It is not automatically blind to visible room light; the device's spectral response and filtering determine what reaches the detector.
Why designers use it
- Decode 38 kHz IR remote control signals with a PIN photodiode + TSOP receiver.
- Detect the presence of a hand in a touchless tap or soap dispenser.
- Track a line on a robot by measuring IR reflection from white paper versus black tape.
- Detect the edge of a paper sheet in a printer's paper-path sensor.
Best for
- IR remote decoding
- Proximity detection
- Line following
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- In direct sunlight without careful optical filtering and modulated emission — solar IR overwhelms the signal.
- When you need long range (> 5 m) — an active LIDAR or ultrasonic sensor handles distance better.
Common mistakes
- Using an IR photodiode without a bandpass filter in outdoor applications, saturating the transimpedance amplifier with solar IR.
- Driving the paired IR LED with DC instead of a modulated signal, making the system blind to slow ambient IR changes.
Where you will find it
- A TV remote control system uses a TSOP4838 integrated IR photodiode + demodulator to decode NEC protocol pulses: the IC filters 38 kHz IR and outputs a clean 3.3 V logic signal to the TV's MCU, while completely ignoring the fluorescent and LED room lighting whose IR content is unmodulated.
- A robot vacuum uses a pair of IR emitter/photodiode modules pointing downward at 45°: when the robot reaches a stair edge, the reflected IR signal disappears because the floor is too far away, and the MCU interprets the drop in photocurrent as a cliff and reverses.
- An automatic hand sanitiser dispenser uses an IR proximity sensor to detect a hand within 5 cm: the modulated 880 nm LED's reflection off the hand is detected by the photodiode, and the MCU fires a solenoid valve for 0.5 s, delivering a measured dose without any surface contact.
A short history
An infrared photodiode is a semiconductor p-n or PIN junction that converts infrared light into an electrical signal. When a photon with sufficient energy strikes the junction, it creates an electron-hole pair; carriers swept by the depletion region's built-in field produce a photocurrent roughly proportional to the incident light, typically operated in reverse bias and used in applications such as infrared remote-control receivers.