If you need to detect an invisible beam, measure proximity, or decode a remote control signal, the infrared photodiode is your baseline sensor. Unlike photoresistors (which are slow and memory-prone) or phototransistors (which have high gain but poor linearity), a photodiode offers nanosecond response times and strict linear current-to-light proportionality. The direct answer for a safe, reliable default part is the Vishay BPW34 or the Osram SFH203. Both cost under $1.00, survive standard breadboard abuse, and cover the 800nm to 950nm spectrum where most commercial IR LEDs and remotes operate.
Component Selection, Symbol, and Pinout
An infrared photodiode is fundamentally a PN junction optimized to absorb photons in the 700nm to 1100nm wavelength range. When an IR photon strikes the depletion region, it knocks an electron loose, generating a tiny current (typically microamps to milliamps) proportional to the light intensity.
Safe Default Part Numbers
- Vishay BPW34: The undisputed workhorse. Large active area (7.5 mm²), peak sensitivity at 900nm, maximum reverse voltage (V_R) of 60V, and low junction capacitance (75pF at 0V). Excellent for general-purpose IR detection and remote control decoding.
- Osram SFH203: Smaller active area but significantly faster rise/fall times (5ns). Choose this when you are building high-speed data links or LiDAR pulse detectors where the BPW34's capacitance would bottleneck your bandwidth.
- Excelitas C30642G: An avalanche photodiode (APG) for ultra-low light. Requires high reverse bias (30V-50V) and temperature compensation. Only use this if standard PIN diodes cannot detect your signal.
Schematic Symbol and Physical Pinout
On a schematic, the infrared photodiode symbol is a standard diode (a triangle pointing to a vertical line) with two inward-pointing arrows striking the junction, indicating light absorption rather than emission.
Physically, most through-hole IR photodiodes come in a black epoxy or clear water-clear package with two leads. The Cathode (K) is the shorter leg and corresponds to the flat edge on the epoxy casing. The Anode (A) is the longer leg. In a reverse-bias circuit, you will apply your positive voltage to the Cathode and route the Anode to your amplifier.
Operation Regions: Photovoltaic vs. Photoconductive
How you bias the diode completely changes its behavior. You must choose between zero-bias (photovoltaic) and reverse-bias (photoconductive) based on your speed and precision requirements.
| Parameter | Photovoltaic Mode (Zero Bias) | Photoconductive Mode (Reverse Bias) |
|---|---|---|
| Bias Voltage | 0V (Short circuit or high-Z load) | 5V to 60V (Cathode positive, Anode negative) |
| Dark Current | 0 pA (Effectively zero) | 1 nA to 50 nA (Increases with voltage/temp) |
| Response Speed | Slow (Milliseconds) | Fast (Nanoseconds to Microseconds) |
| Junction Capacitance | High (e.g., 75pF for BPW34) | Low (e.g., 15pF at 50V reverse bias) |
| Linearity | Excellent at very low light | Excellent across wide dynamic range |
| Best Application | Precision light metering, solar sensors | IR remote decoding, high-speed data, pulse counting |
Complete Application Circuit: Transimpedance Amplifier (TIA)
A photodiode outputs current, not voltage. To read it with an ADC or microcontroller, you need a transimpedance amplifier—a circuit topology that converts an input current to a proportional output voltage using a feedback resistor. While you can put a simple resistor to ground, a TIA using an op-amp keeps the diode at a virtual ground, eliminating voltage-swing capacitance effects and drastically improving bandwidth.
Circuit Specifications
- Sensor: Vishay BPW34 (Reverse biased)
- Op-Amp: Texas Instruments OPA380 (or LM358 for low-speed hobby builds)
- Supply: 5V Single Supply
- Feedback Resistor (Rf): 1 MΩ (Sets gain: 1V output per 1µA of photocurrent)
- Feedback Capacitor (Cf): 15 pF (Compensates for diode junction capacitance to prevent oscillation)
- Voltage Divider (R1, R2): 10 kΩ each (Creates a 2.5V virtual ground reference)
Step-by-Step Build Instructions
- Establish the Virtual Ground: Connect R1 (10kΩ) from 5V to the non-inverting input (+) of the op-amp. Connect R2 (10kΩ) from the non-inverting input to GND. Place a 100nF bypass capacitor across R2 to filter power supply noise. This pins the (+) input at a stable 2.5V.
- Bias the Photodiode: Connect the BPW34 Cathode (short leg/flat side) directly to the 5V rail. Connect the Anode (long leg) to the inverting input (-) of the op-amp. The diode now sees a steady 2.5V reverse bias (5V at cathode, 2.5V virtual at anode).
- Close the Feedback Loop: Connect the 1 MΩ feedback resistor (Rf) between the op-amp's output pin and the inverting input (-). In parallel with Rf, solder the 15 pF capacitor (Cf). Do not skip Cf. Without it, the 75pF junction capacitance of the BPW34 will interact with the 1MΩ resistor to create a pole that turns your amplifier into a high-frequency oscillator.
- Verify Output: The output pin of the op-amp will sit at 2.5V in total darkness. When IR light strikes the diode, current flows through Rf, pulling the output voltage down. A strong IR LED signal might pull the output from 2.5V down to 1.0V, which is perfectly centered for a 3.3V or 5V microcontroller ADC.
Failure Modes and Multimeter Testing
Photodiodes are rugged, but they are not invincible. Understanding how they fail saves hours of debugging.
Common Failure Modes
- Accidental Forward Bias: If you wire the diode backward (Anode to 5V, Cathode to GND) without a current-limiting resistor, it will act as a standard shorted diode, draw massive current, and pop the epoxy package.
- Thermal Runaway: In high-temperature environments, reverse-bias dark current doubles every 10°C. If your circuit lacks thermal management, the dark current can rise enough to saturate your TIA, blinding the sensor to actual signals.
- ESD Damage: While less sensitive than MOSFET gates, the thin depletion region can be punctured by static discharge, resulting in a permanently high dark current (leaky junction).
How to Test with a Digital Multimeter
You can verify a suspect infrared photodiode on the bench in under a minute using a standard DMM.
- Forward Bias Test: Set your multimeter to Diode Test mode. Place the red probe on the Anode (long leg) and the black probe on the Cathode (short leg). A healthy IR diode will show a forward voltage drop between 0.35V and 0.50V. (Note: This is lower than a visible red LED, which typically reads 1.6V to 1.8V). If it reads 0.00V, the junction is shorted.
- Reverse Bias Test: Swap the probes (red to Cathode, black to Anode). The meter should read OL (Over Limit) or open circuit. If it reads a low resistance or a voltage drop, the diode is internally shorted or heavily degraded by ESD.
- Active Light Test: Keep the meter in mV or µA mode. Shine a standard TV remote control directly at the sensor and press a button. You should see a distinct, rhythmic jump in voltage or current on the display corresponding to the 38kHz burst carrier.
Frequently Asked Questions
How do I know if my infrared photodiode is working with a standard multimeter?
Use the Diode Test setting on your multimeter. Touch the red probe to the long leg (anode) and the black probe to the short leg (cathode). A functional infrared photodiode will display a forward voltage drop between 0.35V and 0.50V. If you reverse the probes, it should read 'OL' (open loop). If it reads 'OL' in both directions, the internal wire bond is broken; if it reads near 0V in both directions, the junction is shorted.
Can I use an infrared photodiode to detect visible light?
Technically yes, but poorly. Silicon-based IR photodiodes like the BPW34 have a broad spectral response that tails off into the visible red spectrum (around 700nm). However, the black epoxy package used on most IR diodes acts as a visible-light blocking optical filter. It will absorb most ambient room light while passing 850nm-950nm IR. If you need to detect visible light, you must buy a photodiode with a clear water-clear package or a specific visible-light color filter, such as the Vishay BPW21R.
Why is my infrared photodiode circuit oscillating or picking up 60Hz noise?
High-frequency oscillation in a TIA circuit is almost always caused by missing the feedback compensation capacitor (Cf) across your feedback resistor. The photodiode's junction capacitance creates a phase delay that pushes the op-amp into oscillation. Add a 10pF to 22pF ceramic capacitor in parallel with your 1MΩ feedback resistor. For 60Hz/50Hz mains hum, the high impedance of the TIA makes it act like an antenna. Fix this by shielding the sensor in a grounded metal housing, using a twisted-pair cable for the diode leads, and ensuring your op-amp power rails have 100nF decoupling capacitors placed within 2mm of the supply pins.
