An infrared (IR) photodiode is a semiconductor device that converts infrared light (typically 700nm to 1100nm) into electrical current. If you need a reliable, high-sensitivity default part for a new design, buy the Vishay BPW34 (broadband, high-speed, through-hole or SMD) or the Osram SFH203 (fast response, integrated daylight-blocking filter). Both cost under $1.00 in single quantities and provide excellent linearity for everything from pulse-oximetry to IR remote decoding.
This guide skips the abstract semiconductor physics and goes straight to the bench: how to read the pinout, which biasing topology to choose, how to build a stable transimpedance amplifier (TIA), and how to verify the component with a standard multimeter.
Symbol, Pinout, and Safe Default Part Numbers
On a schematic, the IR photodiode uses the standard diode symbol (a triangle pointing to a vertical bar) with two inward-pointing arrows indicating incoming light. The bar represents the Cathode (K), and the triangle represents the Anode (A).
For physical through-hole packages (like the standard 5mm epoxy dome or the flat BPW34 package), the Cathode is identified by the shorter lead and a flat spot on the plastic rim. The Anode is the longer lead. In surface-mount (SMD) packages, look for a cathode band, a dot, or a chamfered corner on the component body.
Recommended Default IR Photodiodes
| Part Number | Manufacturer | Peak Wavelength | Max Reverse Voltage (Vr) | Dark Current (Id) | Responsivity | Best Application |
|---|---|---|---|---|---|---|
| BPW34 | Vishay | 900 nm | 50 V | 2 nA (at 10V) | 0.62 A/W | General purpose, TIA circuits, smoke detectors |
| SFH203 | Osram | 900 nm | 50 V | 1 nA (at 10V) | 0.62 A/W | Outdoor/ambient light rejection, remote receivers |
| VEMD2020X01 | Vishay | 940 nm | 32 V | 5 nA (at 5V) | 0.55 A/W | High-speed data links, time-of-flight (ToF) |
Operation Regions: Photovoltaic vs. Photoconductive Bias
How you bias the photodiode dictates its speed, noise floor, and linearity. You have two distinct operation regions to choose from based on your application requirements.
1. Photovoltaic Mode (Zero Bias): The diode is left unbiased (0V across the junction). Light generates a small forward voltage. This mode has zero dark current and extremely low low-frequency noise, making it ideal for precision DC light measurement. However, the junction capacitance remains high, resulting in slow response times (microseconds).
2. Photoconductive Mode (Reverse Bias): A reverse voltage (typically 5V to 30V) is applied across the diode. This widens the depletion region, drastically reducing junction capacitance and allowing for nanosecond response times. It also improves linearity at high light levels. The tradeoff is the introduction of dark current (leakage) and shot noise. According to All About Circuits, reverse biasing is mandatory for any AC-coupled or high-speed digital communication circuit.
| Parameter | Photovoltaic (Zero Bias) | Photoconductive (Reverse Bias) |
|---|---|---|
| Bias Voltage | 0 V | 5 V to 50 V (Typical) |
| Junction Capacitance | High (e.g., 72 pF for BPW34) | Low (e.g., 10 pF at 10V reverse) |
| Response Time | Slow (µs range) | Fast (ns range) |
| Dark Current / Noise | Zero dark current / Lowest noise | Non-zero dark current / Shot noise present |
| Linearity | Poor at high illumination | Excellent across wide dynamic range |
Complete Application Circuit: IR Transimpedance Amplifier
A photodiode outputs current, not voltage. To interface it with a microcontroller ADC, you need a Transimpedance Amplifier (TIA) to convert the current to a usable voltage. Below is a complete, breadboard-ready TIA circuit designed for a 3.3V or 5V system using the BPW34 and an MCP6002 op-amp.
Many beginners omit the feedback capacitor (Cf) and wonder why their op-amp is oscillating at 500 kHz. The photodiode's junction capacitance interacts with the high-value feedback resistor to create a pole in the loop response. Cf provides pole compensation to maintain phase margin.
Component List and Values
- U1: MCP6002 (Dual, low-power, rail-to-rail I/O op-amp, 1 MHz GBW)
- D1: Vishay BPW34 (IR Photodiode)
- Rf (Feedback Resistor): 1 MΩ (Sets the gain: Vout = I_photo × 1MΩ)
- Cf (Feedback Capacitor): 2.2 pF (Ceramic, C0G/NP0 dielectric for stability)
- R1, R2 (Voltage Divider): 10 kΩ each (Creates a 2.5V virtual ground reference from a 5V supply)
- C1 (Bypass): 100 nF ceramic across the VCC/GND pins of the op-amp
Wiring Steps
- Connect the MCP6002 VDD to 5V and VSS to GND. Place the 100 nF bypass capacitor directly across these pins.
- Wire R1 and R2 in series between 5V and GND. Connect their midpoint to the Non-Inverting Input (+) of U1-A. This sets your DC bias point to 2.5V.
- Connect the Cathode (K) of the BPW34 to the 5V rail (Reverse Bias).
- Connect the Anode (A) of the BPW34 to the Inverting Input (-) of U1-A.
- Connect Rf (1 MΩ) between the Inverting Input (-) and the Output pin of U1-A.
- Connect Cf (2.2 pF) in parallel with Rf.
- Route the Output pin to your microcontroller's ADC input.
With this configuration, ambient room IR yields a baseline output near 2.5V. When an IR source hits the diode, current flows through Rf, pulling the output voltage down toward 0V. For a deeper mathematical breakdown of selecting Rf and Cf based on op-amp Gain Bandwidth Product, refer to the Analog Devices Photodiode Amplifier guide.
Failure Modes and Multimeter Testing
IR photodiodes are robust, but they do fail. The most common failure modes are ESD damage (which degrades the junction, causing massive increases in dark current) and thermal cracking from excessive soldering heat on SMD pads. Unlike IR LEDs, which frequently burn out from overcurrent, photodiodes are receivers and rarely fail from electrical overstress unless the reverse breakdown voltage is exceeded.
How to Test with a Digital Multimeter (DMM)
You can verify an IR photodiode on your bench using a standard DMM in two steps:
- Forward Voltage Test (Diode Mode): Set your DMM to diode test. Place the red probe on the Anode and the black probe on the Cathode. A healthy IR silicon photodiode will read between 0.35V and 0.45V. This is noticeably lower than a standard visible-light silicon diode (which reads ~0.6V) due to the narrower bandgap required to detect lower-energy IR photons. If it reads 0.0V, the junction is shorted (ESD failure). If it reads 'OL', the internal bond wire is broken.
- Reverse Leakage Test (Photovoltaic Check): Set the DMM to DC millivolts (mV). Place the red probe on the Cathode and black on the Anode. In a dark room, it should read near 0 mV. Point a TV remote at the diode and press a button. You should see the voltage jump by 10 mV to 50 mV as the diode generates photovoltaic current. This confirms the junction is actively converting photons to electrons.
Frequently Asked Questions
How do I tell an IR photodiode apart from an IR LED?
Through-hole IR LEDs and IR photodiodes often look identical (both use 5mm clear or dark epoxy packages). The definitive test is the DMM diode check. An IR LED will emit a faint glow visible through a smartphone camera when forward-biased by the meter's test current. An IR photodiode will not emit light. Additionally, IR LEDs typically have a slightly higher forward voltage drop (~0.45V - 0.55V) compared to photodiodes (~0.35V - 0.45V), though this varies by manufacturer.
Why is my IR photodiode picking up 60Hz/50Hz mains hum?
If your TIA output shows a 60Hz (or 50Hz) ripple, your circuit is picking up electromagnetic interference (EMI) from nearby AC mains wiring, or the photodiode is detecting 120Hz optical flicker from un-filtered fluorescent/LED room lighting. To fix electrical EMI, use a coaxial cable to connect the photodiode to the op-amp, keeping the high-impedance anode trace as short as physically possible. To fix optical flicker, place an optical bandpass filter (like a 940nm narrowband gel) over the diode, or implement a digital bandpass filter in your microcontroller firmware.
Can I use a visible light photodiode for IR detection?
Technically yes, but practically no. Standard visible-light silicon photodiodes (like the BPW21R) are optimized for 400nm to 700nm. While silicon's natural absorption edge extends into the near-IR (~1000nm), visible photodiodes often feature integrated IR-blocking filters (hot mirrors) in their epoxy packaging to mimic human eye response. This filter will attenuate 850nm and 940nm signals by 90% or more. Always select a part explicitly binned for near-IR, like the BPW34.
What is the maximum reverse voltage I can apply to an IR photodiode?
You must never exceed the Maximum Reverse Voltage (Vr) specified in the datasheet. For the Vishay BPW34, this is 50V. Exceeding this causes avalanche breakdown, resulting in a massive, uncontrolled surge of reverse current that will thermally destroy the junction. In practice, a 5V to 12V reverse bias is sufficient to fully deplete the junction and minimize capacitance for 95% of hobbyist and industrial applications, keeping you well within the safe operating area.
