The standard symbol for photodiode is a semiconductor diode (a triangle pointing toward a vertical cathode bar) enclosed in a circle, featuring two inward-pointing arrows representing incoming photons. While the basic geometry is universal, subtle variations exist between international standards, and confusing it with an LED or solar cell is a frequent bench mistake.
The Optoelectronic Symbol Reference Table
Below is the complete reference for the photodiode family and closely related optoelectronic symbols. Use this to decode schematics and CAD libraries.
| Component | Symbol Description | Arrow Direction | In Practice (Typical Use) | Common Part Numbers |
|---|---|---|---|---|
| Standard PN Photodiode | Diode triangle + bar, usually circled. | INWARD (toward diode) | General light sensing, smoke detectors. | BPW21R, SFH203 |
| PIN Photodiode | Identical to standard, sometimes with a wider gap between triangle and bar. | INWARD | High-speed data, pulse oximetry, IR remotes. | BPW34, BPW27N |
| Avalanche (APD) | Diode symbol with a small 'hook' or break at the cathode bar edge. | INWARD | Ultra-low light, LIDAR, fiber optics (requires high reverse bias). | C30642G, SPCM-AQRH |
| Solar Cell | Often drawn as a photodiode, or a circle with +/- terminals and light arrows. | INWARD (if diode style) | Power generation (photovoltaic mode, zero bias). | IXYS KXOB20, SunPower C60 |
| LED (For Contrast) | Diode triangle + bar, circled or uncircled. | OUTWARD (away from diode) | Light emission, status indicators. | WS2812B, L-53SRD |
| Phototransistor | NPN/PNP transistor symbol (no base lead), with two inward arrows. | INWARD | Higher sensitivity, slower speed than photodiodes. | TEFT4300, PT333-3C |
IEC 60617 vs. IEEE 315: Regional and Standard Variants
Which standard applies to your schematic depends on your region and the CAD software you use. Understanding the difference prevents misinterpretation when reading foreign datasheets or legacy military prints.
- IEC 60617 (International / EU / UK): The International Electrotechnical Commission mandates that the symbol for photodiode must be enclosed in a circle. The arrows must strike the center of the diode junction at a 45-degree angle. This is the default in modern Altium and KiCad libraries.
- IEEE 315 / ANSI Y32.2 (US / North America): Historically, the US standard allowed the circle to be omitted for discrete components to reduce drafting clutter, showing only the triangle, bar, and inward arrows. While modern US CAD tools have largely converged with IEC and include the circle, you will still see the 'uncircled' variant on older US military schematics and legacy aerospace PCB silkscreens.
Rows People Get Wrong: LEDs, Solar Cells, and Faded Markings
Even experienced technicians misidentify optoelectronics on a crowded board. Here is how to resolve the most common points of confusion.
1. Photodiode vs. LED (The Arrow Trap)
The most frequent error is mixing up the photodiode and the LED. The rule is based on physics: photons enter a photodiode to create electron-hole pairs (arrows point IN), while photons exit an LED as electrons recombine (arrows point OUT). If the arrows point away from the cathode bar, it is an emitter, not a sensor.
2. Photodiode vs. Solar Cell
Electrically, a solar cell is just a massive photodiode operating in photovoltaic (zero-bias) mode. In schematics, they often share the exact same symbol. The differentiator is the circuit context: if the component is connected to a transimpedance amplifier or reverse-biased to a voltage rail, it is a photodiode. If it connects directly to a charge controller or battery bus, it is a solar cell.
3. Safe Interpretation When PCB Markings are Faded
If the silkscreen symbol for a photodiode on a PCB is faded or missing, do not guess polarity based on physical placement. Use this definitive multimeter (DMM) test:
- Set your DMM to Diode Test mode.
- Place the red probe on one leg and the black probe on the other.
- A reading of 0.3V to 0.5V indicates forward bias (the red probe is on the Anode, black is on the Cathode). If it reads 'OL', swap the probes.
- The Light Test: Switch the DMM to the µA (micro-amp) current mode. Connect the red probe to the Anode and black to the Cathode (forward bias). Cover the component with your hand. If the current reading drops significantly when shielded from room light, you have positively identified a photodiode (standard signal diodes do not react to visible light).
Decision Tree: Which Photodiode Type Do You Need?
Use this decision path to select the correct physical component for your build. The symbol on your schematic will dictate the electrical behavior, but the physical part number dictates the performance.
| Application Requirement | If your circuit needs... | Then select this type... | Concrete Part Recommendation |
|---|---|---|---|
| General Ambient Light / Proximity | Simple ADC reading, low cost, visible to near-IR spectrum. | Standard PN or basic PIN | Vishay BPW21R (Visible) or TEFT4300 (IR) |
| High-Speed / Medical / IR Remote | Fast rise times (<100ns), low capacitance, wide depletion region. | PIN Photodiode | Vishay BPW34 (The industry standard maker part) |
| Ultra-Low Light / LIDAR | Internal gain via avalanche multiplication, single-photon sensitivity. | Avalanche (APD) | Excelitas C30642G (Requires 50V-200V bias) |
| Digital IR Decoding | Reading 38kHz TV remote signals directly into a GPIO pin. | Integrated IR Receiver (Not a raw diode) | Vishay TSOP38238 (Includes built-in AGC and filter) |
Real-World Pinouts and Bench Implementation
For 95% of hobbyist, Arduino, and ESP32 light-sensing projects, the definitive choice is the Vishay BPW34 PIN photodiode. It offers a massive 7.5 mm² active area, meaning it captures plenty of light without requiring complex focusing lenses.
BPW34 Pinout and Physical Identification
- Cathode (-): The shorter lead. On the physical package, this is the side with the chamfered (cut) corner or a small dot printed on the epoxy casing.
- Anode (+): The longer lead. The square corner side of the package.
The Default Implementation Circuit
Do not just wire a photodiode directly to an ESP32 GPIO and expect a clean reading. The raw symbol for photodiode implies a current source, not a voltage source. To get a usable voltage for a microcontroller ADC (0-3.3V):
- Wire the Cathode to your 3.3V or 5V rail (Reverse Bias).
- Wire the Anode to GND through a 100kΩ pull-down resistor.
- Tap the junction between the Anode and the resistor, and route it to your microcontroller's ADC pin (e.g., ESP32 GPIO 34).
- As light hits the diode, reverse leakage current increases, pushing the voltage at the ADC pin higher. Dark = ~0V; Bright light = ~3.3V.
By understanding the exact schematic symbol, verifying standard variants, and testing faded boards with a DMM, you eliminate guesswork and ensure your optoelectronic circuits behave exactly as the physics intends.
