Photodiode Symbol

Definition: The Photodiode symbol represents a light-sensitive semiconductor diode with Anode and Cathode terminals, drawn per IEC 60617 as the standard diode triangle-and-bar with two small parallel arrows pointing toward the device to indicate incident light (the mirror image of the LED, whose arrows point away).

Also known as: photodiode, photo diode, light sensor diode, PIN photodiode, photodetector, optical receiver diode, avalanche photodiode (APD).

What the Photodiode symbol means

The photodiode symbol denotes a PN or PIN junction diode engineered so that photons reaching the junction generate electron-hole pairs, producing a current proportional to light intensity. It is the fundamental fast light sensor of electronics: where an LDR responds in milliseconds, a photodiode responds in nanoseconds, making it the receiver of choice for fibre-optic links, IR remote controls, optical encoders, pulse oximeters and light meters. The two incoming arrows are the entire semantic payload of the symbol — remove them and it is an ordinary diode; reverse them and it becomes an LED.

A photodiode operates in one of two modes. In photoconductive mode it is reverse biased: the light-generated photocurrent flows from cathode to anode through the external circuit, and the reverse bias widens the depletion region for faster response and better linearity. In photovoltaic mode it is unbiased and generates a small voltage like a tiny solar cell (a solar cell is, in fact, a large-area photodiode), which gives lower noise and zero dark current at the cost of speed.

How to identify the Photodiode symbol

Start from the ordinary diode symbol — a solid triangle (anode) pointing into a perpendicular bar (cathode) — then add two small parallel arrows angled at roughly 45°, pointing inward at the triangle. Arrows toward the symbol always mean a light receiver; arrows away always mean a light emitter (LED). Many drawings also enclose the diode in a circle representing the component envelope, particularly in older or European schematics.

IEC 60617 and ANSI Y32.2 / IEEE 315 use the same construction; the practical variations are whether the envelope circle is drawn, whether the arrows are straight or wavy (wavy arrows conventionally suggest radiation as waves, per older practice), and arrow placement. The cathode bar corresponds to the marked stripe or shorter lead on the physical device, exactly as with a rectifier diode.

Live editor identity: Live-library identity: `photodiode`, 40 × 30 canvas units, 2 logical terminals. Match this exact SVG and terminal list when identifying the placeable editor symbol; do not identify a physical package from the schematic outline alone.

Function and usage boundary

In photoconductive mode the photodiode sits reverse biased in series with a load resistor or, far more commonly, feeds the virtual-ground input of a transimpedance amplifier (an op-amp with a feedback resistor), which converts its photocurrent into a clean output voltage: Vout = Iphoto × Rf. Because photocurrent is linear over many decades of light intensity, this arrangement is the basis of virtually every precision optical measurement front end.

Even in complete darkness a reverse-biased photodiode passes a small dark current (nanoamps at room temperature, doubling roughly every 8–10 °C), which sets the noise floor. PIN photodiodes insert an intrinsic layer to enlarge the depletion region for speed and sensitivity; avalanche photodiodes (APDs) run near breakdown to give internal gain for detecting extremely faint signals such as LiDAR returns and fibre-optic pulses.

Usage boundary: Use the Photodiode abstraction only when the intended circuit can be expressed through these logical ports: `anode` (Anode), `cathode` (Cathode). Select a versioned physical profile and exact manufacturer datasheet before assigning package pins, ratings, or fabrication dimensions.

Standards: IEC vs ANSI

The displayed SVG is the live editor implementation, not a licensed reproduction or certification of an IEC or IEEE database glyph. Verify the exact official entry and project edition before issuing work.

Variant availability

• Editor implementation (available): This is the exact SVG and terminal geometry placed by the CircuitDiagramMaker editor.
• IEC 60617 (reference only): IEC 60617 is the official international source. This page does not present the editor SVG as a licensed or certified IEC reproduction.
• ANSI/IEEE 315 (reference only): IEEE 315 is the North American reference listed by IEEE as inactive-reserved; use the edition required by the project.

Logical terminals

The live editor exposes `anode` (Anode), `cathode` (Cathode). These are stable logical terminal IDs for diagram connectivity, not a promise that they equal physical package pin numbers.

Polarity and direction

Polarity or supply meaning is encoded on `anode` = Anode, `cathode` = Cathode. Do not reverse these logical connections, and verify the physical package pin number in the exact device datasheet.

Reference designator

D: D is the editor library convention for this symbol family. Confirm the issued project convention before release.

Where the Photodiode symbol is used

• IR remote-control and light-barrier receivers paired with an IR-emitting LED
• Fibre-optic and free-space optical data receivers, where PIN and avalanche photodiodes recover megabit-to-gigabit streams
• Pulse oximeters and heart-rate sensors measuring light transmitted or reflected through tissue
• Optical encoders, slot sensors and tachometers detecting beam interruption from a rotating disc
• Camera exposure meters, lux meters and ambient-light sensors with transimpedance front ends
• Smoke detectors (photoelectric type) sensing light scattered by smoke particles in a chamber

Key facts

• The symbol is a standard diode with two arrows pointing toward it; the LED symbol is identical but with arrows pointing away — arrow direction is the only difference.
• Photodiodes are normally operated reverse biased (photoconductive mode); photocurrent, not forward conduction, is the signal.
• Photocurrent is highly linear with light intensity over several decades, which is why photodiodes are preferred for measurement over LDRs.
• Photovoltaic mode (zero bias) trades speed for zero dark current — a solar cell is simply a large-area photodiode used in this mode.
• Silicon photodiodes respond from roughly 400 to 1,100 nm with peak responsivity of ~0.5–0.65 A/W near 850–950 nm.
• Dark current (nA-level) flows even in darkness and roughly doubles every 8–10 °C, setting the sensor's noise floor.
• PIN photodiodes add an intrinsic layer for sub-nanosecond speed; avalanche photodiodes add internal gain for very faint signals.
• The standard readout circuit is a transimpedance amplifier: the photodiode feeds an op-amp virtual ground and Vout = Iphoto × Rf.

Common interpretation mistakes

• Treating the Photodiode drawing (40 × 30 canvas units) as a physical footprint. The schematic outline expresses electrical intent; package and panel dimensions come from the selected physical profile.
• Copying `anode`, `cathode` directly to a wire marker or manufacturer package pin number. These are logical Photodiode port IDs and require an exact ordered-part terminal map.
• Reversing a polarity-marked Photodiode terminal because the symbol was rotated or mirrored on the sheet.

Frequently asked questions

What is the difference between the photodiode symbol and the LED symbol?

Both start from the same diode triangle-and-bar. On a photodiode the two small arrows point toward the diode, showing light arriving at the junction; on an LED they point away, showing light being emitted. Functionally they are converses: an LED converts current to light and is operated forward biased, while a photodiode converts light to current and is normally operated reverse biased.

Which way round does a photodiode go in a circuit?

For photoconductive (reverse-biased) operation, the cathode goes to the positive supply rail and the anode toward the lower potential — the opposite of how you would forward-bias a rectifier. Light then generates a photocurrent that flows through the external load. On the physical package the cathode is the marked or shorter lead, matching the bar of the symbol. In photovoltaic mode there is no bias; the anode is the positive output terminal, like a tiny solar cell.

What is the difference between photoconductive and photovoltaic mode?

Photoconductive mode applies reverse bias: the widened depletion region gives fast response (nanoseconds), lower capacitance and excellent linearity, at the cost of dark current and its noise. Photovoltaic mode uses zero bias: the diode self-generates voltage/current like a solar cell, with zero dark current and lowest noise, but slower response. Instrumentation favours photovoltaic mode for precision at low frequency; communications and encoders use photoconductive mode for speed.

What is the difference between a photodiode and a phototransistor?

A photodiode produces photocurrent directly with no gain — typically microamps — but responds in nanoseconds and is very linear. A phototransistor uses light as base drive and multiplies it by the transistor's current gain, delivering 100–1,000 times more current from the same illumination, but with microsecond response and poorer linearity. Choose a photodiode for speed and measurement accuracy, a phototransistor for simple, sensitive on/off light detection.

What is dark current in a photodiode?

Dark current is the small leakage current (typically 1–30 nA for small silicon devices) that flows through a reverse-biased photodiode even with no light present. It rises exponentially with temperature — roughly doubling every 8–10 °C — and with reverse voltage. Because it is indistinguishable from photocurrent, it sets the minimum detectable light level; precision designs reduce it by using photovoltaic mode, cooling, or subtracting a matched dark reference diode.

Sources and verification

• IEC 60617:2026 DB — Graphical symbols for diagrams (International Electrotechnical Commission) — The current official IEC graphical-symbol database and the electrotechnical areas it covers. Exact symbol identity data requires IEC access.; claim scope: standards
• IEEE/ANSI 315-1975 — Graphic Symbols for Electrical and Electronics Diagrams (IEEE Standards Association) — The official catalogue record for North American diagram symbols and reference-designation letters; IEEE lists the standard as inactive-reserved.; claim scope: standards, reference_designator
• IEC 61666:2010+AMD1:2021 — Identification of terminals within a system (International Electrotechnical Commission) — General principles for identifying terminals within systems; it does not replace the exact ordered-device datasheet.
• IEC 81346-2:2019 — Object classes and reference-designation codes (International Electrotechnical Commission) — The official classification scheme used to form reference designations across technical disciplines.; claim scope: reference_designator
• IEC 61082-1:2014 — Preparation of documents used in electrotechnology (International Electrotechnical Commission) — General presentation rules for electrotechnical diagrams, drawings, and tables.; claim scope: standards, usage
• IEC Electropedia IEV 521-04-03 — semiconductor diode (International Electrotechnical Commission) — The diode as a two-terminal semiconductor device with an asymmetrical voltage-current characteristic.; claim scope: terminals, polarity, usage

Geometry fingerprint: 3fb2ef225d2eaab7. A changed SVG, canvas size, or terminal definition invalidates the recorded review.

Related symbols

• Diode symbol
• IR Receiver symbol
• Photoresistor (LDR) symbol
• LED symbol
• Optocoupler symbol
• Photocell / Dusk Sensor symbol

Physical breadboard, PCB, and panel mapping

No universal physical profile is published for this symbol yet. Create a versioned custom part from the exact manufacturer dimensions and terminal map before fabrication.

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