The standard symbol of phototransistor components is an NPN or PNP bipolar junction transistor (BJT) with two inward-pointing arrows indicating light sensitivity. Unlike a standard BJT, the base connection is frequently omitted in the schematic symbol because the base current is generated by incident photons rather than an external electrical trace. However, variations exist depending on whether the designer is following North American or international drafting standards, and whether the specific physical component includes a base pin for electrical biasing.

The Complete Phototransistor & Opto-Symbol Reference Table

Before tracing a circuit, you need to know exactly what you are looking at. The table below maps the most common light-sensitive semiconductor symbols you will encounter on schematics, contrasting the North American and international drafting conventions. Use this as your primary bench reference.

Component IEEE 315 (US/ANSI) Symbol Description IEC 60617 (Global) Symbol Description Key Physical Identifier
NPN Phototransistor NPN transistor in a circle; two inward arrows pointing at the base region; base line often omitted. NPN transistor without a circle; two inward arrows; flat emitter line, angled collector line. Clear or black epoxy dome with a flat edge (e.g., Vishay TEFT4300).
PNP Phototransistor PNP transistor in a circle; two inward arrows; emitter arrow points toward base. PNP transistor without a circle; two inward arrows; emitter arrow points inward. Rare in discrete form; usually found inside specialized optocouplers.
Photodiode Standard diode symbol in a circle; two inward arrows; cathode bar present. Standard diode without a circle; two inward arrows. Looks like a standard LED but typically has a smaller die and clear casing.
Photoresistor (LDR) Resistor zigzag in a circle with two inward arrows. Rectangular resistor box (IEC style) with two inward arrows. Cadmium sulfide disc with a visible serpentine trace on the face.
Optocoupler (e.g., PC817) LED symbol pointing at a phototransistor symbol, enclosed in a single dashed or solid box. Identical internal symbols, enclosed in a solid boundary line indicating a single package. 4-pin or 6-pin DIP IC package; opaque black plastic (no light window).

Regional Schematic Standards: IEEE 315 (US) vs IEC 60617 (Global)

While electricians rely on the NEC or IEC 60446 for wiring colors and physical installations, electronics engineers and schematic designers rely on entirely different standards for component symbols. Understanding which standard your schematic follows prevents catastrophic miswiring on the bench.

IEEE 315 / ANSI Y32.2 (North America)

In the US and Canada, schematics are predominantly drawn using IEEE 315 (which superseded ANSI Y32.2). The defining characteristic of the IEEE symbol of phototransistor components is the enclosing circle. The transistor body, the arrows, and the terminals are all contained within a distinct circle to represent the physical housing of the discrete component. Furthermore, the emitter is indicated by an arrow on the lead wire itself, pointing away from the base for NPN, and toward the base for PNP.

IEC 60617 (Europe, Asia, and Global)

The International Electrotechnical Commission (IEC) standard 60617 is the dominant format in Europe and most modern global CAD software (like recent versions of Altium and KiCad default libraries). The IEC symbol drops the enclosing circle for discrete components. Instead of an arrow on the wire, the emitter lead itself is drawn at an angle, with the arrowhead integrated into the angled line. If you are reading a schematic from a European manufacturer like Infineon or Vishay's EU division, expect the IEC format. For a deep dive into how these standards map to modern EDA tools, refer to the All About Circuits schematic symbol reference.

Warning: Never assume a schematic's regional origin based solely on the language of the text. Many Asian-manufactured development boards use US-style IEEE symbols in their English-translated documentation, while US-based open-source hardware projects often use default KiCad IEC libraries. Always check the title block for the specific standard cited.

Rows People Get Wrong & Identifying Unmarked Components

Even experienced bench technicians misinterpret specific rows in opto-symbol tables. Here are the most common pitfalls and how to safely interpret components when the schematic is ambiguous or the PCB silkscreen has been burned away.

The 'Missing' Base Pin Confusion

The most frequent error occurs when a schematic shows a phototransistor symbol with a base pin, but the physical component on the board (like the popular SFH309 or LTV-817 internal transistor) only has two legs (Collector and Emitter). In schematic theory, the base is shown to represent the node where photon-generated current enters the junction. In physical reality, manufacturers omit the base pin to save space and cost, as the device is meant to be driven entirely by light. If you see a 3-pin symbol but hold a 2-pin part, connect the Collector to the positive rail (via a pull-up resistor) and the Emitter to ground. Leave the schematic's base node floating.

Arrows Pointing In vs. Arrows Pointing Out

A quick glance at inward vs. outward arrows is the fastest way to differentiate emitters from receivers.

  • Inward Arrows: Light is entering the component. This applies to the symbol of phototransistor parts, photodiodes, and solar cells.
  • Outward Arrows: Light is leaving the component. This applies to LEDs, laser diodes, and IR emitters.
If a schematic shows outward arrows on a transistor body, it is not a phototransistor; it is likely a custom light-emitting transistor or a drafting error.

Safe Interpretation When Markings are Faded or Missing

When repairing legacy gear, you will often find a 3mm or 5mm clear epoxy component with no silkscreen and no accessible schematic. To safely identify if it is a phototransistor or a photodiode without desoldering it:

  1. Set your digital multimeter (DMM) to Diode Test mode.
  2. Place the red probe on the suspected Collector (usually the shorter lead or the flat-edge side) and the black probe on the Emitter.
  3. Shield the component from ambient light with your hand. The meter should read 'OL' (Open Loop) or a very high voltage drop.
  4. Shine a bright white flashlight (or your phone screen) directly onto the epoxy dome.
  5. The Result: If the voltage drop on the DMM drops significantly (e.g., from 3.2V down to 0.8V) or the meter begins to beep, you are looking at a phototransistor (or a Darlington pair, which will show even higher sensitivity). A standard photodiode will show a much smaller shift in leakage current, usually remaining in the millivolt range on a standard DMM.

For exact spectral response curves and pinout confirmations, always cross-reference the physical dimensions with manufacturer datasheets, such as the Vishay Phototransistor catalog.

Frequently Asked Questions

What is the exact symbol of phototransistor vs photodiode?

The core difference lies in the semiconductor body depicted. The symbol of a phototransistor features a three-layer junction (Collector, Base region, Emitter) with an emitter arrow, identical to a standard BJT but with two inward light arrows. A photodiode symbol features a simple two-layer PN junction (an anode triangle and a cathode bar) with the same two inward light arrows. In practice, the phototransistor symbol implies internal gain (amplification of the light current), while the photodiode symbol implies a 1:1 photon-to-electron ratio used for high-speed or precision light measurement.

Why does my schematic symbol of phototransistor show a base pin when the part only has two legs?

Drafting software libraries often include the base pin on the symbol of phototransistor components to maintain electrical symmetry with standard BJTs and to allow designers to add a physical biasing resistor if they want to set a specific dark-current threshold. If your physical component (like a standard 2-pin TEFT4300) lacks the base leg, simply ignore the base pin on the schematic. Do not attempt to solder a wire to the side of the transistor casing; the internal silicon die is not accessible.

How do I identify a phototransistor on a PCB when the schematic symbol is missing?

Look for physical clues first: phototransistors typically have a clear, smoke-tinted, or black epoxy dome with a distinct flat edge on the rim indicating the Emitter pin. If the casing is completely opaque black (like an optocoupler), it is likely an integrated module rather than a discrete phototransistor. Electrically, use a multimeter in diode mode combined with a flashlight. A distinct drop in forward voltage when illuminated confirms the presence of a light-sensitive transistor junction. Always verify the component's orientation against the surrounding circuit topology—phototransistors are almost exclusively wired in a common-emitter configuration with a pull-up resistor on the collector.

Does the IEC symbol of phototransistor ever include a Darlington configuration?

Yes. When a schematic requires extreme light sensitivity (such as in solid-state relays or low-light optical switches), the designer will use a Darlington phototransistor. The IEC symbol for this combines two transistor symbols: the emitter of the first (light-sensitive) transistor feeds directly into the base of the second transistor. Both are enclosed in a single boundary box if they share a single physical package, like the popular Lite-On LTV-817 series optocouplers which often utilize Darlington output stages for high current transfer ratios (CTR).