When you are designing an optical encoder, a light-beam break sensor, or an IR remote receiver, correctly interpreting the phototransistor symbol and its physical pinout is the difference between a working circuit and a dead short. Below is the complete reference for schematic symbols, regional standard variants, and physical package pinouts.
Schematic Symbol Standards and Physical Pinout Tables
The schematic representation of a phototransistor combines a standard bipolar junction transistor (BJT) with incoming light arrows. However, the exact drawing changes depending on whether you are following North American (IEEE/ANSI) or international (IEC) standards, and whether the component includes a physical base lead for biasing.
| Component Type | IEEE 315 / ANSI Y32.2 (US) | IEC 60617 (EU/Global) | Key Visual Identifier |
|---|---|---|---|
| NPN Phototransistor (2-lead) | Circle enclosure, NPN arrows, 2 inward light arrows, NO base line. | No circle enclosure, NPN structure, 2 inward arrows, NO base line. | Arrows point IN toward the base junction. No physical base pin drawn. |
| NPN Phototransistor (3-lead) | Circle enclosure, NPN arrows, 2 inward light arrows, base line present. | No circle enclosure, NPN structure, 2 inward arrows, base line present. | Includes a standard base connection line for external biasing. |
| PNP Phototransistor | Circle enclosure, PNP arrows (pointing inward to emitter), 2 inward light arrows. | No circle, PNP structure, 2 inward light arrows. | Emitter arrow points TOWARD the base. Rare in modern discrete designs. |
| Photodarlington | Circle enclosure, Darlington pair (two transistors), 2 inward light arrows. | No circle, Darlington pair structure, 2 inward light arrows. | Two emitter junctions. Used for high-gain, low-light applications. |
While the schematic tells you how to wire the circuit, the physical package tells you where the wires actually go. Because phototransistors often share physical housings with standard LEDs and BJTs, relying on visual package cues is critical.
| Package Type | Common Part Examples | Pin 1 (Left/Short) | Pin 2 (Right/Long) | Pin 3 (Middle) | Physical Landmark |
|---|---|---|---|---|---|
| T-1 3/4 (5mm) 2-Lead | Vishay TEFT4300, Osram SFH 309 | Emitter | Collector | N/A | Flat spot on plastic rim denotes Emitter side. Longer lead is Collector. |
| TO-92 3-Lead | BPW85B, LPT-3034-21 | Emitter | Collector | Base | Flat side facing viewer, leads pointing down. (Note: E-C-B is standard, but verify datasheet). |
| SMD (Side-View) | Vishay TEMT6200 | Emitter (Pin 1) | Collector (Pin 2) | N/A | Pin 1 is usually marked by a small dot or chamfered edge on the mold. |
| SMD (Top-View) | Osram SFH 3100F | Emitter | Collector | N/A | Internal die placement is visible; larger bond pad is typically the Collector. |
Bench Testing: Identifying Unmarked or Faded Phototransistors
If you are salvaging components or dealing with a bin of unmarked 5mm clear or dark-epoxy LEDs, visual identification fails. You must electrically verify the pinout. A phototransistor is essentially a BJT where the base current is generated by photons striking the base-collector junction.
The Multimeter Light-Test Method:
- Set your digital multimeter (DMM) to the resistance (Ω) or continuity range.
- Place the Red probe on one lead and the Black probe on the other. In normal room light, the meter should read 'OL' (Open Loop) or a very high resistance (MΩ range).
- Reverse the probes. It should still read 'OL'.
- Shine a high-lumen white flashlight or an 850nm/940nm IR source directly at the dome of the component.
- Observe the meter. If the resistance drops significantly (into the kΩ range) or the meter beeps for continuity, look at your probe placement.
- For an NPN device, the lead touching the Red (positive) probe is the Collector, and the lead touching the Black (negative) probe is the Emitter.
The hFE Socket Trick: If your DMM has a transistor hFE testing socket, plug the 2-lead phototransistor into the NPN 'C' and 'E' holes (leave 'B' empty). Cover it with your hand to block light; the reading should be near zero. Uncover it and shine a light on it. If the meter registers a massive gain spike (often maxing out the display), you have correctly identified the Collector and Emitter. If it reads zero regardless of light, swap the leads.
Rows and Symbols People Get Wrong
When reading schematics or sorting through a mixed component drawer, makers frequently misinterpret specific symbols and physical traits. Here are the most common points of failure.
1. Inward vs. Outward Arrows (The Optoelectronic Rule)
The direction of the arrows on the schematic symbol dictates the flow of energy, not just current. Outward-pointing arrows indicate emission (LEDs, Laser Diodes, IR Emitters). Inward-pointing arrows indicate reception (Photodiodes, Phototransistors, LDRs, Solar Cells). If you see arrows pointing away from the junction, you are looking at a light source, not a sensor.
2. Phototransistor vs. Photodiode Symbols
A photodiode symbol consists of a standard diode triangle and line with two inward-pointing light arrows. A phototransistor symbol features the three-terminal BJT structure (Emitter, Base, Collector) with the same inward arrows. In practice, a photodiode operates in reverse-bias for high-speed, low-light linear applications (like fiber optic receivers), while a phototransistor operates in forward-active mode to provide internal current gain (hFE), making it better for slower, higher-current switching tasks like opto-isolators and beam-break sensors.
3. The 'Missing' Base Pin on 3-Lead Packages
Many makers buy a 3-lead TO-92 phototransistor (like the BPW85B) assuming they must wire the base pin to a bias resistor. In 90% of hobbyist and industrial switching applications, the base pin is left entirely floating (disconnected). The device is meant to be driven purely by light. The base pin is only brought out in specialized designs where the engineer wants to inject a small DC bias current to shift the ambient light threshold or to use the component as a standard BJT when light isn't present. If your schematic shows a 3-lead symbol but no base connection, simply insulate and trim the physical base lead.
4. Clear vs. Dark Epoxy Packaging
While not a schematic symbol error, confusing the physical epoxy color leads to circuit failure. Clear or water-clear packages are designed to react to the full visible light spectrum (and some near-IR). Dark, opaque, or black-epoxy packages (like the Vishay TEFT4300) contain an integrated optical filter that blocks visible light and only passes infrared wavelengths (typically 850nm to 950nm). If you use a clear phototransistor in an environment with fluorescent or sunlight ambient light, the sensor will saturate and fail to detect your IR remote signal. Always match the epoxy color to your light source spectrum.
For deeper standard definitions, refer to the All About Circuits optoelectronic devices guide, and always verify specific pinout mappings against the manufacturer's datasheet, such as the Vishay TEFT4300 datasheet, as regional cloning of TO-92 packages occasionally results in swapped Emitter and Collector leads.
