If you are trying to identify an unmarked component from a transistor photo or design a light-sensing circuit from scratch, you need a direct answer. For general-purpose infrared (IR) detection, the safe default pick is the OSRAM SFH309FA (high sensitivity, 900nm peak). For visible ambient light sensing, use the Vishay TEPT5700 (570nm peak, matched to human eye response). Both are available in standard 3mm or 5mm through-hole packages for under $0.40 each.

A phototransistor is essentially a standard bipolar junction transistor (BJT) where the base current is generated by photons striking the base-collector junction rather than by an electrical connection. This guide skips the deep semiconductor physics and focuses strictly on bench-level identification, biasing, circuit design, and multimeter testing.

Decoding the Transistor Photo: Pinouts and Symbol Anatomy

When you find an unmarked optoelectronic component and snap a transistor photo to identify it, look for the clear or black-tinted epoxy dome and the lead configuration. Unlike standard BJTs, phototransistors usually only have two active leads brought out to the package, though the schematic symbol often shows three.

Visual Identification Rule: In a standard 3mm or 5mm through-hole package, the Emitter is the longer lead (and usually sits next to the flat spot on the plastic rim). The Collector is the shorter lead. The Base is internally connected to the light-sensitive junction and left floating.

Schematic Symbol Breakdown:

  • Collector (C): The top terminal. Connected to the positive supply voltage (VCC) through a load or pull-up resistor.
  • Emitter (E): The bottom terminal with the standard BJT arrow pointing outward (NPN). Connected to ground (GND).
  • Base (B): Represented with two inward-pointing arrows indicating incoming light (photons). In 95% of hobbyist and industrial circuits, this pin is omitted or left unconnected (NC).

If your transistor photo shows three physical leads (like the BPW85B), the middle pin is typically the base. You can use this base pin to inject a small DC bias current to adjust the dark-current threshold, but for most optical interrupter or ambient light circuits, you will leave it floating.

Operation Regions and Biasing the Base

To use a phototransistor effectively, you must understand its three operating regions. Unlike a standard BJT where you bias the base with a voltage divider, a phototransistor is biased by the intensity of the light hitting the die (measured in mW/cm² or lux).

Phototransistor Operation Regions (Typical 5V NPN Setup)
Region Light Level Vce (Collector-Emitter) Ic (Collector Current) Circuit Behavior
Cutoff (Dark) 0 lux (Complete darkness) ~5.0V (VCC) < 100 nA (Dark current) Acts as an open switch. Pull-down resistor pulls output to 0V.
Active (Linear) 1 to 10 mW/cm² 0.5V to 4.5V 10 µA to 2 mA Current is proportional to light intensity. Used for analog light metering.
Saturation (Fully On) > 15 mW/cm² (Direct IR beam) < 0.2V (Vce_sat) Limited by external load resistor Acts as a closed switch. Output voltage approaches VCC (if using pull-down).

How to Bias It: You do not bias the base with a resistor. Instead, you bias the collector using a load resistor (typically 1kΩ to 100kΩ). A lower resistor value (e.g., 1kΩ) makes the circuit faster (lower RC time constant) but less sensitive to dim light. A higher value (e.g., 100kΩ) increases sensitivity but slows down the switching speed, making it unsuitable for high-speed data transmission like IR remote decoding.

The Decision Tree: Selecting the Right Light Sensor

Do not just grab any clear LED from your parts bin and hope it works as a sensor. Use this decision path to select the exact part number for your application.

Sensor Selection Decision Matrix
If your application requires... Then choose this topology... Concrete Part Number Pick
Detecting ambient room light (turning on a display backlight) Visible-light phototransistor (500-600nm peak) Vishay TEPT5700 (570nm, matches human eye)
High-speed IR data reception (38kHz remotes) Integrated IR receiver module (includes AGC/demodulator) Vishay TSOP38238 (Not a raw phototransistor)
Object detection / optical interrupter (slotted switch) Standard IR phototransistor (850-950nm) paired with IR LED OSRAM SFH309FA (900nm, high gain)
Flame detection (UV/Visible spike) UV-sensitive phototransistor or specialized flame sensor GUVA-S12SD (UV sensor module)

The Verdict: If you are building a standard slotted optical switch, a tachometer, or a proximity sensor for an Arduino or ESP32 project, buy the OSRAM SFH309FA. It offers the best balance of high collector current (up to 15mA in saturation) and fast rise/fall times (approx 15µs) for under $0.40 per unit.

Complete Application Circuit: 5V Optical Interrupter

This circuit creates a digital light barrier. When an object breaks the IR beam between the emitter LED and the phototransistor, the microcontroller GPIO pin goes LOW. When the beam is clear, the pin goes HIGH.

Mains Safety Note: This circuit operates at 5V DC and is completely safe for bench work. If you are using this optical interrupter to trigger a mains-voltage relay or contactor for industrial counting, ensure the low-voltage DC side is galvanically isolated from the AC side using an optocoupler or proper relay isolation. Never share grounds between 5V logic and 120V/240V AC systems.

Materials List:

  • 1x OSRAM SFH309FA (IR Phototransistor, 900nm)
  • 1x TSAL6200 (IR Emitter LED, 940nm)
  • 1x 47Ω 1/4W Resistor (LED current limiting)
  • 1x 10kΩ 1/4W Resistor (Phototransistor pull-down)
  • Microcontroller (Arduino Uno, ESP32, etc.)

Wiring Steps:

  1. Wire the IR Emitter: Connect the 5V rail to the anode (long lead) of the TSAL6200 IR LED through the 47Ω resistor. Connect the cathode (short lead) to GND. This pushes roughly 30mA through the LED, creating a strong, invisible IR beam.
  2. Wire the Phototransistor Collector: Connect the short lead (Collector) of the SFH309FA directly to the 5V rail.
  3. Wire the Emitter and Pull-Down: Connect the long lead (Emitter) to GND through the 10kΩ resistor.
  4. Tap the Analog/Digital Output: Connect your microcontroller's GPIO pin (e.g., Arduino Pin 2 for digital, or A0 for analog) to the junction between the phototransistor Emitter and the 10kΩ resistor.
  5. Align the Optics: Point the clear IR LED directly at the clear phototransistor dome. If using them in a slotted housing, ensure the gap is no wider than 10mm to prevent ambient sunlight from saturating the receiver.

Expected Measurements: With the beam unblocked, measure the junction with a multimeter; it should read close to 5.0V (phototransistor is saturated, pulling current through the 10kΩ resistor). Block the beam with a piece of cardboard; the voltage should drop to < 0.1V (phototransistor cuts off, pull-down resistor grounds the node).

Failure Modes and Multimeter Testing

Phototransistors rarely fail catastrophically unless subjected to overvoltage or extreme heat from a soldering iron. The most common failure mode is epoxy yellowing/degradation from prolonged UV exposure, which filters out IR light and drastically reduces sensitivity, or junction leakage from ESD strikes.

Here is how to test a salvaged or suspect phototransistor using a standard digital multimeter (DMM). For deeper theory on semiconductor junction testing, refer to standard phototransistor testing guides.

Step-by-Step DMM Test:

  1. Set the Dial: Turn your multimeter to the Diode Test mode (the symbol with an arrow and a line).
  2. Dark Test (Reverse Bias): Cover the sensor with your finger or black tape to block all light. Place the Red probe on the Collector (short lead) and the Black probe on the Emitter (long lead). The meter should read OL (Over Limit) or > 2.0V, indicating the junction is blocking current.
  3. Light Test (Forward Bias via Photons): Keep the probes in the same position. Uncover the sensor and shine a bright white LED flashlight or your phone's camera flash directly into the dome. The voltage reading on the DMM should immediately drop to between 0.300V and 0.600V. This confirms the base-collector junction is generating photocurrent and forward-biasing the internal diode.
  4. Short Test: Swap the probes (Red to Emitter, Black to Collector). The meter should read OL regardless of light exposure. If it reads near 0.00V, the internal junction is shorted and the part is dead.
Bench Trick: If you don't have a flashlight handy, point a standard TV remote control at the sensor and press a button while watching the DMM. You will see the voltage rapidly flutter between the dark and light thresholds as the 38kHz IR pulses hit the junction.

Safe Default Part Numbers and Specs

Stop guessing with random clear LEDs pulled from broken toys. Use these verified, mass-produced phototransistors that have stable supply chains and comprehensive datasheets available from major distributors like Mouser and Digi-Key in 2026.

Reference Spec Sheet: Default Phototransistors
Parameter OSRAM SFH309FA (IR Default) Vishay TEPT5700 (Visible Default)
Peak Wavelength 900 nm (Infrared) 570 nm (Green/Yellow Visible)
Max Vce (Breakdown) 32 V 6 V
Dark Current (Iceo) < 200 nA < 50 nA
Collector Light Current 3.5 mA (typ at 1mW/cm²) 1.2 mA (typ at 100 lux)
Rise / Fall Time 15 µs / 15 µs 30 µs / 30 µs
Package 3mm T-1 (Clear/Black tint) 5mm T-1 3/4 (Clear epoxy)
Approx. Unit Cost $0.35 $0.42

For a complete breakdown of ambient light sensor spectral responses, consult the Vishay TEPT5700 datasheet, which provides excellent graphs mapping the sensor's sensitivity curve against the CIE standard human eye response curve.

Final Recommendation: If you are wiring up a breadboard prototype today and need a reliable light-sensing switch, buy the OSRAM SFH309FA. Pair it with a 10kΩ pull-down resistor and a 940nm IR LED, and you will have a robust, noise-immune optical interrupter that interfaces perfectly with 3.3V and 5V microcontrollers without needing additional op-amps or comparators.