The phototransistor market in 2026 has shifted heavily toward surface-mount (SMD) packages and infrared (IR) optimized silicon, driven by automotive proximity sensors and consumer electronics. However, through-hole staples remain widely available for prototyping and repair. If you need a safe default right now, buy the Everlight PT334-6C (through-hole, ~$0.15) or the Lite-On LTR-3208M (SMD, ~$0.10). This guide cuts through the supply chain noise to show you exactly how to select, bias, and test these components on your workbench.
The 2026 Phototransistor Market: What's Available and What to Buy
Over the last few years, the phototransistor market has commoditized 940nm IR sensors while visible-light sensors have become a low-volume, higher-cost niche. When you search distributors like DigiKey's phototransistor category, you will see thousands of SKUs. Most are just minor variations in lens tint, package height, or sensitivity bins. For 95% of DIY, Arduino, and industrial replacement projects, you only need to know four default part numbers.
| Part Number | Manufacturer | Package | Peak Wavelength | V_CE (max) | I_C (max) | Typ. Price (2026) |
|---|---|---|---|---|---|---|
| PT334-6C | Everlight | 3mm T-1 (Through-hole) | 940nm (IR) | 30V | 50mA | $0.12 |
| LTR-3208M | Lite-On | 1206 SMD | 940nm (IR) | 30V | 20mA | $0.08 |
| SFH309FA | Osram | 3mm T-1 (Through-hole) | 900nm (IR) | 32V | 50mA | $0.25 |
| TEPT5600 | Vishay | 3mm T-1 (Through-hole) | 570nm (Visible) | 6V | 20mA | $0.45 |
Pinouts, Symbols, and Operation Regions
On a schematic, a phototransistor is drawn as a standard NPN bipolar junction transistor (BJT) with an inward-pointing arrow representing light hitting the base-collector junction. Crucially, there is usually no electrical base lead; the base is floating, and current is generated entirely by photon absorption.
Think of the base-collector junction like a toll booth where photons act as the toll collectors opening the gate for electron traffic. The more light (photons) that hits the junction, the wider the gate opens, allowing current to flow from Collector to Emitter.
Identifying the Pins
For standard 3mm and 5mm through-hole packages, the pinout is almost universal:
- Collector (C): The longer lead. Connects to the positive supply via a load resistor.
- Emitter (E): The shorter lead, usually aligned with the flat spot on the plastic lens rim. Connects to Ground.
Operation Regions Table
Unlike standard BJTs where you bias the base with a voltage divider, a phototransistor's operation region is dictated by the ambient light intensity and your external load resistor.
| Region | Light Condition | V_CE Voltage | Collector Current (I_C) | Circuit Behavior |
|---|---|---|---|---|
| Cutoff (Dark) | No light / Blocked | Equals VCC | I_CEO (Leakage, ~100nA) | Acts as an open switch. Output is HIGH. |
| Active (Linear) | Moderate light | 0.5V to VCC-1V | Proportional to irradiance (µA to low mA) | Acts as a variable current source. Used for analog light metering. |
| Saturation | Bright / Direct beam | < 0.3V (V_CE sat) | Limited by external load resistor | Acts as a closed switch. Output is LOW. |
How to Bias and Select a Phototransistor for Your Circuit
The most common and reliable way to use a phototransistor is the Common Emitter configuration with a pull-up load resistor. This setup converts the light-dependent current into a voltage that a microcontroller GPIO or comparator can read.
Complete Application Circuit: 5V Digital Break-Beam Sensor
This circuit interfaces a 940nm IR phototransistor with a 5V Arduino or ESP32 GPIO pin to detect when an object breaks an IR beam.
- U1 (Phototransistor): Everlight PT334-6C
- VCC: 5.0V (from microcontroller 5V rail)
- R1 (Load Resistor): 10kΩ (0.25W, 1% tolerance)
- C1 (Decoupling): 100nF ceramic capacitor (placed between GPIO pin and GND to filter high-frequency EMI)
- IR Emitter: 940nm IR LED driven at 20mA (aimed directly at U1)
Wiring Steps:
- Connect the Emitter (short lead/flat spot) of U1 directly to system GND.
- Connect the Collector (long lead) of U1 to the GPIO input pin.
- Connect R1 (10kΩ) between the 5V VCC rail and the Collector of U1.
- Place C1 (100nF) in parallel with the Collector-GND path to smooth out fluorescent light ripple.
Sizing the Load Resistor (R1)
The value of R1 determines whether your circuit acts as a digital switch or an analog sensor. According to Vishay's optoelectronic application notes, the voltage at the collector is calculated as: V_out = VCC - (I_C × R1).
- For Digital Switching (10kΩ): In bright IR light, the PT334-6C might pass 2mA.
5V - (0.002A × 10,000Ω) = -15V. Since the voltage cannot go below ground, the transistor hits saturation, and V_out clamps at ~0.2V (a solid logic LOW). In the dark, I_C is near zero, so V_out stays at 5V (logic HIGH). - For Analog Metering (100kΩ or higher): A larger resistor increases the voltage swing for very low light levels, keeping the transistor in the Active region longer. Use this if you are feeding the signal into an ADC to measure ambient room brightness.
Failure Modes and Multimeter Testing
Phototransistors are rugged, but they do fail on the bench. The most common failure modes are ESD junction punch-through (caused by handling without a wrist strap in dry environments) and lens yellowing (UV degradation from prolonged outdoor sun exposure, which blocks 940nm IR light).
Because they lack a base pin, you cannot test them like a standard BJT. Here is the definitive multimeter test sequence.
Step-by-Step Multimeter Test
- Set your DMM to Diode Test mode. This mode outputs a small current and measures the forward voltage drop.
- Dark Test: Place the Red probe on the Collector (long lead) and the Black probe on the Emitter (short lead). Cover the lens with your finger. The meter should read OL (Open Loop) or a very high voltage, indicating the junction is reverse-biased and blocking current.
- Light Test: Keep the probes in the same position. Shine a 940nm IR flashlight or press a TV remote control button directly against the lens. The meter reading should instantly drop to 0.2V - 0.5V, indicating the junction has turned on and is conducting.
- Reverse Bias Check: Swap the probes (Red to Emitter, Black to Collector). The meter must read OL regardless of whether light is applied. If it conducts in reverse, the junction is shorted and the part is dead.
Market Shifts: When to Ditch Phototransistors for Alternatives
While the phototransistor market remains robust for simple break-beam and proximity tasks, modern designs often require higher speed or better ambient light rejection. If your application is failing due to slow response times or 60Hz/120Hz flicker from overhead LEDs, it is time to look at alternatives.
| Feature | Phototransistor (e.g., PT334-6C) | Photodiode (e.g., BPV10NF) | Digital I2C Sensor (e.g., VEML7700) |
|---|---|---|---|
| Response Time | Slow (~100µs) | Extremely Fast (~5ns) | Moderate (Integration time dependent) |
| Output Type | Analog Current | Analog Current (Requires Op-Amp) | Digital I2C (Lux / White channel) |
| 50/60Hz Flicker Rejection | Poor (Requires hardware filtering) | Poor (Requires complex TIA filtering) | Excellent (Built-in integration) |
| Best Use Case | Object detection, limit switches | IR data comms, high-speed counting | Display backlighting, smart home lux |
Choose the phototransistor when you need a cheap, high-gain switch that can drive a GPIO directly without an op-amp. Choose a photodiode when you are building an IR data receiver (like IrDA) where microsecond timing matters. Choose a digital I2C sensor when you need accurate, human-eye-weighted Lux measurements for automatic display dimming.
