A phototransistor is fundamentally a bipolar junction transistor (BJT) where the base current is generated by incident photons rather than an electrical connection. While beginners often treat them as simple light-activated switches, on the bench they behave as light-controlled current sources. Understanding this distinction is the key to designing stable, noise-immune optical sensors for encoders, ambient light tracking, and object detection.
If you need a quick answer for a prototype: the Vishay TEPT5700 is the safest default for visible light (green/yellow spectrum), while the Everlight PT333-3C is the standard workhorse for 940nm infrared (IR) remote and interrupter circuits. Below is the complete breakdown of how to select, bias, and verify these components without relying on trial and error.
Safe Default Part Numbers and Selection Data
Selecting the right phototransistor requires matching the peak sensitivity wavelength to your light source (usually an LED or ambient room light) and ensuring the collector-emitter breakdown voltage ($V_{CEO}$) exceeds your supply rail. The table below lists the most reliable, widely available through-hole defaults for 2026 bench work.
| Part Number | Manufacturer | Peak Wavelength | Max $I_C$ (mA) | Max $V_{CEO}$ (V) | Typical Price (USD) |
|---|---|---|---|---|---|
| TEPT5700 | Vishay | 570 nm (Visible Green) | 20 | 6 | $0.45 |
| PT333-3C | Everlight | 940 nm (Infrared) | 20 | 30 | $0.12 |
| L-53P3C | Kingbright | 940 nm (Infrared) | 20 | 30 | $0.15 |
| SFH309FA | OSRAM | 900 nm (Near IR) | 50 | 32 | $0.38 |
Symbol, Pinout, and Operation Regions
The schematic symbol for an NPN phototransistor is identical to a standard BJT, but the base lead is omitted, and two arrows point inward toward the base-collector junction to indicate incident light. Physically, most hobbyist phototransistors come in standard 3mm or 5mm (T-1 3/4) epoxy packages that look exactly like LEDs.
Pinout Identification: In a standard 2-pin package, the Collector is the longer lead, and the Emitter is the shorter lead. The plastic rim of the package will have a flat edge adjacent to the Emitter pin. If you are using a 3-pin package (where the base is brought out for optional electrical biasing), the pins are typically Emitter, Collector, Base when viewed from the bottom with the flat edge facing you.
Unlike a standard BJT where you force a base current, a phototransistor's operation is governed by the light intensity hitting the die. Here is how the device behaves across its three primary regions:
| Operation Region | Light Condition | Typical $V_{CE}$ | Typical $I_C$ | Circuit Behavior |
|---|---|---|---|---|
| Cutoff (Dark) | No illumination | $\approx V_{CC}$ (Supply) | $< 100$ nA (Dark Current) | Acts as an open switch; only leakage current flows. |
| Active (Linear) | Moderate / Variable | $0.7V < V_{CE} < V_{CC}$ | $1 \mu A$ to $5$ mA | Current is proportional to light intensity. Used for analog sensing. |
| Saturation | Intense / Direct | $< 0.4V$ ($V_{CE(sat)}$) | Limited by external load | Acts as a closed switch. Used for digital encoders and interrupts. |
Biasing and Application Circuit Design
Because a phototransistor outputs a current ($I_{ph}$) proportional to irradiance, you must convert that current into a measurable voltage. This is done using a load resistor ($R_L$). The two standard biasing configurations are Common-Emitter (pull-up resistor, output taken from Collector) and Common-Collector (pull-down resistor, output taken from Emitter). For microcontroller ADC interfacing, the Common-Collector (pull-down) configuration is generally preferred because the output voltage rises with increasing light, which is more intuitive to debug.
Complete Application Circuit: Ambient Light Sensor
This circuit interfaces a Vishay TEPT5700 to a 5V microcontroller ADC (like an Arduino Uno or ESP32 via a voltage divider if needed, though the TEPT5700 maxes out around 6V so keep $V_{CC}$ at 5V or 3.3V).
- U1: TEPT5700 Phototransistor
- $V_{CC}$: 5.0V DC
- R1 (Load Resistor): $10k\Omega$ (for normal room light) or $100k\Omega$ (for dark environments)
- C1 (Bypass Capacitor): 100nF ceramic (placed in parallel with R1)
Wiring Steps:
- Connect the Collector (long lead) of the TEPT5700 directly to the 5V $V_{CC}$ rail.
- Connect the Emitter (short lead/flat edge) to one side of the $10k\Omega$ resistor (R1).
- Connect the other side of R1 to Ground (GND).
- Place the 100nF capacitor (C1) across the two leads of R1 to form a low-pass filter. This shunts 100Hz/120Hz flicker from AC-powered room lights away from your ADC.
- Route the junction between the Emitter and R1 to your microcontroller's ADC pin.
Failure Modes and Multimeter Testing
Phototransistors are rugged, but they do fail. The most common failure mode on the bench is mechanical lead shear from bending the leads too close to the epoxy body. Electrically, they can suffer from IR degradation if exposed to intense UV light or sustained high-temperature soldering profiles, which permanently lowers their responsivity. Unlike standard BJTs, they rarely suffer thermal runaway because the base current is limited by the external light source, but shorting the collector directly to $V_{CC}$ without a load resistor while under intense illumination can exceed the maximum power dissipation ($P_{tot}$) and melt the die bond.
How to Test a Phototransistor with a DMM
You can verify a phototransistor in-circuit or out-of-circuit using a standard digital multimeter (DMM) in Diode Test mode. You will need a bright flashlight or your phone's screen to stimulate the die.
- Set your DMM to Diode Mode. Ensure the probes are in the correct COM and V/$\Omega$ ports.
- Dark Test (Reverse Bias): Place the Red probe on the Emitter (short lead) and the Black probe on the Collector (long lead). Cover the sensor with your finger to block all light. The meter should read 'OL' (Open Loop) or a very high voltage, indicating the junction is reverse-biased and blocking current.
- Light Test (Forward Active): Keep the probes in the same position. Shine a bright white light directly into the top of the epoxy dome. The meter's internal test voltage will forward-bias the junction as the light generates base carriers. The reading should drop from 'OL' down to a typical diode drop, usually between 0.400V and 0.650V, depending on light intensity.
- Reverse Probe Check: Swap the probes (Red to Collector, Black to Emitter). The meter should read 'OL' regardless of whether light is shining on it or not. If it reads a short (0.00V) or a low resistance in both directions, the internal junction has shorted and the part is dead.
Phototransistors vs. Photodiodes vs. LDRs
Choosing the right optical sensor depends entirely on your speed and gain requirements. According to fundamental optoelectronics principles outlined by Electronics Tutorials, the internal gain mechanism of the phototransistor makes it vastly different from its counterparts.
| Criterion | Phototransistor | Photodiode | LDR (Photoresistor) |
|---|---|---|---|
| Internal Gain | High ($100\times - 1000\times$) | None ($1\times$) | N/A (Variable Resistor) |
| Switching Speed | Slow ($5 \mu s - 50 \mu s$) | Ultra-Fast ($< 10 ns$) | Very Slow ($20 ms - 50 ms$) |
| Linearity | Poor (Logarithmic curve) | Excellent (Strictly linear) | Poor (Inverse logarithmic) |
| Best Use Case | Object detection, IR remotes, encoders | Fiber optics, high-speed data, precision lux meters | Simple night-lights, basic auto-dimming |
| Typical Cost | $0.10 - $0.50 | $0.50 - $3.00+ | $0.05 - $0.20 |
For further reading on component selection and spectral matching, the Vishay Optical Sensors portal provides excellent application notes on matching IR emitters to specific silicon detector geometries. If you need to measure high-speed optical pulses (like a 1Mbps fiber optic link), abandon the phototransistor and use a PIN photodiode with a transimpedance amplifier. But for 95% of maker, robotics, and home-automation optical sensing tasks, a properly biased phototransistor with a bypass capacitor is the most cost-effective and robust solution available.
