A photo transistor is essentially a standard bipolar junction transistor (BJT) where incident light generates the base current instead of an electrical connection. To use one as a basic optical switch, wire the collector to your positive supply (e.g., 5V) and the emitter to ground through a 10kΩ pull-down resistor; the voltage at the emitter junction will swing from near 0V in the dark to near VCC when illuminated. This simple configuration forms the backbone of optical encoders, beam-break sensors, and ambient light detectors.
The Photo Transistor at a Glance: Pinout, Symbol, and Safe Defaults
Before soldering anything to a breadboard, you need to identify the pins and select a part that matches your optical environment. The standard schematic symbol for an NPN photo transistor is identical to a standard BJT, but with two inward-pointing arrows striking the base junction, representing incident photons.
Pinout and Package Identification
Most hobbyist and prototyping photo transistors come in a 5mm T-1 3/4 clear or black epoxy package, or a TO-92 plastic package.
- Emitter (E): Typically the short lead, located on the side with the flat spot on the plastic lens rim. Connects to ground (or the load in a high-side configuration).
- Collector (C): Typically the long lead, on the rounded side of the lens rim. Connects to the positive supply.
- Base (B): If a third pin exists (common in TO-92 packages), it is the middle lead. In 95% of applications, leave the base pin unconnected. It is only used if you need to inject a small electrical bias current to shift the dark-current threshold, which is rarely necessary for modern high-gain devices.
Safe Default Part Numbers for Prototyping
Choosing the wrong wavelength is the most common reason a photo transistor circuit "fails" on the bench. A 940nm IR sensor will barely react to a 600nm red LED. Here are the reliable, high-gain defaults stocked by major distributors like Mouser and Digi-Key.
| Part Number | Manufacturer | Peak Wavelength | Vce (Max) | Ic (Max) | Package / Lens | Best Application |
|---|---|---|---|---|---|---|
| TEPT4400 | Vishay | 570 nm (Green/Yellow) | 50V | 20 mA | 5mm Clear | Ambient light sensing, visible laser tripwires |
| PT334-6C | Everlight | 940 nm (Infrared) | 30V | 50 mA | 5mm Black (IR pass) | IR beam breaks, object detection, encoder wheels |
| LTR-3208E | Lite-On | 940 nm (Infrared) | 30V | 100 mA | Sideways-looking Black | PCB edge detection, slot opto-interrupters |
| SFH 309 | ams OSRAM | 850 nm (Near IR) | 32V | 50 mA | 5mm Clear | Broad-spectrum IR, remote control receivers |
Sources: Vishay Optoelectronics, All About Circuits - Opto-electronic Devices
Biasing for the Job: Operation Regions and Application Circuits
Unlike a standard BJT where you use a voltage divider to set a fixed base current, a photo transistor is biased by setting the collector-emitter voltage and letting the light intensity dictate the base current ($I_B = I_{light}$). The load resistor ($R_L$) you place on the emitter (or collector) determines whether the device operates as a linear analog sensor or a digital on/off switch.
Operation Regions
| Region | Vce Voltage | Collector Current (Ic) | Primary Use Case |
|---|---|---|---|
| Cutoff (Dark) | $V_{CE} \approx V_{CC}$ | $I_{CEO}$ (Nanoamps of dark leakage) | Sensor is blocked; output reads LOW (or HIGH depending on topology). |
| Active (Linear) | $V_{CE} > 0.2V$ | $I_C = h_{FE} \times I_{light}$ | Light metering, analog ambient light sensors, camera exposure control. |
| Saturation (Switch) | $V_{CE(sat)} \approx 0.2V$ | Limited by external $R_L$ | Digital beam-breaks, optical encoders, RPM counting. |
Complete Application Circuit: 5V IR Beam-Break Switch
Let’s build a robust digital beam-break sensor to feed into a 5V microcontroller (like an Arduino Uno). We want the output to snap cleanly from 0V to 5V when the beam is unbroken, dropping to 0V when interrupted.
1. The Emitter (Light Source) Side:
- Component: TSAL6200 (940nm High-Power IR LED).
- Wiring: 5V → Current Limiting Resistor → IR LED Anode → IR LED Cathode → GND.
- Math: The TSAL6200 has a forward voltage ($V_f$) of 1.2V and handles up to 100mA continuous. We’ll target 38mA for a strong beam without thermal issues. $R = (5V - 1.2V) / 0.038A = 100\Omega$. Use a standard 100Ω 1/4W resistor.
2. The Receiver (Photo Transistor) Side:
- Component: PT334-6C (940nm IR Photo Transistor).
- Wiring: 5V → Collector. Emitter → Pull-down Resistor → GND. The microcontroller digital input connects to the Emitter/Resistor junction.
- Math: When the IR beam hits the PT334-6C, it saturates. The datasheet specifies a saturation voltage ($V_{CE(sat)}$) of roughly 0.2V at $I_C = 5mA$. To ensure we hit saturation without exceeding the 50mA max $I_C$, a 10kΩ pull-down resistor is ideal. $I_C = (5V - 0.2V) / 10,000\Omega = 0.48mA$. This is well within the active/saturation boundary for high-gain IR sensors under direct illumination, providing a clean logic HIGH (~4.8V) at the microcontroller pin. When the beam breaks, $I_C$ drops to zero, and the 10kΩ resistor pulls the pin cleanly to 0V.
Bench Testing: Failure Modes and Multimeter Verification
Photo transistors rarely suffer catastrophic thermal failure because their operating currents are so low (usually under 20mA). When they "fail" on the bench, it is almost always due to one of three things: wavelength mismatch (using a visible light sensor with an IR source), lead fatigue from bending the epoxy-encased legs too close to the body, or junction degradation from electrostatic discharge (ESD) during handling.
Here is how to verify a photo transistor on your workbench using a standard digital multimeter (DMM).
Step-by-Step Multimeter Testing
- The Dark Leakage Test (Diode Mode): Set your DMM to Diode Test mode. Place the red probe on the Collector and the black probe on the Emitter. Shield the sensor from ambient light with your hand. The meter should read "OL" (Open Loop) or show a voltage higher than 2.5V, indicating the junction is reverse-biased and blocking current. If it reads near 0.00V or beeps continuously in the dark, the junction is shorted; discard the part.
- The Illuminated Conduction Test: Keep the DMM in Diode Test mode with the probes in the same position (Red on C, Black on E). Shine a light source directly onto the lens. Critical: You must use a light source that matches the peak wavelength. If testing a 940nm IR sensor, a smartphone flashlight will not work. Point a TV remote control at the sensor and press a button, or use a dedicated 940nm IR LED. The DMM’s displayed voltage should drop significantly (often down to 0.4V - 1.2V) as the light generates base current and the DMM’s internal test voltage forward-biases the collector-emitter path.
- The hFE Socket Hack: Many bench DMMs have an hFE (transistor gain) socket. Plug the Collector and Emitter into the ‘C’ and ‘E’ holes of the NPN side, leaving the Base holes empty. In the dark, the meter will read 0 or a random low number. Shine your matched light source on the sensor. The meter will display a numerical hFE value (often between 100 and 800 for high-gain photo transistors). This confirms the device is amplifying the photoelectric base current correctly.
Troubleshooting a "Dead" Circuit
If your multimeter tests the component as good, but the breadboard circuit still reads 0V at the output, check your load resistor placement. If you accidentally placed the 10kΩ resistor on the Collector side (between VCC and the Collector) and tied the Emitter directly to ground, the output voltage at the Collector will invert: it will read 5V in the dark, and drop to ~0.2V when illuminated. While this is a valid "active-low" configuration, it will confuse your microcontroller code if you wrote it expecting an active-high signal. Always verify your topology against your firmware logic.
For deeper theory on semiconductor junctions and photon absorption, refer to the Electronics Tutorials guide on phototransistors, which details the internal gain mechanisms that separate these devices from standard photodiodes.
