#805 3DU5C Phototransistor

A quick review and test of the 3DU5C metal-encapsulated silicon phototransistor, used for light detection in the visible and infrared spectrum.

Build

Notes

The 3DU5C metal-encapsulated silicon phototransistor has a peak sensitivity at 880nm in the infrared spectrum, but it is relatively good sensitivity through the visible spectrum. This makes it very useful in a wide range of applications.

Specifications:

NPN Silicon Phototransistor
Model : 3DU5C
Working Voltage (max) : 10V
Reverse Breakdown Voltage : 15V
Dark Current : 0.3uA
Photocurrent : 0.5-1mA
Power Consumption : 30mW
Peak Wavelength : 880nm
Body Size : 7 x 5mm/ 0.28” x 0.2”(L*D)
Total Length : 28mm/ 1.1”
External Material : Metal
Weight : 3g

The package has equal length leads. The emitter is identified by the metal tab on the can.

3DU5C-package

Comparing Photodiodes and Phototransistors

See also The Art of Electronics 12.6.1 Photodiodes and phototransistors (3rd Edition).

Photodiodes and phototransistors are both light-sensitive semiconductor devices, but they behave quite differently.

Photodiodes:

A diode optimized to generate current when light hits its junction
Light creates electron–hole pairs → produces a small photocurrent.
Used in photoconductive mode (reverse-biased) or photovoltaic mode (like a tiny solar cell).
Characteristics:
- Fast response (nanoseconds to microseconds).
- Low gain — the photocurrent is small because there’s no internal amplification.
- Very linear and precise.
- Wide wavelength range (UV → visible → IR depending on material).
- Can be used for high-speed or analog light-measurement.
Common uses:
- Remote controls (as a raw sensor or inside modules).
- Optical communication receivers.
- Light meters / lux sensors.
- Fiber optics.

Phototransistors:

A transistor (BJT or sometimes photo-darlington) where light replaces the base current.
Light generates base current → transistor amplifies it → much larger collector current.
Characteristics:
- High gain — much more sensitive than a photodiode.
- Slower (microseconds to milliseconds) because transistor junctions store charge.
- More nonlinear behavior.
- Often packaged in IR receiver modules.
- Requires some biasing and has more electrical complexity.
Common uses:
- IR object detection.
- Slotted opto-switches / rotary encoders.
- Simple ambient light detection.
- Counter/frequency detection at modest speeds.

In summary:

Feature	Photodiode	Phototransistor
Sensitivity	Low	High (amplified)
Speed	Very fast	Slower
Linearity	Very good	Moderate
Noise	Lower	Higher
Output current	Small	Large (gain × photo-generated currents)
Best for	Precision & high-speed sensing	Easy, high-sensitivity detection

Test Circuit Design

This is a quick test of the sensitivity of the 3DU5C:

positioned on the high-side of a 3.3kΩ resistor, so voltage measured at the emitter will increase with brightness
a 3-wire voltmeter attached to monitor the voltage

Note: this is similar to the tests described in Make: More Electronics Experiment 4: Measuring Light

Designed with Fritzing: see 3DU5C.fzz.