SensingBench electronicsTest and identify

Phototransistor

Also called photodiode, IR detector, beam sensor.

The receive half of a garage-door safety beam, an optical interrupter, a printer encoder, or a sunlight sensor. A phototransistor leaks more current as light rises. It is not the same part as a 38 kHz remote receiver.

If you are new to this category

A sensor reports a condition such as weight, light, temperature, or position. Many sensors do not switch fully on or off. A kitchen scale bar, for example, changes by thousandths of a volt. Keep the small board that read the sensor if you still have it.

Keep the connector. Write down the wire colors before you cut them. If you do not know which wire is which, the sensor is hard to reuse.

Four wires on a scale sensor

Red and black feed the sensor a steady few volts. Green and white are the tiny signal, measured between them, not from either one to ground. Pressing the bar changes that signal by millivolts. A normal meter on DC volts can show that it responds. Turning it into a number on a screen wants a small amplifier board, often an HX711.

Specification. The right-hand column is what is typical. The label on the part in your hand wins if they disagree.
What to look for	Usual figure
Garage beam	A paired emitter and detector, often 12–24 V, polarized, with a lens. Keep the pair.
Bare part	Two leads, collector and emitter. Polarity matters.

Volts are electrical pressure, amps are how much current is flowing, and ohms are how hard the part resists that flow. A higher voltage than the part was built for is how salvage parts die on the bench. If a cell in the table says “read the stamp”, that stamp is the whole specification.

What people actually use it for

A beam break
A crude light meter
The original safety beam, reused as a safety beam

Leave this one when

You needed remote-control decoding. Use the demodulating receiver.

Buying a new one is the better idea when

A documented phototransistor when gain matters.

How to tell if this one still works

Reverse the thought: diode test one way, then watch resistance or current change under a lamp versus a hand over it.

Tools you will want: Meter, Lamp.

Words on this page

Short meanings, so the rest of the entry is readable if you are not already doing this every weekend.

polarity: Which side is plus and which is minus. Reversed polarity ruins a lot of electronics. The symbol on a power brick is the map.

Where people actually find it

2 notes, most useful first. “Leave it” means the part is in that machine and you should not take it: it is dangerous, worn out, or only useful if you already know that exact repair.

Harvest at a glance
From	Call	How many	How often	Where it sits
Garage door opener	Prize	1	usually	The safety beam, one side.
Photoelectric smoke alarm	Bonus	1	usually	Opposite the LED.

Prize — take this firstGarage door opener1 · usually
In this particular machine
Paired with an IR LED on the other side. Polarized, often 12–24 V.
Where to look
The safety beam, one side.
How to get it out
Keep the pair and the brackets.
The condition to expect
Aligned ones are good. Yellowed lenses clean.
Bonus, if you are already in therePhotoelectric smoke alarm1 · usually
In this particular machine
The receiver.
Where to look
Opposite the LED.
How to get it out
Small.
The condition to expect
Good.