Definition
A three-terminal semiconductor device that conducts current between its emitter and collector when light strikes its base region. The amount of current that flows is controlled by the intensity of the light falling on the device, allowing it to act as both a light sensor and an amplifier in a single component.
Plain English
An electronic part that turns light into an electrical signal. The brighter the light hitting it, the more current it lets through.
Context Anchor
Seen in aircraft electrical and electronic system discussions, especially where a circuit needs to detect light without a mechanical switch.
Derivation
From 'photo' (Greek phos, meaning light) and 'transistor' (a contraction of 'transfer resistor'). The name signals that it works like a transistor, but uses light instead of an electrical signal at the base to control current flow.
Why Pilots Care
Pilots rarely interact with phototransistors directly, but they sit inside the equipment that triggers smoke alarms, reads switch positions, and senses motion in autopilots and instruments. When these components fail, the symptom is a faulty warning or sensor reading.
Intuition Check
"Photo" does not mean a photograph here. It means light is what makes the electronic part respond.
Example Sentence 1
The cargo compartment smoke detector uses a phototransistor to sense when smoke particles scatter light inside the sensing chamber.
Example Sentence 2
During preflight checks the avionics technician verified that the phototransistor in the wingtip recognition light sensor responded correctly to daylight.