A phototransistor is defined as what?

• A. Photocell used to sense the presence of light
• B. Device that senses temperature changes
• C. Device converting light to sound
• D. Light-emitting diode used for signaling

Answer: (A)

A phototransistor is a transistor whose operation is controlled by light. When light shines on it, photons create charge carriers in the base region, generating a base current that modulates the collector-emitter current. This makes the device function as a light sensor with amplification—the output current increases with light intensity, giving a stronger electrical signal than a simple light-sensitive resistor would. It’s not about sensing temperature, nor about converting light to sound, nor about emitting light like an LED.