Phototransistor operation — control mechanism: A phototransistor’s conduction, and thus its output state in interfacing circuits, is governed by the presence or absence (and intensity) of incident light acting as an effective base drive. Evaluate this statement’s accuracy.

Introduction / Context:
Opto-sensors such as phototransistors are common in digital input conditioning, opto-isolators, and light detection. Understanding how light influences their conduction clarifies why pull-ups, biasing, and thresholding are arranged as they are in interface circuits.

Given Data / Assumptions:

• A phototransistor responds to photons that generate carriers, effectively providing base current.
• Many phototransistors omit a discrete base lead; light acts as the stimulus.
• External biasing and load resistors translate collector current into a voltage.

Concept / Approach:
Incident light increases the effective base drive, raising collector current and changing the voltage across the load. In common-emitter configurations with a pull-up, more light typically pulls the output node LOW (more current through the transistor), while in other topologies the logic polarity can be inverted. The essence remains: light presence/absence controls conduction.

Step-by-Step Solution:
Model the device as a BJT with optically induced base current.More light → more carriers → higher collector current.Translate current to voltage with the load to obtain a logic-level signal.Set thresholds so ambient variations do not cause false triggers.

Verification / Alternative check:
Datasheets specify “collector current vs irradiance” curves confirming monotonic response to light intensity.

Why Other Options Are Wrong:
Requiring an external base lead is unnecessary; many packages are base-less. Photodiodes differ in operation (reverse-biased junction current) though both are light-sensitive; the statement correctly concerns phototransistors.

Common Pitfalls:
Ignoring ambient light; failing to add hysteresis; misinterpreting logic polarity based on wiring configuration.

Final Answer:
Correct