Which of the following is an example of an external analog device?

• A. Relays
• B. Capacitors
• C. Thermistors
• D. Microprocessors

Answer: (C)

An external analog device is a sensor that provides a continuously variable signal representing a physical quantity to the control system. A thermistor fits this role because it is a temperature-dependent resistor. Its resistance changes smoothly as temperature changes, and when used in a circuit (for example, as part of a voltage divider feeding an ADC), it produces a voltage that varies continuously with temperature. That ongoing, proportional signal is what the controller uses to monitor temperature.

Relays are switches that respond to a digital control signal and operate in an on/off manner, not as sensors. Capacitors store energy or filter signals and don’t sense a physical quantity to produce a measurable, proportional signal. Microprocessors are digital controllers; they handle digital signals or require analog-to-digital conversion but are not themselves analog sensors.