Temperature alarm circuits – which basic sensor is typically used? In practical electronic temperature alarm circuits (e.g., overheat warning), which component is most commonly employed as the temperature-sensing element?

Introduction / Context:
Temperature alarm circuits require a sensor whose electrical property varies predictably with temperature and is easy to interface. The classic, low-cost choice in many appliances and industrial panels is the thermistor, especially the negative-temperature-coefficient (NTC) type.

Given Data / Assumptions:

• Moderate temperature range near ambient to a few hundred °C.
• Low-cost analog circuits using comparators or microcontrollers.
• Need for large, monotonic resistance change with temperature.

Concept / Approach:

NTC thermistors exhibit a steep decrease in resistance with increasing temperature, modeled by R(T) ≈ R0 exp(β(1/T − 1/T0)). This yields strong sensitivity for threshold detection. They can be placed in a voltage divider feeding a comparator to trigger alarms. Other components listed do not primarily sense temperature: photoconductors respond to light; varistors respond to voltage surges; piezo materials sense stress; a plain transistor can be used as a temperature sensor but is not the common, standalone choice in simple alarm circuits compared to a dedicated thermistor.

Step-by-Step Solution:

Verification / Alternative check:

Household thermostats, battery packs, and HVAC safety circuits frequently employ NTC thermistors for both monitoring and alarm due to their sensitivity and low cost.

Why Other Options Are Wrong:

Photoconductor senses light; varistor is for surge protection; transistor usually needs linearization and biasing and is less common in off-the-shelf alarm modules.

Common Pitfalls:

Ignoring self-heating of thermistors at high currents; use small sense currents and proper thermal placement.

Final Answer:

Thermistor