Temperature and Humidity Sensors
Temperature and humidity sensors are usually designed in two major variations. In one, the sensor acts as a trigger or a switch when a certain level of temperature or humidity is reached, then the sensor is triggered and a specific circuit is activated. Other more complex versions are used to measure the actual momentary temperature or humidity in the ambient air. It does not have a trip point; it has a varying analog response to the level of those two atmospheric readings.
A simple temperature sensor relies on the metallic expansion principles of thermodynamics. As the temperature changes, the metal bends until contact is made or removed from a switching mechanism. This would imply the desired temperature has been reached. More complex temperature sensors, especially for electronic circuits, rely on the voltage drop across a transistor to determine the current temperature. Since the voltage delta of transistor devices is well known as a factor of temperature, it is very easy to determine what the temperature is by measuring base to emitter voltage.
Humidity sensors usually also have a way of measuring the temperature in the air since humidity is the relation of moisture in the air to the current air temperature. Humidity sensors typically rely on a capacitor to determine moisture content. The dielectric element between the two capacitive plates can collect water molecules and have the k-value adjusted according to a known pattern. This affects the voltage in a way that can be sensed and reported. Read more Read less