1.5 Passive and active sensors. A passive sensor is one that does not require external power, whereas an active sensor requires external power. Identify which of the following sensors are active and which are passive.
The physical stimulus (heat) of the measurand (temperature) provides the input power.
"With increasing temperature, the volume of liquid expands and the meniscus moves up the capillary. The position of the meniscus shows the temperature against an inscribed scale."
(From Alcohol thermometer - Wikipedia, the free encyclopedia.)
The thermostat in a liquid-cooled car engine blocks the flow of coolant to the radiator until the engine has warmed up. When the engine is cold, no coolant flows through the engine. By letting the engine warm up as quickly as possible, the thermostat reduces engine wear, deposits and emissions.
As in the case of the thermometer, the physical stimulus provides the input power.
The secret of the thermostat lies in the small cylinder located on the engine-side of the device. This cylinder is filled with a wax that begins to melt at perhaps 180 degrees F (different thermostats open at different temperatures, but 180 F/82 C is a common temperature). A rod connected to the valve presses into this wax. When the wax melts, it expands significantly and pushes the rod out of the cylinder, opening the valve.
The way a typical aquarium pH meter works is by using a pH Sensing Electrode which when introduced in the water creates a difference in the electric potential from the inside of the glass to the water with high or low pH (depending on the amount of ions in the water). This difference in potentials may be measured by a voltmeter. (Aquarium Chemistry: Measuring pH with a Meter.)
The physical stimulus, low pressure inside the jar and high pressure outside, bends the lid toward the inside. No external power required.
Microphones in cell phones are typically carbon microphones. They change their resistance with the pressure wave (sound). They need and external power supply.
The water level in the tub lifts a small inverted cup (the float) attached to a post that passes through the bottom of the dishwasher tank to operate a switch. When the float has raised enough the switch below opens, breaking electrical contact to the fill valve, stopping water from entering the dishwasher's tank and preventing the dishwasher from overfilling.
(From How Dishwashers Work. -Troubleshooting Dishwasher Problems-.)
No external power required.
Most temperature sensors inside refrigerator are actually thermistors, a device that changes its conductivity (or the inverse resistance) with temperature. It does require external power. (Testing A Refrigerator Thermistor On A GE Refrigerator.)
These can be active or passive depending on the design. For example, an accelerometer based on a piezoelectric crystal generates its own electric voltage as output. In the case of a piezoresistive accelerometer the measurement needs an external power supply. (Accelerometers.)