Capacitive Sensor

Capacitive touch screen smartphones

Capacitive sensor - a parametric type converter in which a change in the measured value is converted into a change in the capacitance of the capacitor . ^[one]

A special circuit converts the change in capacitance into a threshold sensor signal (for example, dry contact ). In simplest sensors, this is usually a multivibrator , a frequency (or duty cycle) -voltage transducer, and a comparator . Sometimes, if the change in capacitance in response to the effect is small, it is necessary to install circuits on microcontrollers that are involved in auto-tuning the sensitivity and zero of the sensor.

Capacitive sensors are widely used where it is necessary to control the appearance of slightly conductive liquids, such as water. These are liquid level sensors, rain sensors in cars, sensors in touch buttons on household appliances (there is a lot of water in living tissues), etc.

There are also capacitive liquid level sensors that are widely used to measure the amount of fuel on aircraft . Typically, the sensor is a pair of metal cylinders inserted into each other (sometimes of complex shape, to ensure linearity of the sensor’s characteristics with a complex tank shape) immersed in fuel. The principle of operation of the sensors is based on the fact that the capacitance is directly proportional to the dielectric constant of the insulator, and ε is different for air and fuel (of the order of 1 and 1.8, respectively). As a result, when the tank is filled with fuel, the reactance of the sensor increases. Capacitive fuel meters are fed, as a rule, from the general network of the aircraft with a voltage of 115 V at a frequency of 400 Hz, which is lowered to power the sensors.

The main advantages of capacitive sensors: a high sensitivity threshold and low inertia. Main disadvantages: strong influence of external electromagnetic fields. ^[2]

A specific kind of sensors is capacitive touch screens .