What is a Capacitive Sensor : Working & Its Applications

A simple capacitive sensor has been available commercially for many years for the detection of nonmetallic objects, although they are restricted to short ranges, normally below 1 cm. Generally, a capacitor sensor is one type of proximity sensor used to detect nearby objects through their electrical field effect formed through the sensor. These types of sensors have some similarities with radar in their capacity to sense conductive materials while observing through insulating materials like plastic or wood.

The differences are significant as compared to radar. Capacitive sensors are smaller, simpler, less expensive & use less power. So, this article briefly explains a capacitive sensor and its works with applications.

What is a Capacitive Sensor?

An electronic device that is used to detect the targets like liquids or solids without any physical contact is known as a capacitive sensor. For detecting these targets, the capacitive sensor will produce an electrical field from the sensor’s detecting end. Any target that can interrupt this electrical field can be detected through this sensor.

Capacitive Sensor
Capacitive Sensor

The solid materials which can be detected by a capacitor sensor are paper, plastic, glass, cloth, and wood. The liquids which can be detected by a capacitor sensor are oil, paint, water, etc.

Working Principle of Capacitive Sensor

A capacitive sensor works like a normal capacitor. In this sensor, a metal plate within the sensing face is electrically connected to an oscillator circuit and the target which is detected can act as the next plate of the capacitor. Not like an inductive sensor that generates an electromagnetic field, a capacitive sensor generates an electrostatic field.

Electrostatic Field Generation
Electrostatic Field Generation

The block diagram of the capacitive sensor is shown below. The internal capacitive diagram is shown above. This sensor includes a high-frequency oscillator with a sensing surface that is formed through two metal electrodes. Once an object approaches close to the sensing surface, then it moves into the electrostatic field of the electrodes & changes the oscillator’s capacitance.

Capacitive Sensor Working
Capacitive Sensor Working

Consequently, the oscillator circuit will begin to oscillate & change the sensor’s output condition once it arrives at a certain amplitude. Once the target goes away from the capacitive sensor, the amplitude of the oscillator will reduce, switching the capacitive sensor back to its original position.
This sensor’s typical detecting range is about 1 inch or 25 mm whereas some sensors’ range is extended up to 2 inches.

These sensors detect the superior dielectric constant of an object simply. So, this makes achievable the detection of the material within nonmetallic containers because the dielectric constant of liquid is much higher as compared to the container. So this provides the sensor ability to observe throughout the container & detect the liquid. For better operation, they must utilize in a situation with a quite constant temperature & humidity.

Capacitive Sensor Formula

The capacitive sensor is one kind of device used for capacitive sensing. It is mainly based on the capacitive coupling principle. This sensor can simply detect & measure different things like motion, chemical composition, displacement, electric field & indirectly detect many other variables which can be changed into dielectric constant or motion like acceleration, pressure, fluid composition & fluid level.
A capacitance sensor includes two metal plates which are separated by a distance ‘d’ and area ‘A’. So the capacitance ‘C’ between two terminals can be given through the following expression.

C = ε0*εr*A/h

Where,

‘C’ is capacitance within Faradays

‘εr’ is Insulator’s relative dielectric constant

‘εo’ is dielectric constant for free space

‘A’ is the overlapping area of two plates

‘h’ is the width of the gap between two plates.

Types of Capacitive Sensors

Capacitive sensors offer non-contact target detection. These sensors not only detect the existence or nonexistence of a target; however, they can also detect pressure, flow, spacing, liquid level, etc. This can be simply done for different materials in different industries. There are different types of capacitive sensors which include the following.

Miniature

This type of capacitive sensor is available in a wafer or a cylindrical shape that can be placed in the most compact places. These sensors are mainly used to monitor and control, machine processes & work like detectors used for job counting. To provide the most excellent fit within tight spaces, small sensor heads need an external amplifier. So on this outside amplifier, the potentiometer lets for adjusting sensitivity.

Miniature Sensor
Miniature Sensor

Cylindrical

This cylindrical capacitive sensor is larger as compared to the miniature type sensors which range from ∅6.5 – M12 & M12 – M30. This sensor mainly comprises an adjustable sensing distance, range of housing diameters, and flush & non-flush mounting options. These sensors mainly provide level detection or contact-free proximity detection directly otherwise throughout a container wall.

Cylindrical Capacitor Sensor
Cylindrical Capacitor Sensor

High Temperature

High-temperature capacitive sensors are used where the sensor head is exposed to extreme temperatures. These sensors can still work even in direct contact through hot materials & temperatures to detect liquids temperature levels & bulk goods even in the most severe circumstances.

High Temperature Sensor
High Temperature Sensor

Analog Capacitive Sensor

The analog capacitive sensor simply works like typical capacitive sensors, although it includes different benefits based on how it is utilized. For example, these sensors are brilliant for the selection of material, monitoring of thickness & concentration difference as compared to other uses.

Analog Capacitive Sensor
Analog Capacitive Sensor

Mains-operated Capacitive Sensor Circuit

The Mains-operated Capacitive Sensor circuit diagram is shown below. This circuit can be built with electronic components like a capacitive sensor, R1= 220K, R2 = 47K, R3=1K, D1=TIC106M 600V 5A SCR, LP1is any small Neon Bulb, LP2 is 230V Lamp, BZ1 is 230V Buzzer (Optional), SPST SW1 (Optional) and PL1 is a Male Mains plug and cable.

This circuit works as a high-sensitivity capacitive sensor. Once a part of the human body comes near to the sensor then Buzzers & Lamps can be operated at half of the mains voltage supply. This circuit can be used as an alternative to the Door Alarm circuit or capacitive sensor circuit.

Capacitive Sensor Operated through Mains
Capacitive Sensor Operated through Mains

Once this circuit is used as a door alarm, then the Buzzer or the Lamps are triggered once someone touches the handle of the door from the exterior. The alarm is not self-latching, so the Buzzer/ Lamp will be deactivated once the hand will be removed from the door.

The wide-range sensitivity control ‘R2’ permits the use of the circuit over different types of doors, handles & locks. The device had proven consistent even once a fraction of the lock comes in contact with the wall but does not function through all-metal doors.

This circuit can be used as a simple touch control because it will work once the left-side terminal of ‘R1’ will be touched. In this situation, you can perform without the sensor plate. Alternatively, using a thin aluminum or copper sheet for measuring 30 x 20cm of the sensor, a fraction of the human body can be noticed at a 20cm distance.

This circuit was mainly designed for the operation of 230V AC. If 110 to 120V AC operation is required, then the ‘R1’ value must be altered with 100K. If this circuit does not function properly, try to overturn the mains plug into the socket, as the mains supply neutral wire should be connected to the cathode terminal of D1. Once the circuit is utilized as a door-handle alarm, then a better system will be done with R2 a trimmer otherwise potentiometer in the range of 5K to 10K.

Please note that the load will be driven with half the main voltage supply available. This should cause no problems, as the Lamp will produce sufficient light for signaling purposes and mains-operated Buzzers should work, only producing a bit less noise. The circuit is connected to 230V AC mains, keep away from the circuit once plugged & put it within a plastic box.

Capacitive Sensor vs Inductive Sensor

The difference between the capacitive sensor and inductive sensor includes the following.

Capacitive Sensor

Inductive Sensor

Capacitive sensors detect objects by using an electric field.  Inductive sensors detect objects by using a magnetic field.
In order to detect an object through the capacitive sensor, the object doesn’t need to be conductive.

 

In order to detect an object through an inductive sensor, the object must be conductive.
These sensors are used to detect both metals & non-conductive materials. This sensor is used to detect metal objects.
The working of capacitive sensors is, it measures changes within an electrical property known as capacitance. These sensors work on the change of inductance principle.
Capacitive sensors are available in four types miniature, cylindrical, high temperature &. analog Inductive sensors are available in three types High-frequency oscillation, magnetic and Electrostatic capacity.
It has a lower switching frequency. It has a higher switching frequency.
Accuracy is not good due to the possibility of false triggers. Very good accuracy.
The detection range of a capacitive sensor is from 2 to 50mm. The detection range of an inductive sensor is from 0.8 to 100mm.

Capacitive Sensor Interfacing with Arduino

We know that a capacitive sensor simply detects capacities changes within the surroundings. This sensor responds to different materials which contain certain conductivity like metals & liquids. Some types of sensors also have the chance to regulate the sensitivity. This can be possible through a button or a potentiometer.

This sensor is frequently used to detect water levels within tanks, leaks, or air bubbles within pipes. Additionally, it can also work as a touch sensor, metal detector otherwise like a door contact.

The interfacing of the MPR121 Capacitive Touch Sensor with Arduino is shown below. In this diagram, the MPR121 module is used as a capacitive touch sensor which is a high precision sensor.

This module includes 12 touch buttons & supports I2C communication so that it can be easily interfaced to any microcontroller. This board doesn’t include any regulator so the voltage supply must be in between 1.7 – 3.6VDC.

Pin Configuration of MPR121

The pinout of the MPR121 capacitive touch sensor includes the following. This sensor module includes 6 pins on the left which are discussed below and 12 pins (0 to 11) on the right are touch buttons.

MPR121 Sensor Module
MPR121 Sensor Module
  • VCC is the power supply of a module like 3.3V.
  • IRQ is an Interrupt output.
  • SCL is a serial CLK input for the I2C protocol.
  • SDA is a serial data I/O for the I2C protocol.
  • ADD is an adjusting address for the I2C protocol.
  • GND is a Ground terminal.

The required hardware and software components of this interfacing mainly include Arduino UNO R3, MPR121 proximity capacitive touch sensor module, male to female jumper wire & Arduino IDE. The following interfacing diagram will show how to connect Arduino Uno to the MPR121 module through connecting wires.

Capacitive Sensor Interfacing with Arduino
Capacitive Sensor Interfacing with Arduino

Once the circuit connections are made, Go to the Library manager & need to install the Adafruit MPR121 library. After that, need to upload the below code into the Arduino board then open the Serial Monitor.

/*
Modified on Jan 20, 2021
Modified by MehranMaleki from Arduino Examples
Home
*/
#include <Wire.h>
#include “Adafruit_MPR121.h”
#ifndef _BV
#define _BV(bit) (1 << (bit))
#endif
// You can have up to 4 on one i2c bus but one is enough for testing!
Adafruit_MPR121 cap = Adafruit_MPR121();
// Keeps track of the last pins touched
// so, we know when buttons are ‘released’
uint16_t lasttouched = 0;
uint16_t currtouched = 0;

void setup () {
Serial.begin(9600);
while (!Serial) { // needed to keep leonardo/micro from starting too fast!
delay(10);
}
Serial.println(“Adafruit MPR121 Capacitive Touch sensor test”);
// Default address is 0x5A, if tied to 3.3V its 0x5B
// If tied to SDA its 0x5C and if SCL then 0x5D
if (!cap.begin(0x5A)) {
Serial.println(“MPR121 not found, check wiring?”);
while (1);
}
Serial.println(“MPR121 found!”);
}
void loop() {
// Get the currently touched pads
currtouched = cap.touched();
for (uint8_t i = 0; i < 12; i++) {
// it if *is* touched and *wasnt* touched before, alert!
if ((currtouched & _BV(i)) && !(lasttouched & _BV(i)) ) {
Serial.print(i); Serial.print(” touched”);
}
// if it *was* touched and now *isnt*, alert!
if (!(currtouched & _BV(i)) && (lasttouched & _BV(i)) ) {
Serial.println(” released”);
}
}
// reset our state
lasttouched = currtouched;
// comment out this line for detailed data from the sensor!
return;
// debugging info, what
Serial.print(“ttttttttttttt 0x”); Serial.println(cap.touched(), HEX);
Serial.print(“Filt: “);
for (uint8_t i = 0; i < 12; i++) {
Serial.print(cap.filteredData(i)); Serial.print(“t”);
}
Serial.println();
Serial.print(“Base: “);
for (uint8_t i = 0; i < 12; i++) {
Serial.print(cap.baselineData(i)); Serial.print(“t”);
}
Serial.println();
// put a delay so it isn’t overwhelming
delay(1000);
}

First, this project code verifies whether the capacitive sensor module is connected properly or not to the Arduino Board. After that, it displays the touch & release of every key on the Serial Monitor.

Capacitive Sensor Characteristics

The characteristics of the capacitive sensor include the following.

  • These sensors can sense small or lightweight objects that cannot be noticed through mechanical limit switches.
  • These sensors provide a maximum switching rate for a quick response within object counting-based applications.
  • They can sense liquid targets throughout nonmetallic barriers like plastic, glass, etc.
  • Their operational life is long including an almost boundless no. of operating cycles.
  • The solid-state o/p generates a bounce-free contact signal.
  • Standoff or Range Distance: Usually these sensors have a larger sensing distance as compared to other sensors which range in between 5& 40 millimeters.
  • These sensors provide a combination of linearity, accuracy, stability, bandwidth & resolution which are better than conventional sensors like strain gauge & LVDTs.

Advantages

The advantages of the capacitive sensor include the following.

  • These sensors detect non-metallic targets.
  • They can detect by using certain types of containers
  • Construction is simple
  • A capacitive sensor is adjustable to different types of materials.
  • These sensors detect solid & liquid targets
  • Less cost
  • It has high sensitivity & can be operated through a small magnitude of energy.
  • It is used applicable in measuring pressure, humidity, force, etc.
  • It has a good frequency response & resolution (<0.003 mm)

Disadvantages

The drawbacks of the capacitive sensor include the following.

  • It is very responsive to changes within environmental conditions like humidity, temperature, etc so this will affect the performance.
  • The capacitance measurement is not easy as compared to resistance measurement.
  • These sensors are not accurate as compared to inductive-type sensors.

Applications

The applications of the capacitive sensor include the following.

  • Capacitive sensors detect as well as measure things that are conductive otherwise having dielectric except air.
  • These sensors are most frequently used to determine the change within a conductive target position. However, these types of sensors can also be very effective in measuring density, presence, thickness & non-conductors location. Non-conductive materials such as plastic have a dissimilar dielectric constant as compared to air.
  • Capacitive sensors are used to detect or measure position, proximity, acceleration, displacement, fluid level & humidity.
  • As input devices, capacitive sensing touch screens are used in mobile phones, tablets, digital audio players, etc
  • These sensors replace mechanical buttons.

Thus, this is all about an overview of a capacitive sensor. These sensors are frequently used to measure the change within a conductive target position. However, these can be very effective while measuring density, presence, thickness, and non-conductors location.  Here is a question for you, what is an inductive sensor?

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