Capacitive and Metallic Touch sensor interfacing with arduino uno

This tutorial is about interfacing capacitive and metallic touch sensors with arduino uno. Both the sensors performs the same function but their operating procedure/protocol is different. Touch sensors can be used instead of push buttons. The advantage is we don’t need to insert force in order to press a button. Even we can active a key with out touching it using a touch sensor, yes with out touching it. Touch sensing technology is becoming popular day by day. You might interact with it every day and every hour. Every mobile phone now a days has a touch screen or sensor. Their are many touch sensing protocols/technologies but for this tutorial we will see only the basic one. We will learn about the principle on which they operate and then we will interface the touch sensors with arduino uno. Lets begin with answering what’s the difference between capacitive and metallic arduino touch sensor?

Arduino capacitive touch sensor

Capacitive touch sensor as the name suggests that their is some thing capacitive and capacitive deals with capacitance or in easy words capacitor.

In capacitve sensing when we bring human body near to the sensor a capacitance is build between the sensor and human body. Human body(suppose the tip of finger) and sensor body behaves like the plates of the capacitor and a charge is induced between the plates. This charge is picked by the sensor and is translated as a button press. As we move our finger back from the sensor plate the capacitance is lost and circuit breaks.

The most popular arduino capacitve touch sensor available in the market is one based on TTP223 touch sensor ic. The equivalent circuit of the touch module is given in the below diagram. In the circuit of the TTP223 below if we bring our finger tip near to the touch pad our finger and touch pad builds a capacitor. This new capacitor is in parallel to capacitor C1. Recall capacitor in parallel increases capacitance. So the capacitance increases and a charged is induced in the capacitor. This induction is picked by the sensor as a button press.

Note: We don’t need to touch our finger tip to touch plate. Capacitor can be build by taking our finger near the touch plate. Since the distance needed to build capacitor is in nano meters so usually we touch our finger to the touch plate.

Capacitive touch sensor TTP223 circuit diagram

Arduino Metallic touch sensor

Arduino metallic touch sensor is based on mosfet. The gate of the mosfet is left open. Source of the mosfet is connected to an operational amplifier(LM393). When the gate of the mosfet is touched(finger touched) charge is induced in the gate of mosfet and it starts conducting. At the same time the operational amplifier inverting input threshold breaks up and operational amplifier also starts conducting. A variable resistor on the board is used to set the threshold. Schematic of the board is given below with board on the left.

Arduino metallic touch sensor equivalent circuit diagram

Arduino metallic touch sensor outputs an analog value as well as a digital signal. Analog value depends on the threshold set and the induced charge in the gate of mosfet. Digital however outputs “High” when the gate is touched. Normally it remains “Low“.

Capacitive touch sensor with arduino uno

Here is a practical example on how to interface touch sensor with arduino? In this example i will switch on a buzzer when touch sensor is touched. Buzzer will produce sound for one second and then switches off automatically. I am also printing a small message on serial monitor of arduino ide about the touch session.

Circuit diagram of the project is given below. Touch sensor signal pin is connected to arduino digital pin 2. Buzzer cathode is connected to digital pin 9 of arduino uno. Both the buzzer and capacitive touch sensor is powered through the 5 volt output of arduino regulator.

Capacitive touch sensor and buzzer with arduino

In the code above i first declared the arduino pin 2 and 9 as “Capsensor” and “Buzzer“. In the setup function i declared “Buzzer” as output and “Capsensor” as input. Then i started the serial monitor of arduino ide at 9600 baud rate. In the loop function the statement digitalRead(Capsensor) == HIGH checks the touch sensor signal pin. If found high the control goes in the if statement. Firstly it print a message “Sensor is touched” on serial monitor of arduino ide. To see the text open serial monitor(Tools->Serial monitor or CTRL+SHIFT+M) of arduino ide. Then the buzzer will be switched on. A delay of 1 second keeps the buzzer in buzz state for 1 second. After 1 second the control jumps out of the if statement and buzzer is turned off. The loop function runs for infinite amount of time so when ever the sensor pad is touched a sound is produced for 1 second.

Metallic touch sensor with arduino uno

Metallic sensor can be interfaced with arduino in two different ways either digitally or in analog form. For digital interface with arduino their is noting to be changed. The digital interface circuit diagram and code is given below. The digital interface is similar to capacitive touch sensor.

Metallic touch sensor interfaced digitally with arduino

Metallic touch sensor with arduino

Metallic touch sensor interfaced analog with arduino

In the analog interface their is some change in the circuit diagram. I used arduino analog channel 0 to measure the analog reading. Depending on the analog reading now the buzzer will buzz.

In the above code if the sensor is touched then the buzzer will buzz. Buzzer will buzz depending on the value of the analog reading. The delay instruction is changed accordingly. A constant number 5 is multiplied to the sensor analog reading in order to buzz the buzzer for time which is differential-able.

Project codes