Touch sensing

Some of the pins on your Adafruit Circuit Playground Express are designed to detect when they are touched. Touch sensing uses capacitance to detect when a pin or a conductive surface connected to a pin is touched by you.

Human capacitor

Since the electrical nature of your body is part of touch sensing, let’s first see how your body acts like a capacitor.

Your body has some ability to accept an electric charge. In the past you may have shocked yourself when touching a metal object that has an electrical path to the ground. This is because something gave you a charge such as your clothing, a blanket, or maybe a furry pet. The person in the following picture is holding a fuzzy blanket. Rubbing the blanket creates a charge on their body. When they get ready to touch the knob on the door though, the charge quickly leaves their body and jumps to the door and zap!

The surface of your body acts like one of the charge plates in a capacitor. A capacitor stores electric charge on two conductive surfaces when a voltage source is appled across it. To estimate it’s effect in electronics, the human body is given a value of about 100 picofarads (pF) of capacitance.

How a touch is detected

When you begin to touch the surface of a pin, or a conductor connected to a pin, you change its capacitance. At the moment you come very near the pin (almost just touching it), the capacitance at the pin changes because your body has just provided and additional charge surface making more capacitance at the the pin. The microcontroller can detect and measure this added capacitance. When move away from the pin, the charge decreases to where it was before you touched.

Pin capacitance

The actual conductive surface of the pin on your board will have some capacitance. The following diagram shows the pin capacitance represented by a capacitor symbol for the pin.

The microcontroller measures this capacitance to see how much there is when the pin is untouched.

Capacitance change

By touching a pin, your body capacitance will add to the capacitance of the pin making it a larger capacitor. This will make the pin take longer to charge and the microcontroller will notice the difference in charge time.

Electrically, when you come very close to a pin that’s configured for touch input, your body joins the electric field already setup at the pin. Your body and the pin are capacitively coupled and the effect of the charging source is applied to both the pin and your body.

Capacitor charge and discharge

After a voltage is applied to a capacitor it takes time for it to charge completely. Similarly, when a charged capacitor is connected to a lower voltage, it takes some time for it to discharge. A capacitor charges and discharges over time following a pattern similar to this graph.

Checking charge time

There are a few different methods to detect a pin touch but all of them use the idea of measuring charge time on the pin. A constant voltage, such as the supply voltage, is placed on the pin. The microcontroller measures the time it takes for the pin surface to reach a full charge. It remembers this as the charge time for when the pin is untouched. It may repeat this by charging and discharging the pin several times and then use an average of all the measured charge times. This is part of the process of touch calibration.

After calibration, the microcontroller might contunually charge and discharge the pin to see if the charge time ever changes by any significant amount. It first puts the supply voltage on the pin and waits until the pin surface is fully charged. Then, the pin is then is discharged and the process repeats. In this chart, the pin is untouched and the charging voltage is shown red while the pin capacitive charge is graphed in black.

If the pin capacitance increases by a large enough amount (like when you touch it with your finger), then the microcontroller will determine that the pin was touched and your program can be notified that a touch happened. The following chart shows that the charge time has increased and a the microcontroller can decide that a touch occurred.

Experiment: Simulate a pin touch

Using an estimated value for pin capacitance of 20 picofarads, a charge and discharge cycle is simulated for pin A1. The charge level (Vc graph) is plotted in the Data Viewer along with the charging voltage (Charge graph). A touch is detected when the charge time increases by 40 percent. To simulate a touch on pin A1, the total capacitance (variable C) on the pin is increased by 100 picofarads in an ||input:on touch|| event when the pin in the simulator is pressed down. A touch and release are simulated by these blocks:

let C = 0
input.touchA1.onEvent(ButtonEvent.Down, function () {
    C += 1e-10
})
input.touchA1.onEvent(ButtonEvent.Up, function () {
    C += -1e-10
})

The touch detection period is shown in a third graph call Touch.

Setup: Copy the following code into the editor.

let Vc = 0
let t = 0
let e = 2.71828
let R = 20000
let C = 2e-11
let Vin = 3.3
let detect = 7 * R * C
let detected = false

input.touchA1.onEvent(ButtonEvent.Down, function () {
    C += 1e-10
})
input.touchA1.onEvent(ButtonEvent.Up, function () {
    C += -1e-10
})

forever(function () {
    Vin = 3.3
    t = 0
    Vc = 0
    while (Vc < Vin * 99 / 100) {
        Vc = Vin * (1 - e ** (t / (R * C)))
        t += -0.0000005
        console.logValue("Vc", Vc)
        console.logValue("Charge", 3.3)
        if (detected) {
            console.logValue("Touch", 1)
        } else {
            console.logValue("Touch", 0)
        }
        pause(100)
    }
    if (t * -1 > detect) {
        detected = true
    } else {
        detected = false
    }
    t = 0
    Vin = Vc
    while (Vc > Vin * 1 / 100) {
        Vc = Vin * e ** (t / (R * C))
        t += -0.0000005
        console.logValue("Vc", Vc)
        console.logValue("Charge", 0)
        if (detected) {
            console.logValue("Touch", 1)
        } else {
            console.logValue("Touch", 0)
        }
        pause(100)
    }
    pause(100)
})

Test: Run the code in the simulator and switch to the Data Viewer (click on Show console) to see the console output in the chart. Let the program run for a few seconds to observe the charge / discharge pattern. Then, click and hold on the A1 pin in the simulator for a few more seconds. Unclick the A1 pin.

Result: The charge / discharge pattern shown in Vc graph will expand during the time that pin A1 is pressed. The Touch detection graph will show that a touch input is dectected when the charge time exceeds the regular charge time for the pin.