Buttons and switches

Push buttons, microswitches, tilt and reed switches: how they work, wiring them to an Arduino with a pull-up resistor, and why contacts bounce.

Small square push buttons with four legs, large round coloured buttons, a black microswitch with a lever, a tilt switch and a reed switch in a glass tube next to a magnet

What are buttons and switches?

A push button and a switch do the same job: they join and separate two pieces of metal, the contacts. When the contacts touch, current flows. When they are apart, it doesn’t. The difference is what happens when you let go:

A push button joins the contacts only while you hold it. When you let go, a spring pushes it back. The button of a doorbell, the keys on a keyboard and buttons A and B on the micro:bit are push buttons.
A switch stays the way you set it until you move it again. The light switch in your room and the switch that turns on the Maqueen robot are switches.

Some switches are not moved by a hand but by something else: a ball hitting a lever, a tilt or a magnet. Such switches are the simplest sensors.

How to recognise them

Four parts, each with its symbol below it. A 6 × 6 mm push button from above with four legs: dashed blue lines show that legs 1 and 2, and 3 and 4, are joined inside. A microswitch with a lever and legs COM, NO and NC. A ball tilt switch, a metal can with two legs and a ball inside. A reed switch, two thin metal strips in a glass tube, with a magnet below it. Symbols: a push button with a push stem, a changeover contact with COM, NC and NO, a plain switch, and a switch inside the outline of a glass tube

A small push button (6 × 6 mm) has four legs but only two contacts. Legs 1 and 2 are joined inside, and so are legs 3 and 4. Pressing joins the pair 1 and 2 to the pair 3 and 4. That is why the button goes across the middle groove of the breadboard, as on the Breadboard page. If you aren’t sure which legs are joined, check with a multimeter.
A microswitch is a small push button with a lever. It needs very little force, and its contacts snap over with a quiet click. It has three legs:
- COM (common) is the leg that is always used,
- NO (normally open) is joined to COM while the lever is pressed,
- NC (normally closed) is joined to COM while the lever is not pressed.
A ball tilt switch (tilt sensor) is a small metal can with a ball inside. While it stands upright, the ball lies at the bottom and joins the contacts. When you tilt or shake it, the ball rolls away and the contacts separate.
A reed switch is two thin metal strips in a glass tube. When you bring a magnet near, the strips are pulled together and touch. The glass is thin: don’t bend the legs right next to the tube, but a little further away.

Below each part is its symbol. On the push button’s symbol, the stem with a button on top says that a hand holds the contact. The reed switch has the outline of its glass tube.

What is inside a push button?

A push button cut through the middle, released on the left and pressed on the right. Numbers: 1 button, 2 metal cover, 3 metal dome, 4 contacts, 5 legs. On the pressed button the dome has snapped down and touches the middle contact, and an orange arrow shows the current's path from a leg through the dome to the middle contact

The button is the plastic knob you press with your finger.
The metal cover holds all the parts together.
The metal dome is a thin curved plate. When you press it, it snaps down with a click, and when you let go, it springs back by itself. It is the spring as well.
The contacts: the rim of the dome always rests on the outer contacts, and it touches the middle contact only while it is pressed.
The legs are joined to the contacts. The outer contact goes to one pair of legs, the middle one to the other.

On the right the button is pressed. The dome has snapped down and touches the middle contact, so current flows from one pair of legs through the dome to the other.

Where we use them

At the micro:bit workshop, a push button closes the circuit in the Breadboard lab. In the lesson Switches and the bumper we try out a push button, a microswitch and a tilt switch, and a microswitch becomes the bumper of the Maqueen robot.
At the Arduino workshop we read buttons in Buttons done right, and many of them on a few pins in Many switches on few pins. In Detecting the ball a microswitch counts the ball’s hits, and a ball tilt switch checks that the player doesn’t push the machine.
In our pinball machine buttons work the flippers and start the game, microswitches sit under the targets, and four ball tilt switches make up the tilt.

Buttons and switches at the Arduino workshop

From here on, this page is for the Arduino workshop.

A pin must not float

While the button isn’t pressed, the pin it is connected to isn’t connected to anything. Such a floating pin reads 0 or 1 at random (misconception 10). So the pin is always also connected through a resistor, which holds it in a known state while the button is at rest.

Left: a button between pin D2 and GND, and inside the Arduino a resistor R from 5V to D2, turned on by INPUT_PULLUP. Right: a button between 5V and D2, and a 10 kΩ resistor from D2 to GND, with INPUT

A pull-up resistor (left) connects the pin to 5 V. A released button gives HIGH, and a pressed one connects the pin to GND and gives LOW. The Arduino Uno has such a resistor built in, 20 to 50 kΩ. pinMode(pin, INPUT_PULLUP) turns it on, so nothing but the button goes on the breadboard.
A pull-down resistor (right) connects the pin to GND, and the button goes to 5 V. A pressed button gives HIGH. The Uno has no built-in pull-down resistors, so we add a 10 kΩ one, as in Two traffic lights at a crossroads.

We wire buttons with a pull-up resistor: it takes fewer parts, and we don’t need 5 V on the breadboard. While the button is pressed, only about 0.14 mA flows through the built-in resistor. How the program reads the button is in Exercise 1 of the lesson on buttons.

Contact bounce

When the metal contacts hit each other, they bounce, like a ball dropped on the floor, and settle only after a few milliseconds. The same happens when you let go. A person can’t notice it, but the Arduino reads the button many times every millisecond and sees every bounce:

The voltage on a pin with a pull-up resistor: on every press and release it jumps between 5 V and 0 V several times before it settles

A program without debouncing counts one press as several. At the workshop we debounce in the program: after every change we ignore the button for a while (Exercise 3 of the lesson on buttons), or we use the Bounce2 library. A Hall sensor has no contacts, so it doesn’t bounce.

Debouncing with a capacitor

Contacts can also be debounced without a program: with a 100 nF capacitor (code 104) straight across the button’s legs. The graph shows the voltage on pin D2 during one press and release, with the built-in pull-up resistor. The bounces are an example. In the yellow part the button is pressed, and the dashed lines are the limits of 3 V and 1.5 V: above 3 V the Uno surely reads 1, and below 1.5 V it surely reads 0.

Two graphs of the voltage on D2 over 32 ms. Top, without a capacitor: on the press at 4 ms and the release at 20 ms the voltage jumps between 5 V and 0 V several times. Bottom, with 100 nF: on the press it drops to 0 V and rises to at most 0.4 V in the gaps between bounces, and on the release it stays near zero while the contacts bounce, then rises slowly and passes 3 V at about 25 ms

The press: the first touch of the contacts empties the capacitor at once. While the contacts bounce, the pull-up resistor charges it, but slowly: in a 0.3 ms gap the voltage rises only to 0.4 V, far below 1.5 V. The Arduino sees one clean press.
The release: every new touch empties the capacitor again. The voltage starts to rise only when the contacts settle, and it passes 3 V 3.2 ms after the last bounce.

How fast it charges is set by the time constant τ = R · C = 35 kΩ · 100 nF = 3.5 ms (how a capacitor charges is explained on the Capacitor page). The built-in resistor can be anything from 20 to 50 kΩ, so τ is 2 to 5 ms. That is fine for buttons, but the release always comes a few milliseconds late. At the workshop we therefore debounce in the program, where we choose the time ourselves.

NO or NC?

A microswitch can be wired with its legs COM and NO, or COM and NC. The program can read either, but they differ when something goes wrong. If the wire to a microswitch wired to NO breaks, the Arduino thinks nobody is pressing it, and nobody notices. With NC a broken wire looks like a press that never ends, so you see it at once. That is why the tilt in Detecting the ball treats a broken wire as a switch that has been open for too long.

So switches that guard something important, such as the one on a washing machine’s door or at the end of a machine’s travel, are wired so that a broken wire stops the machine.

Many buttons on a few pins

The Uno has only 18 free pins, and in the pinball machine the display, the lights and the sound need pins as well as the switches. So the buttons go into a matrix of rows and columns, so that 12 keys need only 7 pins. How the matrix works and why it needs diodes is explained in Many switches on few pins.

How much current can the contacts take?

Contacts wear, and a stronger current wears them faster: a small spark jumps between them as they separate. Rough values for parts like ours:

Part	Highest current through the contacts
6 × 6 mm push button	about 50 mA at 12 V
reed switch	about 0.5 A, but less when the load is a coil
microswitch with a lever	a few amps

The real values are in each part’s datasheet. So a push button or a reed switch only tells the Arduino that something has been pressed. A motor, a coil or a bright bulb is switched by a transistor, not by the button’s contacts.

What did we learn?

A push button joins the contacts only while you hold it, and a switch stays the way you set it.
A small push button has four legs: 1 and 2, and 3 and 4, are joined inside. A microswitch has COM, NO and NC.
A pin with a button needs a pull-up or pull-down resistor. With INPUT_PULLUP a pressed button gives LOW.
Contacts bounce for a few milliseconds. We debounce them in the program or with a capacitor.
A contact wired to NC also reports a broken wire. A button only reports, and strong currents are switched by a transistor.

Quick quiz

1. What is the difference between a push button and a switch?

A push button joins the contacts only while you hold it, and a spring pushes it back when you let go. A switch stays the way you set it.

2. A small push button has four legs. Why does it go across the middle groove of the breadboard?

Legs 1 and 2, and 3 and 4, are joined inside. Across the groove each pair ends up in a different half of the breadboard, so the button joins two different columns only when you press it.

3. A microswitch is wired with its legs COM and NC. When are the contacts joined?

While the lever is not pressed. When you press it, NC separates from COM.

4. A button is wired between D2 and GND, with INPUT_PULLUP. What does digitalRead(2) give while it is pressed?

LOW. A pressed button connects the pin to GND, and while it is released the built-in pull-up resistor holds the pin HIGH.

5. Why does a program without debouncing count one press as several?

The metal contacts bounce a few times when pressed and released, so they join and separate several times within a few milliseconds. The Arduino reads the button many times every millisecond and sees every bounce.

6. Why don't we switch a motor straight with a 6 × 6 mm push button?

The contacts of a small push button take about 50 mA, and a motor needs much more. The contacts would spark and wear out quickly. The button reports to the Arduino, and a transistor switches the motor.