The Touch Sensor

ComponentSensing5 min

The Touch Sensor is the simplest input the EV3 has: a button that is either pressed or not. That sounds trivial, but it is how a robot knows it has hit a wall, reached the end of a track, or been told to start by a person.

released

pressed

The red button out, and the same sensor with it pushed in. These two states are the entire output of this sensor — there is nothing in between.

Blocks reference

Block	What it does
wait until <[1 v] is pressed? :: sensors>	Holds the program here until somebody presses the sensor.
<[1 v] is pressed? :: sensors>	Reports true or false. Drop it into a condition to make a decision rather than a wait.
[1 v] when [bumped v] :: events hat	Starts a whole stack of its own. The dropdown chooses the moment: pressed, released or bumped.

Three different events

A button is not only “pressed”. One press is three things: the moment it goes down, the time it stays down, and the moment it comes back up. Watch what a single press does to three programs at once.

when program starts

forever

if 1 is pressed? then

change count by 1

versus two hat blocks

1 when pressed

1 when bumped

0is pressed? in a loop0when pressed0when bumped

In a loop: 0 answers from one pressBumped: exactly one

Nobody is touching the sensor. All three programs are watching it.

released

The middle counter is the one that surprises people. Nothing is wrong with it — a loop really does check that fast, and every check really is a separate answer.

Nothing there is broken. A loop really does get round hundreds of times a second, and each time it asks is pressed? the honest answer is still yes — so if that loop plays a sound or counts something, it does it hundreds of times from one finger. The two hat blocks each fire once, and they fire at different moments: pressed the instant the button goes down, bumped only when it comes back up.

The three options, and what each is for:

Pressed — the button is down right now. Good for “hold to run”.
Released — it is up again. Good for acting when somebody lets go.
Bumped — pressed and released. This is what you want for “click to start”, because it will not fire repeatedly while a finger stays down.

The classic bumper

when program starts :: events hat
set movement motors to [B v] and [C v] :: movement
start moving [straight: 0] :: movement
wait until <[1 v] is pressed? :: sensors>
stop moving :: movement

The robot drives until something presses the sensor. Note that the movement is started unmeasured on purpose — the sensor decides when to stop, not a distance.

Why it matters

Touch sensors are everywhere in machines you cannot see into: a lift knows the doors are shut, a printer knows the lid is closed, a washing machine will not spin until it is latched. They are safety devices as much as inputs.