Feeling for Things

Try It — Walking in the Dark

Imagine the power is out and you are trying to cross a room you know well. Your eyes are useless. So you put a hand out in front of you and start walking.

Actually do it. Stand a few steps from a wall, close your eyes, put one hand out, and walk slowly toward it until you touch it.

You did not check once and hope. What were you doing the whole time you were walking?

Sort your sensors

Tip your kit’s sensors out on the table. Without looking anything up, sort them into two piles. You decide the rule.

Learn It — Keep Checking Until

Digital — What you have today

A digital sensor is like a light switch. Two readings, nothing between:

• 0: not touched
• 1: touched

A touch sensor is a mechanical switch. Pressing it pushes two contacts together and completes a circuit.

Analog — Introduced in project 14

An analog sensor gives a whole range of numbers, from 0 to 4095. Light sensors and rangefinders work this way — “how bright,” not “bright or dark.”

You will meet these later. Today, everything is 0 or 1.

Why msleep cannot help you here

Every time you needed the robot to wait, you used msleep(). That will not work now.

The program cannot read a sensor while it is sleeping. If you tell it to sleep for two seconds and the sensor is pressed after half a second, the robot never notices.

You need it to keep checking — over and over, as fast as it can. That is what a loop is for.

Comparing two values

To check something, you compare two values. These are the comparisons you can use:

The while loop

Say it in plain English before you say it in C:

while it is raining
{
    my umbrella is open and over my head
}

close the umbrella

The umbrella stays up the whole time the condition is true. The moment it stops raining, you fall out of the block and close it.

Reading a touch sensor

digital(0) reads the digital sensor plugged into port 0. It gives you a 0 or a 1.

// While not touched
while (digital(0) == 0)
{
	// Keep driving
	motor(0, 100);
	motor(3, 100);
	// Tiny pause to avoid overworking the controller
	msleep(10);
}

motor(0, 0);
motor(3, 0);
// Touched: Stop
msleep(30);

Do It — Drive Until Bump

Step 1 — Mount the sensor and read it live

Mount your touch sensor on the front of the robot so it hits things before anything else does. Plug it into a digital port and write down which one.

On the Wombat, open the sensor list. Press the sensor with your finger and watch the number.

• My sensor changes between 0 and 1 when I press it

Step 2 — Drive Until Bump — in your hands first

Create a new project called Drive Until Bump. Plan it as pseudocode first, then write it.

// 1. Print a message so I know it started
// 2. While the sensor is NOT pressed, drive forward
// 3. When it is pressed, fall out of the loop
// 4. Stop

• Wheels stop when I press the sensor with my finger
• It works on the floor against a wall or a book

Step 3 — Put your variables back in

You spent Project 9 learning why bare numbers are a bad idea. Do not abandon that now.

int left  = 0;
int right = 3;
// Front touch sensor
int bump  = 0;

while (digital(bump) == 0)
{
	motor(left, 100);
	motor(right, 100);
	msleep(10);
}
motor(0, 0);
motor(3, 0);
msleep(30);

Note that bump and left both hold 0 — but they mean completely different things. One is a digital port, one is a motor port. The names are what keep them straight.

• My program uses variables and still works

Step 4 — Flip the condition and predict

Change == 0 to == 1. Before you run it, write down what you think will happen.

My prediction:

Now run it. Then run it again while holding the sensor pressed the whole time.

What actually happened, both times?

Explain why, using the words “true” and “false”.

Put it back to == 0.

Step 5 — Bump and Go Home

Here is what makes a touch sensor genuinely useful: a wall is a place your robot can always find.

New project. Drive backward until the sensor presses against the starting box wall, then drive forward a set distance into the box and stop.

// 1. Drive backward while the sensor is not pressed
// 2. Exit the loop when it touches the wall
// 3. Stop and settle
// 4. Drive forward a set distance
// 5. Stop

• My robot finds the wall and ends up in the same place every time

Step 6 — Add a stop and settle

A robot that has been driving is still moving when the loop ends. Momentum does not care about your program.

Put a proper stop between the loop and whatever comes next:

motor(0, 0);
motor(3, 0);
msleep(30);
// Let the robot actually stop moving
msleep(500);

Run it with and without the pause and compare.

What difference did the pause make?

Step 7 — Wait for a button before starting

The Wombat has one physical push button and three soft buttons on screen, named a, b, and c. Each has a function that returns 1 when pressed and 0 when not.

printf("Press A to start\n");

while (a_button() == 0)
{
	// Wait: do nothing at all
	msleep(10);
}

// Run the mission

Add this to the top of a program. Now your robot waits for you instead of driving off the second it compiles.

• My robot waits for a button press before it moves

This loop has nothing inside it but a tiny pause. Is it still doing something? What?

Step 8 — Mission 13 Advanced — all three cubes for 13 points

Use the touch sensor to make your approach repeatable. If your robot can find a wall or a cube by contact instead of by timing, every placement after that starts from a known spot.

Where in your run does the touch sensor help most — finding the cubes, or finding your way back?

• All three Unstraight Cubes are in one stack or pyramid
• It is still standing after the robot backs away

Step 9 — Run it five times

Did the stack fall over on any run? What knocked it — the robot leaving, or the stack itself?

Score It — Checkpoint

My score

Finish the loop

Write the condition for each one.

Spot the bug

Each of these is wrong. Say what happens when you run it.

Can you do it again?

• I can explain what makes a sensor digital and what its only two values mean
• I can read a live sensor value on the Wombat before I write any code
• I can write a while loop with the braces and no semicolon after the condition
• I know the difference between = and == and I check it every time
• I know what goes inside the loop and what goes after it
• I test a new sensor program with the robot in my hands first
• I can use contact with a wall to give my robot a position it can trust

Think about it

Since Project 3, every distance your robot has driven was really a length of time. What does a touch sensor give you that a stopwatch never could?

A touch sensor only tells you something when you are already touching it. Name one thing on the field you would want to stop at, where bumping into it first would be a bad idea.

Your loop asks a question over and over. So far the question has always been about a sensor. What else could a robot count or measure that a loop could watch?

Next

A wall tells you where you are, but only if there is a wall. Most of the field has nothing to bump into.

In Project 11 — Counting Wheel ticks, you point the same loop at a number instead of a switch — and your robot can finally measure how far it has actually gone.

KIPR · Botball Explorer — Discovery Projects · © KISS Institute for Practical Robotics 1997–2026