Seeing Light and Dark

Try It — How Dark Is It?

Your touch sensor answers one question: am I TOUCHING something? Yes or no. Nothing else.

This one is different. Plug the reflectance sensor — the little one, sometimes called a tophat — into an analog port. Then open the Sensor List on the Wombat and watch the number while you hold it over things.

The sensor shines a tiny light down and measures how much bounces back. Use that to explain why the numbers came out the way they did.

Learn It — Pick Your Own Dividing Line

• Values around 1000: white, lots of light bounced back.
• Values around 3000: black, most light absorbed

Your numbers may not match anyone else’s. Different sensor, different mounting height, different room lighting.

Your threshold

A threshold is the number you pick as the dividing line. Below it, call it white. Above it, call it black.

The usual starting point is halfway between your two readings:

threshold = (white + black) / 2

// For example: (800 + 3500) / 2 = 2150

That is a starting guess, not a final answer. You will adjust it.

Reading it in code

Your third kind of loop condition

The loop has not changed at all. Only the condition it watches.

Switch

In project 10, you watched a switch with two possible values.

while (digital(bump) == 0)
{
	drive();
}

Count

In project 11, you watched a motor position counter that climbed steadily.

while (gmpc(left) < 4000)
{
	drive();
}

Now, watch a brightness

Now, you are watching a brightness number that jumps around.

while (analog(line) < threshold)
{
	drive();
}

Read the new one out loud: “While the sensor is seeing something lighter than my threshold, keep driving. The moment it sees something darker, stop.”

Starting on a light

The light sensor does one job, and it is the job every Botball robot needs: it waits for the lights to come on, then starts the match.

// Wait for light on port 3, then continue
wait_for_light(3);

• You do not have to mount it on the robot.
• It runs its own calibration routine on screen — follow the steps it gives you.
• Call it near the very top of your program.
• Any moderately powerful flashlight should work with the light sensor, including a phone flashlight. Point the light at the sensor and watch the reading change.
• More light means a lower reading, which is the opposite of what most people guess.

Do It — Find the Line, Then Use It

Step 1 — Mount it and set your threshold

Fix the reflectance sensor to the front underside of your robot, pointing straight down, about 1/4 inch clear of the mat. Then take fresh readings from the mounted position — they will not match what you got holding it in your hand.

Step 2 — Find the Line

Create a new project called Find the Line. Drive forward until the sensor sees black, then stop.

// 1. Print "Looking for the black line"
// 2. While the sensor reads less than my threshold, drive forward
// 3. Fall out of the loop when it reads more than the threshold
// 4. Stop everything

Use variables for your motors and your threshold — you have known better than bare numbers since Project 9.

• My robot stops on the black line

Step 3 — Three lines, three distances

The real test of a threshold is whether it works when you did not tune it for that exact spot.

Set your robot at three different distances from a line and run the same program each time, changing nothing.

Step 4 — Drive to black, back up to black

Now do it twice in a row. Drive forward to a line, stop, then reverse until you find a line again.

What did you have to do to get off the first line before looking for the next one?

• My robot finds a line going forward and another going backward

Step 5 — Start on a light

Add wait_for_light() to the top of a program and follow the calibration routine on the Wombat’s screen.

Now your robot sits still until someone shines a light at it.

• My robot waits for a light before it runs

Why would every robot at a tournament need to start this way, rather than someone pressing a button on each one?

Step 6 — Mission 14 Bonus — A cone IN THE ZONE · 7 pts

Use your line-finding program to drive to the Loading Zone boundary, then place the cone.

• A cone is IN the Loading Zone and the other is still OFF the line

Step 7 — Mission 18 — Botguy and both cones for 28 points

Which order will you deliver them in, and why that order?

• Botguy is IN the Loading Zone
• One cone is FULLY WITHIN — no part over the boundary
• The second cone is FULLY WITHIN too
• Nothing got knocked out while placing the last one

Step 8 — Run it five times

Score It — Checkpoint

My score

My sensor card

IN or FULLY WITHIN?

Say whether each one satisfies IN, FULLY WITHIN, both, or neither.

Fix the threshold

Can you do it again?

• I can explain how an analog sensor differs from a digital one
• I can mount a reflectance sensor at the right height and angle
• I can take white and black readings and work out a threshold
• I can write a loop that drives until the sensor sees black
• I know which way to move my threshold when it stops too early or too late
• I can start a program with wait_for_light()
• I can tell IN from FULLY WITHIN and place an object to satisfy the stricter one

Think about it

Your threshold works perfectly in your classroom. The tournament room has huge windows and much brighter light. What might happen, and what would you do about it on the day?

Your robot now has three ways to know where it is: bumping something, counting ticks, and reading brightness. Which one would you trust most, and does the answer change depending on the job?

Right now the sensor only tells your robot to stop. What if it kept reading while driving, and steered based on what it saw?

Next

That last question is the whole of the next project. In Project 15 — Following the Line, the sensor stops being a stop sign and becomes a steering wheel.

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