Seeing Light and Dark

Try It — How Dark Is It?

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

This one is different. Plug the color sensor into sensor port 3 and write that on your card. On the brick, open Port View on port 3, and press the center button until the mode reads reflected light (the brick calls it COL-REFLECT). Then watch the number while you hold the sensor 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

Reflected light is a range reading, not a switch — the other pile from your Project 10 sort, finally in play.

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

Your threshold

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

The usual starting point is halfway between your two readings:

Make it a variable called threshold, set at the top with the rest — you have known better than bare numbers since Project 9.

Your third kind of question

The checking has not changed at all. Only what it watches.

Read the new one out loud: “keep checking the brightness — the moment it drops below my threshold, the sensor is seeing black. Move on.”

Starting the match

Your match start has been a tap on the touch sensor since Project 10. The same brightness trick gives you a second option: point a color sensor up, and wait until its reading jumps — the moment a light comes on over it, or a flashlight hits it.

Which start your tournament uses is a rules question — check with your coach. The blocks are ready either way.

Do It — Find the Line, Then Use It

Step 1 — Mount it and set your threshold

Fix the color sensor to the front underside of your robot, pointing straight down, about half an 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

New project called Find the Line. Drive forward until the sensor sees black, then stop: start moving, wait until reflected light < threshold, stop moving, settle. Threshold and steering as variables at the top.

• 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.

• My robot finds a line forward, then finds a line in reverse, without fooling itself

Write the order of checks you ended up with, in plain words.

Step 5 — Start on a signal

Add a match start to the top of the program — your force-sensor tap, or the point-a-sensor-up light start from Learn It if your tournament uses lights. Test it: the robot must sit dead still until the signal, then go.

• My robot waits for the start signal, then runs

Step 6 — Mission 14 Bonus — a cone IN THE ZONE7 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 cones28 pts

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

A teammate’s robot misbehaves. Say which way their threshold should move — remember: with this sensor, black is low.

Can you do it again?

• I can explain the difference between a switch reading and a range reading
• I can pick a threshold from my own two readings and adjust it when it is wrong
• I remember that black is a low number, and my hexagon points the right way
• My robot can find a line from anywhere, without knowing where it started
• I can find a line twice in a row without the robot fooling itself
• My program waits for a start signal before it moves

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 wheel degrees, 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.