LEARN THE SCIENCE

A laptop can do millions of different things. A microcontroller does one job, very reliably, using almost no electricity. There is one in a microwave, one in a car key, and dozens in the car itself.

The micro:bit is a microcontroller you can reprogram. Its 25 red LEDs can scroll sensor numbers, so you can read them at the table with no laptop. Button A runs random bounce and button B runs Spot Clean. When you load code, it stays. Switch Dusty on a week later and the program starts right away.

Code is a list of steps the computer follows exactly, in order. Dusty’s code uses three big ideas. A loop repeats steps forever. An if checks something and decides what to do. A variable is a box that remembers a number, like the threshold.

A micro:bit pin gives about enough electricity to light a small LED, roughly 1/100th of what a motor wants. Wire a motor straight to it and you get a broken micro:bit, not a slow robot.

The moto:bit board has a circuit called an H-bridge: four electronic switches in the shape of the letter H, with the motor in the middle. Close one pair and the motor spins forward. Close the other pair and it spins backward. The micro:bit only says which pair.

To run a motor at 40%, the board flicks the power fully on and off hundreds of times a second, on 40% of the time. The motor acts like it gets 40% of the power. This is called PWM. At 40%, Dusty drives about 7 centimeters per second.

Each cliff sensor has two tiny bumps. One is an infrared LED that shines invisible light straight down. The other measures how much light comes back.

On the table, the light bounces straight back up. Past the edge, it shines into empty air and almost nothing comes back. That sudden drop is how Dusty knows the table ended.

The sensor works best about 3 mm above the table and gets unreliable past 6 mm. Two US pennies stacked up are almost exactly 3 mm, so they set the height. The sensors sit 62 mm in front of the wheels, which gives Dusty a head start to stop.

A button is digital: pressed or not, yes or no. The sensor is analog: it has a whole range of answers. The micro:bit turns it into a number from 0 to 1023. For Dusty’s sensors, a low number means lots of light came back, and a high number means almost none did.

A threshold is the line between table and cliff. On white paper the table might read 200 and the edge 900. Dark wood might read 650 on the table. A fixed line at 500 would make Dusty think the dark table was already a cliff.

So Dusty calibrates. Press A while it sits still, and it reads the table and sets the threshold 200 above that. Same code, any table.

When two gears touch, their teeth pass one for one. The motor gear has 12 teeth and the big gear has 36, so the motor goes around 3 times for one turn of the big gear.

A small 10-tooth gear is stuck to the big gear. It drives the 18-tooth roller gear, which slows things down again by 18 ÷ 10 = 1.8. The two slow-downs multiply: 3 × 1.8 = 5.4.

The 130 motor spins about 8,000 to 10,000 times a minute. Divided by 5.4, the roller turns about 1,500 to 1,800 times a minute, with enough turning strength to push through a pile of cereal.

Nothing stops instantly. A robot keeps rolling while its code notices the edge and while the motors slow down. A faster robot covers more ground in that time and has more motion to get rid of.

Dusty’s sensors are 62 mm ahead of its wheels. That head start is the room it has to stop. The prediction: at slow speed it needs about 8 mm, and at full speed about 38 mm, so it should never fall.

The experiment tests four speeds (25, 50, 75 and 100), 20 runs each, measuring how far the sensor goes past the edge and counting falls. If the numbers climb toward 62, the chart can predict the speed where Dusty would finally fall.

The first Roomba had no map. It drove straight, turned a random amount at an obstacle, and drove again. Bounce around long enough and you cover the whole room. It sold about a million a year. That is Dusty’s button A.

Spot Clean on button B sweeps a 1 foot square in rows, like mowing a lawn. The brush is 52 mm wide and the rows are 40 mm apart, so each row overlaps the last by 12 mm and no stripe is missed. 305 mm ÷ 40 mm is about 8 rows.

The spill race: 5 grams of cereal in a taped square, Spot Clean against random bounce for the same time, then weigh what each caught.

In the speed test, the thing you change is motor speed. The things you measure are stopping distance in millimeters and falls out of 20. Everything else stays the same: the table, the charged power bank, the 3 mm sensor height, and the running start.

To find an average, add up the runs and divide by how many there are. Then make a chart with speed along the bottom and stopping distance up the side.

Saying "Dusty cleans pretty well" is an opinion. Saying "Dusty picked up 3.8 grams out of 5, which is 76 percent" is a measurement. Science runs on measurements.