What we are making: a plan, and the measurements it rests on.

The brief

Build a device that senses one thing, decides something about it, and does one thing in response — with the screen saying what is going on. Pick from the list below. This one is not free choice; the free one is in May.

Choose one

1. A parking sensor. Ultrasonic in; buzzer and screen out. Beeps faster the closer you get.
2. A room monitor. Temperature and humidity in; screen out, with a warning when it goes past your number.
3. A door minder. Ultrasonic in; servo out. Something gets close, the servo moves.
4. A light that knows. Light sensor in; relay and screen out. Dark enough, and it switches on.

Write these four things down

1. The sentence. ‘When the sensor reads this, the output does that.’
2. The parts. All of them, from your own box.
3. The pins. Which component goes on which pin. Write it down now and you will not be guessing in session 14.
4. What the screen says. Both lines, in both states. Sixteen characters each — count them.

Then measure, before you decide anything

Wire your sensor on its own and print its value. Write down what it reads normally and what it reads when the thing you care about is happening. Your threshold goes between those two numbers.

If the two numbers are close together, say 300 and 310, your device will be unreliable no matter how good your code is — and you have found that out now, with three sessions in hand, instead of in session 16.

Before you leave

• Your four answers and your pin list.
• Two readings from the real thing, and the threshold you chose from them.
• Teacher sign-off.