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What you need:

As required for听Electric candle, plus:

Piezo buzzer
Kitchen foil
Cardboard
Chopsticks

Previously, we gained mastery over light and the basics of circuits. As your devices get more complex, it will make life a lot easier if you can turn the circuit on and off without dismantling it. That means adding a switch.

Switches come in many flavours, depending on what you want them to do. But what they all have in common is that they make or break a circuit, letting you control how electricity flows. For our purposes, we need look no further than kitchen foil.

Like you did for the Electric Candle, make a circuit with an LED, resistor and 9-volt battery. Then disconnect a crocodile-clip wire from one leg of the LED, and clip it to a piece of foil. Take another crocodile lead and give it a second piece of foil to grip. Connect the other end of that lead to the LED where you just broke the connection. Touch the pieces of foil together to complete the circuit. Suddenly, turning the light on and off is easy.

But that isn鈥檛 all you can do. To illustrate this, I built a device that tells me when my toast is ready. This requires something that makes a sound when the toast pops up. I used a piezo buzzer, so called because it harnesses piezoelectricity. When you apply electricity to a piezoelectric material, it deforms. The buzzer is designed so that this makes the material vibrate, producing sound.

Most hobby electronics operates in the 3 to 12-volt range, meaning your piezo can handle the 9-volt battery with no need for a resistor. If you are ever in doubt, you can usually written on the packet.

Assemble the battery, buzzer and wires on cardboard, then find some sticks. Anything about the height of your toaster will do. I used a pair of chopsticks. Now tape a foil switch pad to the end of each stick. Glue the sticks upright on the cardboard, ensuring the foil pads are the same height as the toaster鈥檚 lever when it pops up.

Leave a small gap between the pads. If we cover the toaster lever with

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foil, this will touch both pads when the lever springs up. In other words, the moment the toast is ready, the buzzer will sound. Piping hot toast every time.

With only a basic circuit and a switch, the possibilities are already vast. Try sticking foil on the edge of a door and on its frame for a 鈥�do not disturb鈥� light that comes on when you close it. Or make a flashing bike light, with one part of the switch on the wheel and the other on the frame, so the circuit is briefly complete once per rotation.

Or how about a fairground steady hand game, where you guide a wire loop over a shape without letting them touch. If the buzzer sounds, it is your turn to make the toast.

To download a printable version of this page click here

Thanks to Imperial College Advanced Hackspace for use of their facilities

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What you will need next time
Jumper cables
An electronics breadboard

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Next in the series

1. Introduction

2. Electric candle

3. Toast notifier

4. Desktop traffic light Use a 鈥渂readboard鈥� to build a complex circuit

5. Propeller car

6. Magic eight ball

7. Theremin

8. Sound-sensitive disco ball

9. Rubbish sweeper

10. Biscuit bot

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