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Make a reed switch

By Mike, 14 September 2012 Activity

Split pin on a table

Can you work out how to turn a humble split pin into a magnetic switch?

Attracted to doing some hands-on science? Why not use a magnet to create a ‘reed switch’?

sharp hazard iconelectrical hazard iconSafety: Readers must get an adult to help with using tin snips. Remember to disconnect circuits after five seconds to prevent wires overheating.

You will need

  • Strong magnet
  • 3 x wires with crocodile clips
  • 6 volt battery
  • LED
  • 10 cm length of 1 cm diameter clear plastic tubing
  • Split pin (make sure it is attracted to your magnet)
  • Tin snips

What to do

  1. Use the tin snips to cut the legs from the split pin, giving you two metal strips.
  2. Clip one end of a wire to the positive terminal of your battery and the other to the longest leg of your LED.
  3. Clip another wire to the short leg of the LED and the end of one of the metal strips.
  4. Clip one of the crocodile clips to the second metal strip, and join the other end to the negative terminal of the battery. If you briefly touch the two strips together, the LED should glow.
  5. Push one of the metal strips and its crocodile clip into one end of the plastic tubing.
  6. Insert the other strip into the other end so it overlaps the first without touching it. If it doesn’t reach, cut a small amount from the end of the tube.
  7. Bring a magnet close to the tubing. What happens to the strips? What happens to the light? What happens when you take the magnet away?

What’s happening?

This is commonly referred to as a ‘reed switch’. The magnet pulls on the strips until they come into contact with one another.

Electricity is described as a flow or movement of charged particles in a current between two points. The overall charge or ‘voltage potential’ of each point has to be different. When they are joined by a conductor – such as a wire – the difference (called the voltage) between the points causes charge to flow from one to the other. If there is a break in the conductor, there is no way for the current to flow from one point to the next. The reed switch acts like a tiny drawbridge.

In some cases, there is a coil of wire around the tube that contains the strips. When electricity flows through the wire, it creates a magnetic field that also causes the strips to move together and join the circuit. This can be used in circuits called ‘relays’, which are circuits that have switches operated by a second electrical circuit.

Applications

Because the reed switch is sealed inside a tube, it can be used in hazardous, dirty or wet environments that would otherwise ruin other switches. All it takes is a moving magnet to turn it on and off.

Reed switches were used in groups as reed relays to store information for telephone systems in the early to mid 20th century. Old keyboards also used reed switches – they used to have magnets in the keys which would cause a current to flow when they were tapped.

Today, reed switches are still used in devices such as burglar alarms and some electronic instruments. Speed sensors on some bicycles rely on a small magnet in the wheel to record its rotation. Some laptops even have reed switches that put it into sleep-mode when the lid is closed.

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