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Switches

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Everyone:
Switches
Fuses
Microswitches
RCD
Reed
Relays
Tilt

Everyone

Switch Properties

Switches are used to make or break a circuit. Here are some common varieties and their names.

NC Normally Closed
NO Normally Open
COM Common or shared connection
Poles correspond to the number of circuits that can be switched.
SPST Single Pole Single Throw (The simplest basic on / off switch)
DPST Double Pole Single Throw (Two of the simplest basic on / off switches)
SPDT Single Pole Double Throw (One change-over switch)
DPDT Double Pole Double Throw (Two change-over switches)

Circuit Diagram Symbols

The diagram shows single and double pole switches.

Puzzle

Using two SPDT switches, draw a diagram to show how stair lights work. It should be possible to turn the light on or off both at the top and bottom of the stairs.

Push Button Switch

These come in two varieties: push-to-make (NO) and push-to-break (NC).

Switch Bounce

When a switch is closed, the contacts often bounce causing the circuit to be closed and opened several times until the bouncing has stopped. For something like a lamp or hoover, this does not matter at all but in circuits that count or rely on pulses, switch bounce can stop the circuit working properly.

The switch needs to be de-bounced and this circuit can be used ...

When the switch is pressed, Reset goes low.
If the switch bounces, on the first bounce, the flip-flop is reset.
Later bounces have no further effect so the switch bounce has been removed.
When the switch is released, Set goes low.
Once again, the first bounce sets the flip-flop.
Later bounces have no effect.

More Debouncing Circuits

The one on the left with the AND gate is less likely to work well.
The NOT gate is also a Schmitt trigger making these circuits more immune to noise.