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Single Pole Double Throw Switch (SPDT) - One of Many Types

The diagram on the right uses a Single Pole Double Throw (SPDT) Switch.

The common terminal is the middle terminal in the SPDT Knife Switch or if you are using a household switch, it would be the brass colored terminal. (the other 2 would be silver colored).

This circuit demonstrates what happens when the SPDT switch is moved back and forth.

Light A goes on and B goes off, B goes on and A goes off and so forth.

This popular switch has many practical applications: the transmit/receive button on a "2-way" radio, the "high/low beam" switch for your car headlights, the pulse/tone dialing switch on your telephone, and so on.

The diagram on the right depicts what is probably the most common use for the SPDT switch - the 3 way light switching circuit. Electricians incorrectly call the SPDT switch a "3 way switch". The proper terminology should be "three terminal switch". However the term 3-way switch has stuck and it's a misnomer we'll just have to live with.

So, how does this work? Let's say that Switch 1 is at the bottom of a stairway and Switch 2 is at the top. Suppose Switch 1 is in a 'down' position (B & C connected) and Switch 2 is in an 'up' position (D & E connected). The light bulb is off. Now someone comes to the bottom of the stairs and flips Switch 1 'up'. If you follow the circuit you can see why the light bulb would now turn on because A & B and D & E are connected. When the person reaches the top of the stairs, Switch 2 is flipped 'down', E & F are now connected and so the light bulb goes off.

Another person shows up at the bottom of the stairs and flips Switch 1 'down', connecting B & C thereby turning the light on again. The person reaches the top of the stairs, flips Switch 2 'up' connecting D & E and the light bulb goes off. Notice that in the case of the second person, a downstroke turns the bulb on and an upstroke turns the bulb off. If you have such switches in your house OR if you have purchased household wall switches for this circuit, you now see the reason why they do NOT have the words on and off printed on them.