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Relays & Switches

Pole = A movable arm in the switch.

Throw = The number of contacts that are affected by a single movement of a pole.

SPST (Single-pole, single-throw): Turns a single circuit on or off.

SPDT (Single-pole, double-throw): Controls the flow of current to two different circuits. When one contact is open the other is closed and vise versa.

DPST (Double-pole, single-throw): Allows simultaneous opening or closing of two sets of contacts. Both contact arms are mechanically linked so that they move with a single switch action.

DPDT (Double-pole, double-throw): Connects one set of contacts to either of two other sets of contacts.

Relays

NO = Normally Open, COM is connected to this when the relay coil is on.
COM = Common, always connect to this, it is the moving part of the switch.
NC = Normally Closed, COM is connected to this when the relay coil is off

if you want the switched circuit to be on when the relay coil is on, connect NO & COM
if you want the switched circuit to be on when the relay coil is off, connect NC & COM