Relays  :  use a low amperage circuit to switch on a higher amperage circuit. They are very common on vehicles, and although there are many types, they are very similar in how they work. 




5 pin relay









86 positve side of control with 85 as the negative. 30 is battery supply for the switched circuit and 87a is normally closed switch circuit,87 is the other switch

in the above picture the black is the negative pin 85,the bottom two pins are the positive powewrd 86 and 30,
top red clip is the switch when closed with 87a in the middle  and 87 pin to the light bulb with the green clip

Circuit Off                     Circuit On

86 0 V                           86: 12.09 V

85 V                               85 0 V

30 12.09mV                 30 12.8 V

87a 12.09V                  87a 0 V

87 0 V                          87 12.8V

first two pics show the relay used in class with an assigned switch to simulate headlights turning on and off.
last picture is of a four pin relay
..works similar to the five pin except the 5 pin has another switch circuit
(http://www.reuk.co.uk/Automotive-Relays.htm)


testing the relays


Resistance of circuits...86 to 85 88.2 Ω

                                  30 to 87a 0.2 Ω

                                   30 to 87 0.2 Ω

Control circuit that switches the points: __85__,86

Switched points that carry the higher amps: _30_, 87(87a)

If 12volts are used, the amperage would be: 12 V / 88.2 ohms = 0.136 A

in the drawing below the relay control is the switch for the light bulb.in this case it is off.

following it one of a two light bulb circuit, as the points are switched one of the two bulbs will be on,in this case the high beam because the point is closed