Useful for controlling motors and lamps when either the current is too much for a TIP120 transistor or you don't want the voltage drop across the transistor or you need bi-directional current through the switch.

Pinouts for common relays

Jameco 174377 ($2.50)
Coil: 12V, 270 ohms, 44mA
Contacts: 5A
Comments: Great for bi-directional motor control. Sold at the Robot Store.

Jameco 174431 ($1.50)
Coil: 12V, 400 ohms, 30mA
Contacts: 15A
Comments: Great general purpose relay. Can carry high current. Sold at the robot store.

Radio Shack 275-249 ($3.99)
Coil: 12V, 200 ohms, 60mA
Contacts: 5A
Comments: Drive with 2N3904 or TIP120 transistor. Can handle lots of contact current. Use to reverse direction of a motor. Remember that it is drawing 60mA of coil current from your battery when energized. Buy at any Radio Shack.

Radio Shack 275-232 ($2.49)
Coil: 5V, 250 ohm, 20mA
Contacts: 1A
Comments: Can drive straight from a Stamp pin. Control motors, lamps, solenoids...anything which requires less than 1A of current. If load is greater than 1A, relay freezes open or closed and contacts melt. Most motors at stall will draw more than 1 amp and could not be used with this relay.