555 Timer.

This DIY kit will help with your soldering skills, while you learn about the standard 555 timer IC.
The kit learns you how the IC can be used to get a set time for powering the relay ON.
It intergrades a voltage-follower NPN transistor and Zenner as a stable voltage regulator.  
So you can power it with a wide voltage range and no need to worry about the timing varying between the on-off times of the Relay.

555 Timer Specification:

Capacitors Values: 330uF, 220uF, 100nF, 470uF
Resistors Values: 1K, 10k, 47k, 47k
LEDs: 1x 5mm LEDs
Diode: 3x 1N4001 Diodes and 8v2 zenner 
Transistor: 2x NPN Transistor BC337
Connectors: 1x 2Pin 1x 3Pin
Relay: 12V Relay 
Timer IC: NE555P Timer IC