Using NE555 pls note ...............draw the cct diagram as you're reading....... thanks( 555 timer pin1.....pin8..)loop pin2 to pin6,loop pin4 to pin8.pin1 to negative.pin3 o/p.connect capacitor Cx(10uf 25v) pin8( ) and pin6(-) .connect variable resistor Rx 2.4megaohms and diode In4007 to pin1 and pin2.connect diode 4007 from pin2 to pin3 output.connect 12v zeaner diode from pin1 to pin4.from 12v dc source connect two resistors 2.2k and 10k ,2.2k to pin1 and 1k to pin 4.connect pin3 to base of TIP41, collector of tip41 to th relay coil than 12v to relay coil . connect diode b/w the relay coil.Finally conect your load to normally open and common the relay.Time to switch on depend on value of Cx or Rx.I normall use this for automatic transfer switch ,inverter etc