A time delay relay is a relay that stays on for a certain amount of time once activated. This time delay relay is made up of a simple adjustable timer circuit which controls the actual relay. The time is adjustable from 0 to about 20 seconds with the parts specified. The current capacity of the circuit is only limited by what kind of relay you decide to use.
C1__________10uf 16V Electrolytic Capacitor
C2__________0.01uf Ceramic Disc Capacitor
R1__________1 Meg Pot
R2__________10 K 1/4 Watt Resistor
U1__________555 Timer IC
S1__________Normally Open Push Button Switch
MISC_______Board, Wire, Socket For U1
1. R1 adjusts the on time.
2. You can use a different capacitor for C1 to change the maximum on time.
3. S1 is used to activate the timing cycle. S1 can be replaced by a NPN transistor so that the circuit may be triggered by a computer, other circuit, etc.
Source: Electronics Lab