This time delay relay circuit is built with IC NE/SE555, produced by Intersil which contains a precision timer. Stability to temperature variations is 0.005 %/oC. In the circuit diagram, the IC works as a monostable multivibrator. It is set by operating the key Dr1. With Dr2 you can reset the assembly any time you desire.
Time delay relay operation
Pin 3 provides (after setting) a positive rectangular pulse, whose amplitude is approximately equal to the 12V power supply voltage. This output should not be charged with more than 200 mA over a period of time.
Pulse width can be calculated with the formula:
t = (R2 + Rp1)*Cs1
In this formula t is in seconds if the R2 and Rp1 values are set in ohms and Cs1 is in Farads. Cs1 is the switched capacitor to potentiometer P1 by the S1 switch. Lighting time is set roughly in the decade with S1, while P1 serves to fine tune.
S1a = 0.1 … 1 s
S2b = 1 … 10 s
S2c = 10 … 100 s
S2d = 100 … 1000 s
S2e = 1000 … 10000 s
For the Reed relay, by whose contacts the lamp is connected and disconnected, the following conditions must be met: operating voltage does not exceed 12 volts and current draw is not greater than 200 mA. If Reed relay’s working voltage is below 12 volts, then you can connect a resistance Rx in series with the relay’s coil.
Eventually it can be used a triac in place of the Reed relay. In this case the gate is controlled by Rx (here Rx = 150 ohms).
You should to consider a proper insulation of the time delay relay installation and wiring.