This circuit is build with the 555 IC and will automatically turn off the power after 20 minutes. You can use the circuit to turn off the porch light after you lock the house or similar other uses.
The 555 timer is operated as a monostable and a momentary push on S1 switch makes the output go high which triggers the triac and makes power available in the socket.
The IC output goes low again when C2 has charge up to 2/3 of the supply voltage. This process takes about 20 minutes. C2 should have low leakage otherwise it will charge very slowly and in cases of excessive leakages may not charge to full value at all. Power supply for the timer is provided by half wave rectifier D1, voltage dropping resistor R1, zener diode D2 and filter capacitor C1.