Auto On Off Timer Circuit, Feature : Durable

Auto On/Off Timer is a simple and inexpensive timer, widely used to switch on/off devices. Auto On/Off Timer has a settable timing between 3min to 30min. The circuit is built around counter IC4060 (IC1) and inverter IC4049 (IC2). The circuit switches on automatically when power is on, and it switches off when the settable time is over.
About the Circuit :  
IC1 has on-chip oscillator whose frequency is determined by external RC network connected to pins 9, 10 and 11. When power is on, pulse at junction R4-C2 resets the counter and counting starts. The output at pin3 of IC1 is low, which is given to inverter input pin3 of IC2. This low voltage input becomes high at the output of inverter at pin2. Transistor T1 turns on and relay is energized. T1 remains on until output at pin3 of IC1 becomes high. When the counting reaches to last bit or the time is over, output at pin3 of IC1 becomes high. As a result, the pulse at pin2 of IC2 goes low and T1 turns off. At the same time relay becomes off and it remains off until pin3 of IC1 goes low again. Here LED D1 and D2 are used to give visual indication. D1 gives the indication of timing pulse generation, whereas glowing of D2 indicates timer on indication. Calibration: The time delay between 3min to 30min is set with the help of preset P1.