Many of us face problems when we leave our house as if we want in the engine, so that when we get home the water is filled in the tank head or we want in other appliances such as light fan etc. So, for solve this problem we have describe a simple circuit that will be on your device or devices remotely and will automatically shut down after a preset time interval.
Many circuits have been developed so far to solve this problem but the main advantage of this circuit is it uses readily available components and this circuit is not expensive also because this circuit is made use without microcontroller.
Circuit working
Connect the cell phone with the head phone or headset properly into the audio jack of the mobile phone. Now, when calling the mobile connected to the device at the receiving end, the DTMF IC decoder will provide a high pulse at the pin 15 after detecting the valid tone pair. This will activate IC2 pin 2 and its output will go from high to low. This in turn causes the output pin 3 of IC2 to be high and the transistor T2 connected to it begins to drive because this relay energizes and the device is turned on. The period of time during which the device remains on can be adjusted with the help of variable resistor VR1 and capacitor C4. By increasing or decreasing the value of the variable resistor and the time period of the capacitor can be increased or decreased. This can be calculated by the following formula-
Time period = 1.1 * VR1 * C4 in seconds
In this circuit, the transistor T1 will only drive when all pin outputs 11, pin12, pin13 and pin15 are simultaneously raised with the aid of diode D1 to D5. This can only be done by pressing the number 7 through the DTMF decoder. The DTMF signal sending from the mobile end of the dial is decoded by the DTMF decoder and its respective BCD value appears on the output at pin 11, pin12, pin13. That is why on the device we are using the number 7.