Pre set off timer automatically turns off the devices after set time. Many of us have habit to fell asleep while watching TV or while listening music and the device remains switched on for the whole night. For such problems, you can use this simple circuit which will turn off the device automatically after the set time and will also engage an alarm. You can also vary the set time according to your requirement. This can also be used by students who are preparing for competitive exams they can use it as alarm clock.
Fig. 1: Prototype Of Pre Set off Timer Circuit On Breadboard
This simple circuit is built around four transistors. Connect the power supply and press the push to on switch momentarily to on the device. When switch is pressed transistor T1 and T2 start conducting and because of that relay energizes. LED1 glows to indicate that device connected to relay has become “on”. At this time voltage at mid pint of R1 and R2 is approx. 5V. At the same time transistor T3 is also conducting but transistor T4 does not conduct. So the buzzer and the LED2 connected to it will not glow.
When set time period is over relay de-energizes to switch “off” the device connected to N/O terminal of the relay. Now buzzer sounds and LED2 glows to indicate that set time period is over. With the help of VR1, you can set the time period from few minutes to half hour.