Date of completion: 2014
This very simple circuit is yet another I constructed for school, where some devices were "eating" PP3 batteries very rapidly, especially because the devices were not always turned off at the end of a lesson (grrr!) I swiped the design (OK, "adapted it") from a website somewhere! The circuit solves the problem by having a push switch providing 10 minutes of power, after which the button needs to be re-pressed or the device switches off. Switch-off reduces the current consumption to less than 1 microamp, which is negligible for a PP3 battery.
Design
Pressing the push-switch charges up the large capacitor and switches on the n-channel MOSFET (2N7000), which has a very low on-resistance. Gradually the capacitor discharges through the 2.2 Mohm resistor, until the gate voltage is low enough that the MOSFET switches off suddenly.
Circuit Diagram
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