[quote author Crossroads] "You are pretty close with your design idea.
It needs 2 transistors. A P-channel MOSFET that acts a "high side" power switch.
Its gate is pulled high to turn it off.
When the DS3231 Interrupt line goes low, that turns the P-channel on.
The Arduino then drives a simple NPN on to hold the gate low and does whatever the arduino does. When done, it stops driving the NPN, letting the pullup resistor pull the gate high and turn off again.
The NPN is needed so that the pullup resistor voltage does not power the Arduino thru an IO pin's protection diode.
Need to us a Low Rds, Logic Level P-channel FET. A thru hole part could be:
http://www.digikey.com/product-detail/en/NDP6020P/NDP6020P-ND/1055922"
Hi
Sorry for asking about a thread of last year. I am a newbie and very interested in what is descripted in this thread. I am building a feeder for pheasants triggered exactly at sunrise and sunset (my gratitude to forumites Bricolo and Etibou). As a low power device, I opted for a pro mini working twice 10 seconds each day. The rest of the time, I want to power it off, hence my great interest in this thread.
If You can please help me, I am unsure of how to design how to sink down via a NPN the gate of the Mosfet, as descripted by Mr Crossroads.
Elements : pro mini, PCF8563 RTC, TPS782 Voltage Regulator 3,3V, NDP6020P p Mosfet, P2N222a transistor.
.
As You can see on the picture, the RTC sends via INT/SQW a low interrupt to the p Mosfet gate, that way closing the fet and powering the arduino. As the duration of the interrupt is short (don't know how long), immediately, the Arduino should ASAP trigger via pin 7 a NPN to keep sinking the gate low as long as the arduino is doing its tasks.
What I am unsure of is how to properly sink the gate low ??
Formerly, the INT/SQW applied a high impedance (3 V, 2AAA) with a pull up (10K) linked to the VIN of the battery (3 AAA cells). So, the sinking of the gate should also overcome this pull up.
Is my using only a 10K resistor (R4) enough ?
Marginally, as a newbie, I am always wondering which resistors picking, mostly copying though I am supposed to know the Ohm law. If You can please check the other resistors ? And any nonsenses ?
I hope that opting for a P2N222a transistor is correct ?
Thanks a lot for Your valuable contribution in this forum and Your advices.

[/quote]