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I am trying to design a datalogger that should run for a long time on one set of batteries. I have a RTC module that has a interrupt wire that is high when powered on and goes low when a alarm time occurs. When this happens I want to power on the main circuit, measure and then send a signal from the arduino to power off again.

I have tried to just run via a p-channel mosfet, but my electrical engineering skills are not sufficient to design a latch circuit for it. I didn't manage to get it to turn on when the power turns on...

I would be thankful for some help. nearly working circuit

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  • \$\begingroup\$ 1. Schematic or block diagram please? 2. How long does the interrupt from the RTC last? 3. How often does it repeat? 4.How long does a measurement take? 5. Is there an output from the "main circuit" that indicates the end of a measurement? 6. What's the relationship between the "data logger" and the "main circuit"? \$\endgroup\$ – EM Fields Aug 8 '15 at 12:41
  • \$\begingroup\$ I added my simulation circuit. The interrupt lasts for 1 second. The measurement is fast, but it might take more than a second to start the ic and do the measurement. The arduino will have one output dedicated to doing the power off. \$\endgroup\$ – Flyhard Aug 8 '15 at 13:21
  • \$\begingroup\$ You'll also need a signal of some sort to the Arduino to let it know that the measurement has been started/made so that it'll know when to turn off whatever it's supposed to. \$\endgroup\$ – EM Fields Aug 8 '15 at 14:40
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This circuit should do what you want:

enter image description here

IC2 represents your RTC module; I just picked one with an IRQ output.

When the IRQ line goes low, the latch (IC1) is set (pin 3 high). Pin 6 is then low, which turns on the P-channel MOSFET nd supplies power to the main circuit.

When the Arduino sets its output low, the latch will reset, and the power will be turned off.

(Note: I didn't include the decoupling caps on each of the IC's, they should have 100 nF caps between Vcc and ground.)

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