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The situation is as follows: I have a 3.3V regulator that I would like to turn on and off by a momentary switch connected to its "CE" pin. This regulator has an input from a 3.7V battery and an output to a microcontroller (3.3V).

My question is, what is the simplest possible circuit that can be used so that pressing the switch once will turn the regulator off and then on again?

I cannot use software with the microcontroller as my intention is to turn the system off completely and I would not be able to turn it back on.

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    \$\begingroup\$ Use the switch to turn it on; have the MCU hold it on as long as it wants to be, then turn itself off. \$\endgroup\$ – Chris Stratton Dec 26 '20 at 0:57
  • \$\begingroup\$ I said I can't use the MCU. And the switch is momentary, that means that it only lets the current through when it is held down. If you let go, it opens the circuit. \$\endgroup\$ – Wave Wolf Dec 26 '20 at 4:18
  • \$\begingroup\$ It's not nonsense at all but something widely done. The button gets things started, the MCU starts up all but instantly and holds the enable active. \$\endgroup\$ – Chris Stratton Dec 26 '20 at 4:20
  • \$\begingroup\$ What is the part number of the regulator? \$\endgroup\$ – Bruce Abbott Dec 26 '20 at 6:02
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    \$\begingroup\$ You use the button to kickstart things and to tell the software to it's time to turn things off. \$\endgroup\$ – Chris Stratton Dec 26 '20 at 21:02
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You can use a JK Flip-Flop to achieve this. By setting the J and K inputs HIGH, you will be able to use the Flip-Flop as a toggle device to set the CE pin on your LDO HIGH or LOW, depending on the toggle state. A couple of notes:

  • Remember that a JK Flip-Flop will maintain it's previous state when the power is turned off. Having a circuit to reset the Flip-Flop would be helpful if this is not desirable.
  • Be careful with debounce. Add capacitors to the input if needed to combat this.
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    \$\begingroup\$ And how will you power the flip flop? Really you aren't achieving anything you couldn't with the MCU itself, and you add the bouncing sensitivity. \$\endgroup\$ – Chris Stratton Dec 26 '20 at 14:28
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    \$\begingroup\$ @ChrisStratton The OP couldn't use a microcontroller, stated in his post, so this was a suggested alternative. And the Flip Flop can be powered by the 3.7V battery. \$\endgroup\$ – Jay Dec 26 '20 at 16:47
  • \$\begingroup\$ @Jay I liked your proposal, I will think about it to make a good design. \$\endgroup\$ – Wave Wolf Dec 26 '20 at 17:26
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    \$\begingroup\$ The op's belief that they can't use the MCU is simply mistaken. \$\endgroup\$ – Chris Stratton Dec 26 '20 at 21:03
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    \$\begingroup\$ @ChrisStratton I won't continue to commit on the best way for the OP to achieve the desired results, but they clearly stated that they wanted a hardware based solution, not a software based one. I do agree that you have more flexibility with a microcontroller, but that's not what the OP wants. \$\endgroup\$ – Jay Dec 27 '20 at 0:55

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