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I'm building a LED flashlight powered by a 3.7v lithium battery that must be ON when I push the button OFF when the button is released.

Problem

The only push button that I have in stock works other way round: by default is always ON (the circuit between its 2 poles is connected) and when I push it interrupts the circuit entering in OFF mode.

What is the proper way to reverse its logic to the desired (ON when I push, OFF when I release)?

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    \$\begingroup\$ What does "convenient" mean to you? What are the voltage and current levels? How much room do you have for extra circuitry? How much standby current can you tolerate when the flashlight is off? \$\endgroup\$ – Elliot Alderson Feb 14 at 15:08
  • \$\begingroup\$ @ElliotAlderson With convenient I mean, without overcomplicating the project with too much circuitry or expensive components. I'm not sure about eventual standby current leakage tolerance, I can do some test anyway keeping it lower possible is better. \$\endgroup\$ – AndreaF Feb 14 at 15:18
  • \$\begingroup\$ OK, what do you mean by "overcomplicating", "too much circuitry", or "expensive"? We can not read your mind, and these words have vastly different meanings to different people. \$\endgroup\$ – Elliot Alderson Feb 14 at 15:19
  • \$\begingroup\$ Easiest by far is probably to attach a small relay. Though you could of course also use a transistor. The mechanical solution will likely be far more important than the electrical solution. \$\endgroup\$ – Lundin Feb 14 at 15:20
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    \$\begingroup\$ The cheapest possible solution is almost certainly to buy the correct switch. Your question doesn't make sense. \$\endgroup\$ – Elliot Alderson Feb 14 at 17:44
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Something like this should work. The standby current draw is less than a microamp. Depending on the battery size, the battery might last almost as long as without a load.

"Jonk" loves these kinds of problems, he will likely have something better.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ 10M you serious? If you get moisture anywhere remotely near that flashlight, it will go haywire. And lets not even mention EMC. Cough and the LED will flicker. \$\endgroup\$ – Lundin Feb 14 at 15:41
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    \$\begingroup\$ @Lundin - If this is a product, use the correct type of switch. If this is temporary hack, try this circuit. \$\endgroup\$ – Mattman944 Feb 14 at 15:48
  • \$\begingroup\$ Hack or not, it won't be a reliable one. I'd much rather take shorter battery life in exchange for an actually working flashlight, by picking 10k or so. \$\endgroup\$ – Lundin Feb 14 at 15:58
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    \$\begingroup\$ I once designed a <1 nano-amp trans-impedance amplifier with a 300M feedback resistor. Yes, it needed to be shielded very well and protected from moisture. Agree that protecting a flashlight from moisture may not be practical. But, this is not a practical question anyway. \$\endgroup\$ – Mattman944 Feb 14 at 16:21
  • \$\begingroup\$ I built the circuit, put it in a throwaway plastic food container. V1 = 3 AA Ni-MH batteries, M1 = ZVN4424A, R1 = 100 ohms, D1 = common white LED. Works fine. Can't get the LED to flicker, even if I put it near an AC power drill, or touch the circuit. \$\endgroup\$ – Mattman944 Feb 20 at 9:08

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