Electronic – Need a soft latching power on/off that starts OFF


I have spent a few days looking for a soft latching power on/off switch. I have breadboard-tested several designs, including at least these three:

Design #1

NE555 One button on/off switch

Design #2

One button on/off switch

Design #3

Two buttons on/off switch


I tested all 3 of them with and without load. As load I used a lux meter project (Arduino Pro Mini, BH1750 lux sensor and Oled display). The 555 alternative (design #1) is on the breadboard at the right. Power cables are at far right:

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All of them work (with/without load), but … all of them start frequently in ON position when connected to the power supply (with/without load, tested physically inserting/extracting the positive power cable from the breadboard).

Just think what may happen when main power returns after a blackout.


Do you have a switch design that guarantees starting at OFF when power is apply, just like a mechanical on/off switch? Bonus: One that does not oscillate when you keep the button pressed?

(I really like two separate buttons, if possible).

Best Answer

Have you looked at this circuit.

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If you wire it in the auto-off position, you are guaranteed to startup with no power. It uses one push button to turn on(momentary), turn off(long press) and reset(momentary). If you look throught their website you will find different circuits for different needs but i think this should do it. I have simulated it in spice and it works just fine. The 10 uF cap adjusts the time you need to press the button to shutdown the system(10uF @5v supply=3seconds). You could also use a digital pin of a microcontroller to latch it to off. Pretty neat.