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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

Testing:

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:

enter image description here

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.

Question:

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).

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Have you looked at this circuit.

enter image description here

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.

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  • \$\begingroup\$ Very interesting. Clever, but I need something less power-hungry. \$\endgroup\$ – user83628 May 8 '17 at 17:36
  • \$\begingroup\$ I'm sorry. My previous comment was intended for Stretch's post. \$\endgroup\$ – user83628 May 8 '17 at 17:50
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Separate ON/OFF

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Very interesting. Clever, but I need something less power-hungry. \$\endgroup\$ – user83628 May 8 '17 at 17:51
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Here is a simple circuit. The S-1009 gives you bulletproof reset regardless of startup glitches or slow rise time- it is an open drain output that parallels the OFF switch.

The gates can be 74HC or 4000 series depending on your operating voltage (74HC is good to 6V absolute max). With 4000 series the circuit will operate to 10V.

Quiescent consumption is of the order of 300nA, which should offer some improvement over a mechanical relay.

R3 slows the turn on to help reduce glitching if there is some capacitance in the load.

Preferably use switches with grounding straps or add some resistance (a few hundred ohms to 1K) in series with each to help with ESD.

schematic

simulate this circuit – Schematic created using CircuitLab

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