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

latchpowerswitches

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

Design #2

Design #3

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:

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).

Best Answer

Have you looked at this circuit.

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.

Related Solutions

Electronic – How to restrain this soft latching power switch from eating too much current

The FETs can hog current upto 500 mA. For higher load, it is very easy to find FETs with low Vgs threshold. Before pressing the switch SW1, M1 will be off since Vgs is zero. Once switch is pressed, M3 turns on followed by M1 followed by M2. M2 holds the ground to gate of M1 there onwards. When SW2 is pressed, M2 releases the Gate of M1, making the Vgs of M1 zero again, thereby cutting out 5V_OUT. 7805_5V is regulator output. Needless to say, the switch section will consume significantly less current to be suitable for portable applications. I hope, power dissipation across 7805 is looked upon.

(Vin - 5V)/0.7 Watts

schematic

^{simulate this circuit – Schematic created using CircuitLab}

Electrical – Latching circuits – problem above input voltage

Circuit 1: VGS max is rated as 25 volts for Q2. At 30 volts, your gate source junction is breaking down. Circuit 2: When you push the button to latch off, P channel voltage is momentarily pulled up to [power input - power input * 15K/(100K+15K)] as @sstobe says. At 16 volts, this is about 2 volts, so you never turn the MOSFET off (VGS(Th) can be 2 volts from data sheet). You have to pull the gate voltage up to within less than this value.