I’m trying to make a power switch which only turns on the circuit if it’s pressed twice like double tapping a phone screen.
Is there a way to make it completely mechanically (i.e. the power button does not require any power to operate)?
If not, how can it be made electronically so that the switch circuit uses minimum power.
Thanks,
What kind of power is being switched to turn on “the circuit” above ? What kind of power is available to run an electronic switch ?
A way to do it electronically would be to have a simple micro in a sleep mode and awaken on the 1’st switch push. It then times while waiting for the 2’nd switch push. If that happens “in time”, the micro switches power on via a FET or SSR. If the 2’nd push comes too late (thus not a double tap), or not at all, the micro goes back to sleep. Some form of hardware switch debouncing will likely be needed.
I suspect a 556 could do the above too. Mebbe.
To do it mechanically … I can only hint at a method. The activation is via pneumatics. The 1’st switch push injects air into a chamber w/a small leak. If the 2’nd push happens “in time”, enough air pressure is built up to push a latching switch. Wait too long and the leak bleeds off enough pressure that a 2’nd push doesn’t activate the latching switch. Of course it’ll take a double push to turn the power off.
How much power are you switching?
I designed a device that does exactly this and works pretty much how Mee-n-Mac describes it. It consumes a few microamps (I’d have to go look at my documentation to remember exactly how much) of standby current and switches about 100mA IIRC after receiving a double click on the input switch.
I never figured out how to market it outside its original niche, but If you’re interested, I can build another one. Won’t be free, but we can probably come to an agreement 
If you want to do it mechanically, I can’t think of a way, but a common way to avoid accidental switch activation is to use a safety switch that requires two motions to activate. Like this: http://www.digikey.com/product-highligh … eries/3136
Sonu_su:
…If not, how can it be made electronically so that the switch circuit uses minimum power.
Thanks,
D-flip flops and JK-flipflops (set up to work like a d-flipflop) can be used to work like a pulse divider. Or basically divide a pulse frequency by 2. The release of the button after the 1st press would make the output change state. The second press and release would turn the output off again to original state. Is that sufficient? Or do you want the equivalent of a ‘double click’ action to change the output on. 2x press-and-release to turn something on. If so then you need a 2nd D- or JK flipflip stage to divide the pulses by 2 again. Which is no extra parts as these IC’s usually have multiple (2) per package.
The output can turn on a transistor or mosfet to drive heavier currents, which could even be a relays-switch to fully isolate it it from the load.