Question regarding micro powerup

Hello all,

I recently stumbled across this great forum. After a few weeks lurking and learning a lot I have my first post.

I'm waiting for some hardware to arrive before I can test something but I thought I would throw the question up to the Guru's on here for feedback.

For simplicity I will explain my (basic) circuit requirements.

I have a pic micro (TBC) and want this to be totally physically isolated from the battery when instructed by the micro. (it's a long story and is not for current consumption reasons.)

If I put a momentry push switch on the +supply line of the micro, what would the minimium time be that this would need to be pressed before the micro would power up, execute some code that would pull a pin high. This would fire a transistor to pull in a relay. A set of contacts on the relay would then be in parallel with the momentry switch, therefore when the momentry switch is released the relay contacts would still bypass the switch and keep the micro powered.

The code would execute and if necessary pull the relay drive pin low. The relay would drop out and all would be 'physically' isolated from the battery.

The only way to power up would be a momentry press switch actuation.

Please note. The battery has to be physically isolated so a relay is the only option.

I hope this makes sense

Thanks for any feedback.

If I put a momentry push switch on the +supply line of the micro, what would the minimium time be that this would need to be pressed before the micro would power up, execute some code that would pull a pin high. This would fire a transistor to pull in a relay. A set of contacts on the relay would then be in parallel with the momentry switch, therefore when the momentry switch is released the relay contacts would still bypass the switch and keep the micro powered.

Click to expand...

Sounds a lot like Mouse Trap.

Instead of using a relay, I'd recommend a mosfet. With a relay you'll need a transistor anyway to make sure the feedback from the relay doesn't kill the pic. Might as well skip the relay althogather.


Power on time is going to depend on the type of resonator being used, voltage, pic configuaration, etc etc. If the osc was already running all that would happen within a few instructions, if you use a high speed crystal you can expect to wait up to a few milliseconds. Ceramic resonators are quicker then xtals, not sure about RC clocks. Either way, sounds like it would happen before you could ever release the button anyway,

Wow, what a novel idea - have a button power-on the pic, which then triggers a transistor that powers itself! Probably common knowledge, but it sure gives a great way to power on a pic, do some task, and power it back off. Great for little battery-operated toys!

Thanks!!!

Thanks for the feedback guys.

Still waiting for the hardware to turn up to see if the principle would work.

It's not a mousetrap but I like the idea

The requirement I have is to totally isolate the battery from the complete circuit when it gets the command.

Thanks.