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Auto poweron/poweroff is a perennial.
Here's a post message I posted in April based on something posted in ....
This is based on a circuit I have in successful use in a  low volume
production specialist item.
I could give you (Jinx) a PCB to populate to try it if you wished.

This draws NO measurable current when off, starts on a button press and
subsequently allows the button to be used as an input.


        Russell McMahon

________________________________________

> I'd like to
> 1)  Turn on the device by pressing a button
> 2)  Later be able to have the PIC turn itself off
> 3)  While the PIC is on, have it be able to detect "ON" button presses
>        again
> 4)  Only use one IO pin (so gnd or +5v to switch off, high-impedance
>      input  to detect button and stay on)

Here's an attempt to meet your full specification.
I haven't tried it and it's not fully designed yet but I think a little
thought and/or playing would make a reliable working circuit from it.

There are too many requirements on the single pin to allow a simple digital
0/1 full rail signal level to be used.
Roman Black recognises this in his SCR design and "cheats" by making the
turnoff function independent of the pin - in his cct the PIC reduces its
current consumption below the SCR hold on current. This is a clever and
legitimate solution but will not work in cases where there is other
circuitry drawing current in excess of the SCR holding current.

My solution also uses a "trick" - but using the PIC pin to do it.
Transistor TR2 is held on by R3+R4 BUT the voltage at the junction of R3 &
R4 is a LOGICAL low level which the PIC can read. When the button is
operated after startup the PIC pin sees a logical high.

To turn the circuit off the PIC (or other cpu) takes R3/R4 junction to
ground. Proper design may wish to see a small series resistor to avoid
possible damage from the 100uF capacitor discharging into the pin.
The true low level turns off TR2 and initiates shutdown. Somewhere along the
way the PIC will lose the ability to maintain an active low output and C1/R3
must have a long enough time constant to hold the transistor off after this.

The time that the voltage into the regulator (on capacitor C7) stays
actively high will be an important factor in turn off.
Suitable care and thought would definitely turn up a working combination but
odds are that as shown the component values are wrong.

If the regulator has a substantial output capacitor but a smaller input cap,
the PIC may be designed to stay alive after TR3 is turned off. This would
ensure the circuit is well shut down before the PIC expires.

PIC "death throes" need to be carefully thought through - as is always the
case in this sort of circuit.

Z1 stops PIC pin being taken illegally high.
R5 provides turn off for TR2 in absence of other drive sources.
R3/R4 divider, which holds TR2 latched on, must provide low enough output to
place a logical 0 on PIC pin, while R7/R4 divider provides a logical high.
R7 sized to provide enough level for a logical 1 - higher voltages will be
clamped by Z1. D3 left over from a prior circuit - does no harm but Z1
probably sufficeth at 4v7.
C1 value should be as low as reasonably possible consistent with turn off
timing requirements. R4 can be made much larger (depending on transistor
beta, load current supported  etc) - allowing scaling up of other resistors
allowing smaller C1



        Russell McMahon






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