Just curious, can you simplify the circuit using a regulator that has an
on/off switch ?

For example

http://cache.national.com/ds/LM/LM9074.pdf or

http://rocky.digikey.com/WebLib/Sharp/Web%20Data/pq1txx1m2zp.pdf

Tal

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Russell McMahon
> Sent: Friday, September 12, 2003 8:33 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [PIC:] Auto shut off
>
>
> 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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