Out of curiosity (I like to have examples for when standard practices
don't meet the requirements) what about this application precludes
using a small plastic strip installed between the battery or cell and
the contact that is pulled out when one wants to activate the unit?
Given the button, mechanical action is necessary to start it, so why
must it be a push motion instead of a grab, pull, and discard motion?

I don't know if this idea would work or not, but note that some PIC
pins don't have protection diodes, but can still be used as I/O.
Attach the power input to an I/O without protection diodes, then use a
button to temporarily power the PIC which turns the I/O output high.
The I/O then shunts the power into the positive rail.

I haven't thought through the physics of running the I/O backwards
like that - it may be a non-starter right out of the gate.  In other
words, "This exercise left to the reader."

-Adam

On 1/18/07, Peter Todd <pete@petertodd.ca> wrote:
> I've been tasked with making a very simple device that will turn an LED
> on for one minute every day. I'm using a 16f683 with a DS32khz for the
> timekeeping and have designed a nice circuit that seems to use about 5uA
> sleeping and keeps accurate time.
>
> The application also needs to have a button, that when pressed starts
> the device up and starts it's led on, led off routine. Before that I
> want the device to be completely off, drawing no current from the
> battery, after pressing the button, it should apply power to the DS32khz and
> PIC. This should be a latch, so pressing the button again has no further
> effect.
>
> I've come up with a little bistable flip-flop that seems to do this, see
> http://gw.petertodd.ca/~pete/bistable.png When power is first applied
> the capacitor along with the resistor makes sure that MOSFET T2 is off,
> this causes T1 to be turned on. When the switch is pressed, T1 turns
> off, T2 turns on and the load is activated. Pretty simple.
>
> Of course this uses 0.3uA when off due to discharge of the 10Mohm
> resistor. I'm also worried that EMF could trigger the circuit,
> especially with that 10Mohm resistor.
>
> Anyone built anything similar? I know a lot of Maxim RTC parts say that
> the internal batteries are protected by an "electronic seal" that only
> activates upon the first application of power.
>
> --
> http://www.petertodd.ca
> --
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