piclist 2001\01\22\192802a >
Thread: Auto off circuit solution
picon face BY : Russell McMahon email (remove spam text)

>There seems to be a lot of interest in an auto off circuit. I will offer
>this circuit for use, with a brief explanation. The intent is to offer a
>solution that gives clear detail for the archives. Hopefully people will
>find and use it, and quit bothering us with repeated questions.

Gordon Varley has suggested an excellent solution.
I would like to suggest something which works as well for me and which has
the advantage of working with lower battery voltages.
The FET solution has the advantage of potentially less control working with
much higher on currents with

I use something almost identical to Gordon's circuit BUT use a bipolar pnp
transistor instead of the FET.
This allows the circuit to work down to very low voltages.

I am using this in a design which operates a PIC 16F84 from 3 x NiMH button
cells at a nominal 3.6volts. It should work down to 2 volts or less , with
transistor Vbe being the main limit.

Replace FET with appropriate pnp transistor. I use Zetex E-line series (TO92
like) with current ratings up to several amps and current gains of 100+ at
1A plus.
eg ZTX749, ZTX750 etc but other types will work. The high gain and low
saturation of the Zetex devices make them often well worth the slight extra
price. At typical PIC circuit currents (10's of mA) saturation drop should
be well under 0.1 volts.

Connection: Replace FET in Gordon's circuit with PNP with Emitter to
Battery+ [was FET source] , Collector to output [was FET drain] , Base to
22k/10k boundary [was FET gate].  Resistor values could be changed somewhat.
Say 22K -> 100k or even 1M just to hold transistor off when unpowered. The
10k may be able to be increased to as much as 100k depending on upper
transistor Beta and load current. I use a BC337 for the lower transistor but
that's just because it's my default small npn - most anything would do

Instead of the on/off switch I take a diode from a normally low keypad line,
so that pressing the appropriate key wakes up the system without affecting
the normal keypad function.

With this circuit I get an off current of "unmeasurable" (under 0.1 uA.).
I found that PIC quiescent current was too high for my needs no matter how
much care I took and this solved the problem perfectly.
If essentially zero off current is desired It is vital that in the off state
there are NO current paths from battery to ground that do not pass through a
transistor collector-emitter path.

Adding a capacitor to the 10K / on-off switch junction will allow the
circuit to remain on for a short period after the key/button is released
giving the processor a longer period in which to take control.


       Russell McMahon

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