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'[EE]: Auto off circuit solution'
2001\01\22@132749 by John Pearson

Sorry for pestering.

I have not been able to access the archives for some time. I get an error
message about the server being overloaded, or something.

Thanks for the solution.


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2001\01\22@140202 by jamesnewton

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The archive at
is working at WELL under its maximum capacity.

James Newton (PICList Admin #3) 1-619-652-0593
PIC/PICList FAQ: or .org

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2001\01\22@145031 by jamesnewton

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There are at least 4 archives.. see

James Newton (PICList Admin #3) 1-619-652-0593
PIC/PICList FAQ: or .org

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2001\01\22@192802 by Russell McMahon

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>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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2001\01\24@014504 by Russell McMahon

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>I need an auto-off circuit, that can be started with a momentary push
>button, but I don't need to turn the circuit off manually. Just let it
>time-out and turn off. Would this allow for any fewer parts. As far as I
>can see, I just wouldn't have to monitor point A, saving only one i/o line
>and a resistor.

I don't use the monitor point in my application - I assume that the user
presses the button to start the unit and then wants it to run. Alos, in my
application, the push button is part of a small number of PIC input keys so
I know when the user is pressing it (same result as monitoring poibnt A I
guess). The program makes decisions about shutting down based on user input
or lack of it.

As shown the circuit will release essentially as soon as the button is
released. Adding a capacitor at the circuit side of the push button will
provide a "low" when the button is released. This will charge until the
transistor turns off. Delays of perhaps a few seconds maximum are achievable
this way. Using the PIC to hold the circuit on is a more reliable method.

>Does the bc337 just isolate the pic i/o pin (B) from the higher than Vdd
>voltage of the battery?

Essentially yes - but care is needed if you want to use a PIC pin  here
directly . The lower transistor collector can swing happily above PIC Vcc
with no ill effects.
If you switch the base of thge upper transistor with a PIC pi  it must not
ever be pulled more than 0.6 volts above PIC Vcc. if this happened the
internal protection diodes would take over and hold the upper transistor on
(or partially on) . The only acceptable pin for highe rthan Vcc switching
would be an ipen collector one - eg RA4 on 16F84 and numerous other PICs.
Given the small size and low cost of a single transistor and resistor the
saving hardly seems worthwhile. If you did this you would need a different
start circuit (pill down top transistor base via a resistor).

>If so, could a logic level fet be used instead of
>the ztx749/750? I am not sure what logic level fets do, I just saw them in
>a catalog without any specs.

A logic FET is simply a FET which can be fully turned on by "logic level"
drive signals eg 5 volts. Some operate from as little as 3 volts. N channel
FETs are available as logic FETs buyt P channel FETs (used in the upper
transistor position in this circuit ) are essentially not available as logic
FETs for technical reasons.

The original circuit used a FET for the upper "transistor".
An N Channel  logic FET could be used for the lower transistor.

A major advantage compared to using a bipolar transistor here is that the
FET draws no tuen on drive current and a long delay turn off circuit could
be implemented using a modest sized capacitor plus a resistor to discharge
it. Turn off would be a little "soft" as the FET turned off and the circuit
would need to be able to handle this.


               Russell McMahon


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