piclist 2001\01\24\014504a >
Thread: Auto off circuit solution
picon face BY : Russell McMahon email (remove spam text)

>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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