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'[PIC]: EE challenge PIC && 12V battery'
2001\01\14@183056 by GMorbe

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part 1 596 bytes content-type:text/plain; (decoded 7bit)

Hello Roman, All

I think i go the discrete approach because of components availability
reasons.
The use of a capacitor for the hold time is clever but i dont like it,
because the PIC would die down slowly by discharge unless you take special
action to avoid this.
Meanwhile i found a solution around the diodes to decouple the button
information from their switch on function. By connecting the buttons to
the - potential its possible to decouple them with resistors only. The
values of them needs to be well defined however. See my picture.

Germain Morbe




part 2 7068 bytes content-type:image/gif; (decode)


part 3 131 bytes
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2001\01\14@215128 by Dan Michaels

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GMorbe wrote:
>Attachment Converted: C:\WIN31APP\EUDORA\ATTACH\PICTX.gif
>

I wonder that you might not have a minor problem with your ckt.
The typical leakage current into PIC input pins is ~1uA, so 10M
series Rs would seem to be too high.

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2001\01\15@012946 by Germain Morbe

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Dan,

you may be right, the values have to be well defined. The shown values are
not tested yet, but are a rough calculation to start with. See the time
stamp of my posting. Thank you, i will check it this morning. :)

Germain Morbe

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2001\01\15@022721 by Roman Black

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GMorbe wrote:
>
> Hello Roman, All
>
> I think i go the discrete approach because of components availability
> reasons.
> The use of a capacitor for the hold time is clever but i dont like it,
> because the PIC would die down slowly by discharge unless you take special
> action to avoid this.
> Meanwhile i found a solution around the diodes to decouple the button
> information from their switch on function. By connecting the buttons to
> the - potential its possible to decouple them with resistors only. The
> values of them needs to be well defined however. See my picture.
>
> Germain Morbe

Hi Germain. I'm not sure I like the design. Your 12v
battery voltage is permanently connected to the two
PIC pins, which:
1. is way out of voltage spec!!
2. will drain battery into the PIC protection diodes.

Why not replace the 10M with 1N4148 glass diodes, these
are cheaper and smaller than resistors, and will
solve the two problems...
-Roman

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2001\01\15@034910 by Michael Rigby-Jones

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{Quote hidden}

He would also need to provide pullups from the PIC side of the diodes to
Vcc, otherwise there would be nothing to pull the PIC pins to a logic one if
a button wasn't pressed.

Mike

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2001\01\15@052040 by Simon Nield

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you can ditch the left hand transistor and it's associated bits if you turn one of the button
reading ports round to be an output once you have read it. Just use the PIC pin to pull the line
low... would need to ditch the 10M resistors too though.

replacing the 10Ks with say 220k would reduce off current, if your transistor has enough gain.

connecting 1n4148s or similar from the button pins (anode) to vcc of the pic (cathode) will ensure
the voltage on the pic pins remains within spec, although if you check the datasheet hard enough you
might find that the protection diodes are able to perform this function safely (personally i would
use external diodes)

(the radio is constantly powered at the moment btw, which I assume is just an error in your
schematic ?)

a handy tip for optimising low power stuff: measure the current being drawn from the battery with a
well calibrated meter and hack your code to make the unit operate either constantly on or off (you
need to do both) then try disconnecting bits of the circuit you think you can optimise and see if
they actually make a significant change to the current draw. if there is no big change then don't
waste your time. (useful for instance in verifying if the 'off' current of your circuit is
acceptable / improvable)

regards,
Simon

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2001\01\15@092127 by Germain Morbe

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Simon,

according to Roman ( and he4s right by the way ) the portpins are not real
oct because of the protection diodes as you found also. Therefore pulling
the base low after a button was pressed would work, but releasing it to high
state ( what means 12V in this case ) would probably not. Also changing the
10M resistors to a significant lower value would result in exessive current
drain while in off state.

Roman, i will decide to use the diodes or not after i finished my
messurements. I took the idea for the simplification out of a MCHIP appnote
about interfacing a pic to the AC powerline via just one resistor. I found
if  thatone worked my should also.

Germain

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2001\01\15@102012 by Simon Nield

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germain:
>but releasing it to high
>state ( what means 12V in this case ) would probably not.

doh. of course it wont. my apologies. could use could use a series diode to fix it of course, but
then the circuit complexity is creeping up again without offering any advantages over what you
already had.

regards,
Simon

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2001\01\15@163539 by Gennette, Bruce

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Be carefull with that one resistor to full AC voltage - you'll get away with
it on 110V RMS systems, but 230V (peak way over 400V) systems *WILL* destroy
the resistor in time.  The solution is to use 2 equal value resistors in
series so that each experiences less than 300V peak (their design limit).

This is a global discussion group so designs and FAQs *SHOULD* always take
into account that most of the world now uses the nominal 230V standard.
(Are you listening Microchip ?).

Bye.

> {Original Message removed}

2001\01\16@050620 by mike

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On Tue, 16 Jan 2001 08:36:46 +1100, you wrote:

>Be carefull with that one resistor to full AC voltage - you'll get away with
>it on 110V RMS systems, but 230V (peak way over 400V) systems *WILL* destroy
>the resistor in time.  The solution is to use 2 equal value resistors in
>series so that each experiences less than 300V peak (their design limit).
..or use metal glaze resistors, which are deigned for high voltage
use.
>This is a global discussion group so designs and FAQs *SHOULD* always take
>into account that most of the world now uses the nominal 230V standard.
>(Are you listening Microchip ?).

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