True, the zener requires some calcs, but I was suggesting
the zener as one of other alternatives since the 7805
seems so overkill.  Personally, for a flashing LED ckt,
no matter how revolutionary, I'd just use 4.5V, which
can be obtained from 3 batteries or a 4.5V wall-wart,
etc.

BTW, I'm starting to get used to the "Mr. Dude" thing. :-)

Cheers,
-Neil.



-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Roman Black
Sent: Friday, July 19, 2002 2:35 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC]: Flashing LED Episode II (small attachment)


Kieren Johnstone wrote:
>
> I fixed the MCLR-wrong-polarity thingy, added a resistor, and a zener
diode
> to regulate instead.  But, a question (which my books don't appear to
answer
> properly!) - would I use a 5.1V zener to drop the voltage to 5.1V, or a 2V
> zener to drop the voltage (from 6V) to 4V?; I would assume that the latter
> is true, because a website told me they have a "specified voltage drop" -
> but to quote Mr. Dude, "a 5.1V zener".
> Just to clear that up..


I'm not sure Mr. Dude's suggestion is the best for
your needs. :o)

Using a 5.1v zener with a 6v battery raises many
problems, and you really need a good understanding
of ohms law etc to "tune" it for each circuit.

Since you seem to be starting out with PICs and
with circuit design, I suggest something you can use
on lots of circuit, because the moment your "flashing
led (TM)" is working you will want to build other
stuff with your PIC 16F877.

Maybe use a 12v plugpack (wall wart) and the 7805
that you have already. Another suggestion is to put
a 150 ohm resistor between the +12v and the INPUT
of the 7805. This is very handy when you are first
starting out and will *probably* stop you frying your
expensive 16F877 when something gets connected wrong
(which seems very likely!)  :o)

Is there any reason you're not using a 16F84 or 16F628?
These program in about 20 seconds, compared to about 2
minutes to program a 16F877. You will soon get sick of
THAT wait.
-Roman

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