I was thinking about using 12V  4 or 7 watt bulbs but, you're right that
adds up to a lot of power in a hurry. If 40% were on using 7 watt bulbs
that would be 1176 watts using a 5 x 7 matrix! Think I better scale this
down and do some experimentation to see what bulb size I really need. I
want the display easily readable from a distance of 50 - 75 feet at night
(I think the night part will help a bunch).  Thanks for giving me a start
on this, will probably be back with more questions since I'm fairly new to
the software end of PICs.




                    Donald L
                    Burdette              To:     PICLIST@MITVMA.MIT.EDU
                    <dlburdette@JU        cc:
                    NO.COM>               Subject:     Re: High Current Serial
                    Sent by: pic          Display Driver
                    microcontrolle
                    r discussion
                    list
                    <PICLIST@MITVM
                    A.MIT.EDU>


                    11/13/99 11:43
                    PM
                    Please respond
                    to pic
                    microcontrolle
                    r discussion
                    list






You haven't mentioned what bulbs you intend to use, but I'm guessing you
want 1/4 or 1/2 watt bulbs.  At 12 Volts, that's about 20 or 40 mA per
bulb.  I'm also guessing you'll use a standard 5x7 matrix per character.
At 12 characters, that's 420 bulbs.  If worst case is 40% of bulbs on
(I'm guessing here!), that's 42 or 84 watts, and 3.5 or 7.0 amps.

While these numbers are not unreasonable, they are not trivial to work
with.

Multiplexing sounds great.  Since you'll have a 7x60 matrix, you'd have
60+7 drive pins to work with rather than 420.  However, I don't know that
a 1/60 duty cycle is reasonable for incandescent lamps.  I think you'd
want about 12*(60^0.5) = 93 volts drive (somebody correct me here if i'm
wrong).  Of course, at that voltage your average current would be less
than 1/2 or 1 amp.

If I were doing this, I think I'd go for 1/4 W bulbs, grouped into 4
groups of 3 characters.  That would give me a 28 x 15 matrix, and 1/15
duty cycle, requiring about 46.5 volt drive (round to 48?).  I'd use
74HC595 shift registers, 4 for rows and 2 for columns.  I'd make the rows
the low side, with a 1K resistor driving the base of a ZTX453 transistor.
 Columns would be the high side, with ZTX553's, 1K resistor to a ZTX453
being driven by the '595 (see the current thread on high side drivers).
These parts are all available from Digi-Key.  Suitable substitutes could
be found easily.

 My code would look kind of like this:

        Shift out 4 bytes to row drivers (each row is a full byte to make
software easier)
        Shift a 1 into first column driver
        Strobe the '595 output latch pins to update the output data
        Wait for 7 mS

        Shift out next 4 bytes for row drivers
        Shift the 1 to the next column driver
        Latch the outputs
        Wait 7 mS

        ...

This would give about 9.5 Hz refresh rate, which Wagner says is adequate.
 You could trim the 7 mS down if you see flicker.

Let us know what you end up with!


Don