piclist 1998\10\07\051425a >
Thread: 4 digit auto-ranging frequency-meter.
face BY : Regulus Berdin email (remove spam text)

Dave VanHorn wrote:
> >You can still minimize the parts by having the 7segment common anode,
> >make all the colllectors of the NPN transistors tie to the +5V supply
> >and reverse/invert drive to the LED (to the negative side).  This way
> >the base resistors is not needed anymore.
> >
> >Reggie
> Are you SURE they will turn off? Look at the pin specs for minimum
> high output voltage, and leakage current. I've got my doubts about the
> current design, but IIRC, they can drive closer to ground than to VCC.

Yes I'm sure, 25 ma for sinking, V(OL) = 0.6V . I had used this
configuration on some of my designs.

In case you misunderstood my point, here is an ascii schematic of what I

          O Vdd
          |               |               |               |
          |               |               |               |
        |/              |/              |/              |/
RA0 O---B|      RA1 O---B|      RA2 O---B|      RA2 O---B|
        |\              |\              |\              |\
          V               V               V               V
          |               |               |               |
          O               O               O               O
   digit select 1  digit select 2  digit select 3  digit select 4

RB1 O---/\/\/\-----O Segment A
RB2 O---/\/\/\-----O Segment B
RB3 O---/\/\/\-----O Segment C
RB4 O---/\/\/\-----O Segment D
RB5 O---/\/\/\-----O Segment E
RB6 O---/\/\/\-----O Segment F
RB7 O---/\/\/\-----O Segment G


<361B2271.339D259B@skyinet.net> 7bit

See also: www.piclist.com/techref/index.htm?key=digit+auto+ranging
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