4 bit (dual) LED display

PHOBoS

Something I, and probably many others, have wondered about is how to use more than one LED display (for numbers > 9).
I know there are special counter chips that can directly drive displays and can be linked but that's not very useful if you just want
to display a decimal value of a binary number. You could use a PIC or something similar which might be the easiest solution for larger displays.

But I did came up with a simple solution for 4 bits.
My idea was to directly use the input bits for numbers 0..9 and for larger numbers add 6 to it using an adder.
This means that 10 (1010) will become 16 (10000) and by using only the last 4 bits the result is 0.
10 (1010) + 6 = 16 (10000) -> 0000 = 0
11 (1011) + 6 = 17 (10001) -> 0001 = 1
12 (1100) + 6 = 18 (10010) -> 0010 = 2
etc.
So what's needed is a circuit that detects when the input > 9 then add 6 with the adder and at the same time
display a '1' on the second display. (the one on the left)

However it turned out to be even easier to do but still by using an adder.
The only (available) CMOS adder I know is the 4560 and there is something 'special' about this one.
It's an NBCD adder which means that for numbers larger than 9 the output doesn't just count up in binary
but instead starts over from 0 while generating an extra (carry) bit. (it's base-10).
So it detects when the input > 9 then it starts from 0 again and has an extra output bit Shocked

That's what I wanted to do in the first place and this chip already does it all for me Laughing

The only thing to add is something to turn the second display on ('1').

the schematic:
I drew 2 versions, one for CA and one for CC displays. On the left side the adder with the carry out connected to
a transistor to display the '1' (it's connected to segments b and c of the display). The 4-bit outputs of the adder are
connected to the display driver chip (CD4543). A nice feature of this chip is that it can be used for both CA and CC displays
depending on the state of the Phase input (pin 6).

You could display 00..15 instead of 0-15. For this you would have to turn the b and c segments of the second display
permanently on and segments a,d,e,f off for numbers > 9.
And something else I just thought of is making input B1 (pin 14) of the adder high which should result in 1-16 instead of 0-15.

I haven't tested the circuit because I don't have the chips yet but I did already solder it to perf Rolling Eyes

(maybe I'll 'borrow' the chips from another project for testing)

I designed it to use with the fibonacci sequence generator, but made a seperate thread so it doesn't get buried in there.
If someone knows an easy way to display an 8 bit number let me know Wink