Hi Vasile,

I welcome your comments on improving the design of my circuit.

> Why you need 7 seg AND leds ? Can't you do it only on=20
> 7seg display ?

The 7-segment display (red color) is to constantly display a number. =
Even
the decimal point is used as a separate LED indicator. The other two =
LEDs
(red & green) are required to indicate key press and mode changes.

> 9 buttons AND 8 relays coud be connected on 6+1 pic pins using a
> simple 3 to 8 decoder and a 3 x 3 matrix for buttons. That +1 pin is
> available also for other jobs.=20

Well, driving the 8 relays with 3 to 8 decoder would mean using two =
chips
since I have to also use a relay driver like ULN2803. Instead, I have =
used
UCN5841A (from Allegro), which combines a serial-in parallel-out shift
register and relay driver for 8 relays. This requires 3 pins, but two =
of
them are also used by the other shift register.=20

>  ... often when you need serial/parallel=20
> extra devices is definitely the sign you must change=20
> to another microcontroller. ( excep designs dealing=20
> with dosens of relays and thousents of leds )

I agree. In fact I have been seriously thinking of using 16F870 (28 pin
skinny DIP) instead of 16F628 for this project. I get 22 I/O pins and =
more
importantly, I can try out K=FCbek Tony's nice suggestion of using A to =
D
converter and only one pin to detect closing of 8 or more PB switches; =
the
added advantage being that there will be only 3 wires connecting the =
main
PCB and the keypad.

Like it happens to many others, my problem is that I conceive the =
design,
write the code and make the circuit in the breadboard stage work as
expected. Then new ideas flash in the mind. The design is modified, =
code is
rewritten and new features are added. The wonderful people on this list =
with
their suggestions greatly contribute to this process. This goes on for =
a
while. When the ideas for the project dry up, the order for the PCB is
placed!

Regards,

Bala

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