Multiplexed LCD question

[Igor]

Hi,

I want to connect a multiplexed LCD to a microcontroller,
obviously to save I/O pins, I've been looking into LCD
controllers and found that to drive ~32/4 multiplexed
segments will require a chip that will cost at least two
bucks. Does anybody know of a cheaper way to drive the LCD
(the multiplexed one) directly from uC, without using
tri-stated pins? What's your opinion about the following
(simplified) curquit:

---------
uC | Seg 1 -------------------------------
|---------------| |
| ... | LCD |
| Seg N | |
|---------------| |
| -------------------------------
| |Com1 ... | ComN
| Com 1 | |
|-----------------[transistor 1] |
| ... | |
| Com N | |
|-----------------------)--------[transistor N]
| | |
--------- -----------[Ground]

- where one common line is "active" per frame and there
is no need to generate complex voltage wave forms, like LCD
controllers do. Is there any problem with this?

Thanks,
Igor

 

[James Meyer]

- where one common line is "active" per frame and there
is no need to generate complex voltage wave forms, like LCD
controllers do. Is there any problem with this?

Thanks,
Igor

That circuit will result in an average voltage applied to the LCD that
is some level of DC. DC drive will work, but it will eventually degrade the
LCD. Try it. The LCD may eventually fail, but it may also last so long that
you have stopped using whatever it is you built.

Jim