Re: Driving tri-state LED matrix

On May 30, 7:42 pm, Tomás Ó hÉilidhe <t...@xxxxxxxxxxx> wrote:

Is there anyway I can get this to work... ? I'm thinking of using one
BJT and one FET, and also a pull-up/down resistor. I'll make up a
circuit and post a picture.


OK here's what I've got:

http://users.imagine.ie/toe/disp3.JPG

A few points about it:

* Because the source of the NMOS isn't going directly to ground, I
have to get a FET that has a 4th pin for the bulk.
* The 10k is a suitable resistor value to put the transistor into
saturation.
* The 200k is a pull-down resistor to ensure that the NMOS gate is
kept low when the uC pin is set to high-impedence. This resistor
should be as high as possible to ensure that it doesn't provide a
current path that will turn on the PNP transistor all the time, but at
the same time it has to be significantly smaller than the output
resistance of the uC pin to ensure that the entire 5 V is dropped
within uC, leaving close enough to 0 V across the 200k resistor.

Any thoughts? (..other than the repetitive "get yourself a uC with
more pins")
.

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