Re: [lcd4linux] GPO circuitry

[Aragon G. <ar...@ph...>]

| By Michael Reinelt <re...@eu...>
|                                          [ 2004-02-15 19:29 +0200 ]
> Every resistor can be switched on and off with a transistor, and every 
> resistor allows a specific current. If you choose the resistors in a way 
> like a "binary system", so that the current will be 10, 20 and 40 mA, 
> you can control the current from 0 to 90 mA in steps of 10 mA.

How will you select each resistor?


> This is very similar to the GPIO schematic on the web page, except that 
> we use a 74HCT573 as a latch.

Yes.  The 4514 has a builtin latch which outputs to a decoder.


> I'm not shure, but what I've read is that the CD4514 is a demultiplexer. 
> This means that it has 16 outputs, but only one can be active at a time! 
> Are you shure that this is what you want?

Pretty sure.  For the backlight trick, each of the 16 outputs is connected
to a resistor of different value (after going through a diode to prevent
pull-down).  The output of each resistor is connected to the base of a
transistor (I'm thinking 2N2222).  The output that is switched on will
determine which resistor power flows through onto the transistor's base
(thus providing a variable current level at the transistor's collector where
the load (LED) will be connected).

Or am I mistaken?


Regards,
Aragon