Re: [lcd4linux] GPO circuitry

[Michael R. <re...@eu...>]

Hi Aragon,

>>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?

Trial & Error? :-)

You have 5 V voltage. a typical LED needs about 1 V (hou have to find 
out yourself. Maybe the backlight has already current limiting resistors 
built in. I'm pretty shure they have, because otherwise you'd not be 
able to connect it directly to +5V).

Maybe it's better to go for it the other way round: just measure the 
current for backlight when connected to +5V and thus running at 100%. 
Say this current would be 100mA (just a guess! I really don't know!)

This means the backlight has an equivalent resistance of
5V / 100mA = 50Ohm

To limit the current to 50 mA (which may give about half the intensity) 
another resistor of 50 Ohm would be necessary.

use this 50 Ohm as the "middle resistor", and choose 25 and 100 Ohm for 
the other resistors.

so you would have the maximum resistance (except from infinity where all 
transistors are switched off) of 100 Ohm (only the transistor connected 
to 100Ohm switched on) and a minimum resistance of about 14 Ohm (all 
transistors switched on, therefore all three resistors 25, 50 and 100 in 
parallel)


>>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).

Well, thats quite similar to the scheme I'm talking about, with two 
differences:

a) I'm saving some resistors and the demultiplexer by binary combining 
some values (in my case, more than one transistor could be switched on 
at a time!)

b) I'm controlling the current "after" the transistor. Shure, I need 
three of them, where you need only one, but with my schematics the 
current (and therefore the intensity) depends only on the resistor (and 
can be easily adopted), where you have to take the transistor and all 
this current-amplification stuff into account. Shure, it's possible. But 
   I did calculate such transistor schematics about 15 years ago :-)



-- 
Michael Reinelt                                   Tel: +43 676 3079941
Geisslergasse 4                                   Fax: +43 316  692343
A-8045 Graz, Austria                          e-mail: re...@eu...