neepie:
It sounds like you are not using a direct connection to power.
Ahh, now i got it. I thought you were just talking about the LCD not the backpack. But still, there is a 100ohm resistor on the lcd. Thats actually not enough(?)
Depends on the backlight. Measuring the current is the best way to get a good idea
about the power demand.Did some measuring and seems like the backpack has a transistor for backlight. Seems like the jumper connects the transistor base to Vcc via 4.7k resistor.. So when i remove the jumper, LCD gets no ground.
It is possible that the jumper connects the base to vcc by resistor. But that disagrees with your
earlier statement of:The jumper seems to cut off positive pin from the backlight.
Different backpacks have different circuits and the jumpers on them
don't all do the same thing.
I've seen these:
Remove to force backlight to be off, inserted to allow PCF8574 control
Remove to force backlight to be on, inserted to allow PCF8574 control
Remove backlight to allow PCF8574 control, inserted to force backlight off
Remove backlight to allow PCF8574 control, inserted to force backlight on
It depends on the circuit.Also depending on the circuit the signal needed from the PCF8574 can
be positive or ground to turn on the backlight.
(with the correct constructor, the library takes care of that for you)Yet its not clear how does the chip control backlight by software. Will be looking more later.
The PCF8574 drives an output port pin that goes to the base of the transistor.
This turns on the transistor,
which depending on the transistor type and circuit will activate the power or ground of the backlight.These backpacks are really simple circuits on a simple double sided board,
so you can draw up the entire schematic in a few minutes by just looking at the board.
(might be a bit tougher if it is soldered to the LCD, but still doable with an ohm meter)
That is what I would recommend doing if you want to interface an arduino pwm pin to the circuit.
Depending on the curcuit it might be a simple as connecting a wire to one of the jumper header pins
or it may require soldering a wire to a trace and cutting another trace.
This is not something to just guess at, since a wrong connection could fry the AVR chip or
the PCF8574.