Re: Raspberry Pi support for Rev-2 & BL

Hi Paul

On 22.01.2013 21:50, paul_c wrote:
> Picking an LCD module at random, 
> http://www.pololu.com/catalog/product/772 the specs for the LED 
> backlight tells us that some 4.2V at 120mA is required. The Raspberry 
> Pi will deliver 3.3V at around 2mA (it is possible to increase this, 
> but total current loading across all pins must be taken in to account). 
> To drive a 120mA load, a FET or transistor must be used - See the above 
> link for an example of using a FET, or the following for a transistor:
> http://www.raspberrypi-spy.co.uk/2012/08/16x2-lcd-module-control-with-backlight-switch/ 
> or if you prefer a more traditional schematic, 
> http://www.susa.net/wordpress/2012/06/raspberry-pi-relay-using-gpio/
> A transistor is probably the easiest to source and use.
> 

After making myself more familiar with analog circuits (again) I now
know why my backlight didn't work.

My current problem is that all circuits you referenced above use NPN
transistors that light the backlight if the BL is 'high'. The original
patch however, turns the BL pin 'low' if backlight is enabled.

Do you know which circuit was originally used by the you/author of that
part?

Otherwise I will recommend to use circuit [1] and modify the code to
work with that. The usual backlight circuit from [2] must not be used
with a Pi.

[1]
http://lcdproc.sourceforge.net/docs/lcdproc-0-5-6-user.html#hd44780-ftdi-backlight
[2]
http://lcdproc.sourceforge.net/docs/lcdproc-0-5-6-user.html#hd44780-connections-backlight.circuit

Regards,
Markus