Re: Raspberry Pi support for Rev-2 & BL

Hi Markus

On Tuesday 22 January 2013, Markus Dolze wrote:
> BTW: When I tried to use the switchable backlight on my display, it
> didn't work. Likely because of the 3.3 V signal level vs. 5 V power
> supply. Can you point me to a working circuit?

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.


Regards, Paul.