I have bought some time ago a 5" LCD with HDMI input and touch panel connected via usb (through GB32F103 (STM32F103 Chinese clone 😄 ) and XPT2046). I had some adventure with making this touch to work (this is on one of my github repos 😄 ). I hacked my LCD because I got inspired on official Raspberry Pi forum by topic https://www.raspberrypi.org/forums/viewtopic.php?f=44&t=126321 . Well, actually I did this to show that this is possible and quiet simple (for me :evil: )

I thought of few ways to control backlight, but actually only two are the best ways to do this:

1. Control via EN input the backlight step-up controller
2. Replace switch with small signal relay (G5V/G6K/G6J/N4100/etc.) and drive relay by transitor from GPIO

I'm going to focus on the 1st way, because I don't have a switch on my LCD and I think it's the most proffesional way of doing this - I don't like idea of cutting load (in LED diodes form) from dedicated switched DC/DC controller for powering LEDs.

Reverse engeenering circuit :) , fortunatly chip that power-up LEDs is marked well, and it was easy to find by google "4103 sot23-6" (sot23-6 - this is name of this chip chassis). Just fast look at datasheet while looking at pcb and - yes, this is this one :) . PT4103 have EN input, that driven low will disable chip functions, and driving EN high (more that 1,5V) will enable the chip operation.

EN input is connected to 5V rail from power socket (microusb). That's mean chip is constantly enabled, I'm also lucky because there is already pull-up resistor 10k (R26 in my pcb version). So just solder a thin wire (for reference this one is called on aliexpress "wrapping wire" 😄 ) to the 10k resistor from EN pin side, power up LCD module and touch with wire to usb socket chassis (chassis is connected to GND/0V), backlight shoud turn off - if not then (in worst case) there would be a spark and wire may burn out (and backlight might also never turn on again 😉)... But I'm confident of my knowledge and expirence 😄

This is how the circuit is made on my LCD board (blue wires). The green part is what I added. I used RK7002 N-MOSFET transistor with Vgs(th)=1,5V , and resistor have around 51kOhms - I had this one value in 0603 chassis, can be anything between 33k..47k..75k . 😄 .

I have cutted a small area on pcb from GND fill ,and used this to solder wire to GPIO, gate of transistor and resistor.

Some photos:

Final look:

Movie on Youtube with testing how EN behaves: https://www.youtube.com/watch?v=TtfMw2QhIUY

I have installed wiringPi for this testing my hack because it have a command line tool that allow to control GPIO pins.

I use GPIO 21 of BCM numbering style. I have tested this on RPi2. You can use any free GPIO pin that fits you.

First setup the GPIO pin as output:

gpio -g mode 21 out

Now to switch on backlight set GPIO pin to low state - to 0V :

gpio -g write 21 0

TO turn off backlight set GPIO pin to high state - to 3,3V :

gpio -g write 21 1

And movie on Youtube with results of my hack 😄 https://www.youtube.com/watch?v=lP1gdCcOtek