RPi GPIO 7" HDMI LCD BACKLIGHT CONTROL

I bought a Wimaxit M728 - China HDMI LCD 7" Touch Display for RPi from Amazon, but after many attempts, I haven't had any luck controlling the backlight on/off and dimming from software. So I reverse engineered the LCD board and searched for a simple way to control brightness directly from an RPi GPIO pin.

The concept behind this instructable is applicable (with some modifications) to various LCD displays, but this particular one is valid only for this display.

The first two sections are a rapid overview on what to modify without any explanation. Other sections explain the circuit in details.

As of now, this has not been fully tested.

What is shown here may contain errors!

Desoldering the DC/DC converter broke it, so I have to wait for a spare to terminate everything. Anyway, I've tested the concept and it seems to work fine.

This job requires a steady hand and experience using cutter and soldering iron. Doing this is at your own risk!

• Small drill bit / cutter
• Soldering iron
• Thin wires (AWG24/30 wire wrapping wire is perfect)

To enable the backlight from an RPi GPIO, we need to disconnect the enable signal coming from the on-board controller to the backlight buck boost converter (pin 4 of the SOT23-6 IC, mine is marked HYEQC).

In images i've removed my IC for debug purposes.

If your breaks or fries, an AP5724 is a suitable replacement, but it needs to be tested.

1. Disconnect original enable signal: It's possible to remove the connection from the via with a small drill bit, or cut the trace with a knife/cutter.
2. Wire your own enable signal: solder a thin wire directly to pin 4 of the boost converter and wire it to the wanted RPi GPIO (ok with logic levels).
3. Drive the wired GPIO high or low from your RPi script

Enable signal: backlight ON 3.3V ; backlight OFF 0V

The on-board controller is using a PWM signal to partially shunt some current from the converter feedback loop to control the brightness. We can remove the 15k resistor (R3) and use its pad to apply both a DC or a PWM signal (in the 0V - 3.3V range. Lower the voltage, the higher the current shunted from the feedback of the controller, the higher the brightness).

If DC voltage is used, 15k resistor and filter capacitor can be removed. Backlight is proportional to the applied voltage (from 0 to 3.3V)

If PWM is applied, use a 15k resistor with a filter capacitor. The original controller uses 0V - 3.3V @ 826Hz circa. Pay attention that the duty cycle is 'inverted', so for 30% duty, the brightness is 70%.

Some notes on the pcb layout

Some notes on the backlight boost converter

AT THE MOMENT THAT'S NOT FULLY TESTED. DO IT AT YOUR OWN RISK.

Please let me know if I'm wrong with something or if you have more information!