The backlight of the 2.8" PiTFT has 4 LEDs in series and it draws ~75mA at all times, controlled by a transistor. The PiTFT 3.5" display has 6 LEDs in a row, and we use a boost converter to get the 5V from the Pi up to the ~20V needed to light up all the LEDs.
There might be times you'd like to save some power and turn off the backlight. The screen and touchplate will still work, you just can't see anything. We designed the board with the STMPE610 touchscreen controller which has 2 extra GPIO and tied one of them to control the backlight. You can use the command line to control the backlight.
By default, the backlight's on...but you can control it in two ways!
PWM Backlight Control with GPIO 18
If you want precise control, you can use the PWM output on GPIO 18. There's python code for controlling the PWM but you can also just use the kernel module and shell commands.
You'll need to make sure the STMPE control is not 'active' as the STMPE GPIO overrides the PWM output.
sudo sh -c 'echo "0" > /sys/class/backlight/soc\:backlight/brightness'
(Or if you are running an old kernel before the backlight object, try sudo sh -c "echo 'in' > /sys/class/gpio/gpio508/direction")
OK now you can set the GPIO #18 pin to PWM mode using WiringPi's gpio command
With these basic shell commands, you can set the GPIO #18 pin to PWM mode with 1000 Hz frequency, set the output to 100 (out of 1023, so dim!), set the output to 1023 (out of 1023, nearly all the way on) and 0 (off)
gpio -g mode 18 pwm gpio pwmc 1000 gpio -g pwm 18 100 gpio -g pwm 18 1023 gpio -g pwm 18 0
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If you'd like to not have #18 control the backlight, simply cut the solder jumper, the tiny trace between the two large gold pads marked Lite #18 |
On / Off Using STMPE GPIO
Another option is to just turn it on and off using the extra GPIO created by the touchscreen driver
Thanks to the raspberry Pi overlay system, this GPIO is already set up for you in a file called /sys/class/backlight/soc:backlight/brightness
To turn the backlight off run
sudo sh -c 'echo "0" > /sys/class/backlight/soc\:backlight/brightness'
To turn it back on, run
sudo sh -c 'echo "1" > /sys/class/backlight/soc\:backlight/brightness'
For older versions of PiTFT Kernel
On older versions of the PiTFT kernel/overlay, the GPIO was not tied to the backlight device. Start by getting access to the GPIO by making a device link
sudo sh -c "echo 508 > /sys/class/gpio/export"
ls -l /sys/class/gpio
For some really old versions, the GPIO pin was #252 not #508 so substitute that if you're running something from 2014 or earlier
Once you verify that you see GPIO #508, then you can set it to an output, this will turn off the display since it will output 0 by default
sudo sh -c "echo 'out' > /sys/class/gpio/gpio508/direction"
Then turn the display back on with
sudo sh -c "echo '1' > /sys/class/gpio/gpio508/value"
or back off
sudo sh -c "echo '0' > /sys/class/gpio/gpio508/value"
