Prusa has introduced a new firmware version for their line of 3D printers. One of the cool new features is the capability to control the display’s backlight intensity from the settings menu. There is a big chance that this option will not work if your 3D printer was produced early 2019 or before. The reason is that the display board is old and doesn’t support backlight dimming. But we can fix this quite easily. We need 2 resistors, one NPN transistor, some wires, and soldering skills.
In my board the EXP1 and EXP2 connectors are straight and the solder pads are between the red PCB and the alphanumeric display. That’s why I had to de-solder the display, which turned out complicated and I damaged the board a bit. Once the display was de-soldered I removed the thermal isolation on pad 16 of the red PCB – I later connected there the collector of the controlling transistor.
I used a BC847C transistor in an SMD package, one 100R, and one 100k resistor. The circuit is quite simple and 4 wires are needed to connect it.
- +5V connects to display pin 15
- Transistor collector connects to display pin 16
- PWM_LED connects to connector EXP2, pin 1
- The ground – pick up one ground connection – either display pin 1 or something else.
Reconnect the display to your printer and go to Settings/Brightness. You can set the backlight mode – always strong, always dim or auto – the backlight stays bright for 10s and goes to the dim setting. Once you touch the rotary wheel it’s bright for another 10s.