Truly LCD front panel: the backlight

Here is the follow-up of the post where I described how I took out the front panel of a router (yes, a router) and found a way to interface it with Arduino or other development board. It should be noted that the front panel electronics use 3.3V levels, therefore the popular 5V Arduino boards cannot drive the front panel. Using level shifters would complicate things and increase the possibility of something going wrong, so I ended up using a 3.3V STM32 blue pill development board. This is programmed from Arduino IDE, so the code I write is compatible with Arduino development boards.

While I was sampling various pins of the front panel connector with a logic analyzer, I noticed a strange protocol on pin 18. I was able to trace the PCB track from pin 18 near an area that seemed like a DC-DC converter. It directly drove an integrated circuit marked T43. Searching for it revealed some LDO linear voltage regulators, but this was not the case. Pin 18 carried a digital protocol that would be of no use for an ordinary voltage regulator. But without information I could only write code that would mimic the protocol I sampled. Things changed once the GPL source code has been made public. The signal on pin 18 had a meaning. It was necessary to turn on/off and dim the backlight. Upon powering the front panel on the breadboard, the backlight stayed off. You can turn it on by setting pin 18 high but if you want to adjust its level you must send two bytes using a custom serial protocol. Before getting to the code let’s see an overview of the pins and connections on the breadboard.

Front panel connector adapter on breadboard