erratum-photodiode.md

Fix the ambient light sensor on rev 1.0 boards

The ambient light sensor in the form of a photodiode on D1 was added and placed as one of the last components before ordering the first batch of PCBs. As such I haven't actually calculcated how much of a voltage drop the diode would cause and overlooked that the ADC of the ESP32 could not actually measure the difference between 3.3V and 3.1V accurately.

To remedy this and move the measured voltage to the other side of the spectrum closer to GND a small modification to the circuit board is necessary. The modification is incorporated in revision 1.1 and the change is as follows:

That means:

• If already assembled, desolder R1 and D1.
• Cut the trace between the footprints of R1 and D1 with a sharp tool.¹
• Solder D1 back in reverse direction, so the diode's anode is on the cathode side of the footprint (in the silkscreen box).³
• Put a 1MΩ resistor over the previously cut trace, connecting the diode's anode and the trace going to the ESP32.²
• The +3.3V pad of the R1 footprint remains unpopulated!

Pictures: