If you are playing with LoRa devices it is pretty sure you have some ESP32 LoRa boards, and probably a ESP32 based TTGO T-Beam. This board is quite interesting because it includes a GPS, making very easy to compute distances of messages sent and received through LoRa chips.
TTGO T-Beam ESP32 based LoRa Board
But this board has a little problem: most of the time it emits an annoying high pitch noise making its usage very uncomfortable: some times it can even produce headaches. After a small search in Internet I realized my board isn't the only one producing the noise, so it is a common problem, more frequent when using the board without battery inserted or, while using the battery, reaching low battery levels (when current gets higher).
TP5400 application circuit
How couldn't it be otherwise the culprit is the small coil in the Li-ion charger circuit, a TP5400 chip usually used in power banks. It drives the coil at around 6 - 8 kHz and the particular coil soldered in the board acts as a speaker, producing the annoying noise.
The coil next to TP5400
The solution is to change the coil. Just replace it with another 10 uH coil with the same footprint using hot air and flux. That's all. A silent T-Beam.
Miguel A. Vallejo, EA4EOZ
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