The STM32F100 is the workhorse microcontroller in this sonde, featuring an ARM-Cortex M0 core. It is running at 24 MHZ

Above it we can take a look at an TPS61200 boost converter, two TLV70030 LDOs (one additional LDO being on the rear side) and a P-Channel MOSFET who turns on the sonde, triggered either by a pushbutton or via NFC.

Speaking of NFC, the NFC coil is visible on the bottom of the sonde quite prominently, featuring 12 turns over all 4 layers of this 1 mm thick PCB. There is no NFC interface chip, only one DC-coupled and one AC-coupled path. the return channel is formed by an N-Channel MOSFET who shorts the coil through an resistor