This guide has references to the radiosonde_hardware repo, and is partially based on it. Huge thanks to the author for providing the description.
This page mainly mentions differences between old and new revisions, details can be found on the page above.
The component naming also refers to this document, as well as some pin definitions.
Open the enclosure and take out the PCB. On the back side of it (the one without battery holder), in the middle of the coil on the bottom, there should be a PCB model, that starts with RSM...
- PCB model with a last digit of '4' - newer ~2023 version (for example
RSM414,RSM424) - PCB model with a last digit other than '4', for example '1' or '2' - older version (for example
RSM412,RSM421etc.)
This guide describes hardware used in the RSM414 newer version of the sonde.
PCB - Front photo
PCB - Rear photo
The boost converter - probably TPS61200 from Texas Instruments, is used in a standard circuit. It gets enabled via Q501 P-MOSFET. Q501 state can be changed via hardwired logic to switch the sonde power.
The same as previous revision.
It is exactly the same, though an accurate description maybe useful for somebody.
Previously mentioned Q501 P-MOSFET is controlled via a smaller N-MOSFET Q502. The sonde is ON if the Q502 has a HIGH state on the gate, and it powers off when the gate closes.
The sonde can be switched on via NFC 13.56MHz carrier signal on it's coil, which sends a rectified signal to the Q502 gate.
The button S501 on the bottom of the sonde can turn on the sonde via R507 to bring the Q502 gate to logic HIGH signal. The button status can also be checked with uC via R509/R513_C524 voltage divider. The middle leg of the button voltage divider is connected to the uC IO PA6. The button voltage gets slightly higher when it's pressed.
The battery voltage can be measured by the MCU via R508/R512_C523 voltage divider. The middle leg of this divider is connected to the PA5 pin of the MCU.
Once the Q502 gets a HIGH signal, it stays open via R505, which sends voltage from the battery to the MOSFET gate.
The sonde can be turned OFF by setting PA9 MCU pin HIGH, which opens Q503 N-MOSFET, that shorts the Q502 gate directly to the ground. This closes the Q501 and with that the boost converter EN and VIN pins don't receive power anymore.
As you can already know, it is a new STMicroelectronics STM32L412RBT6 in a LQFP64 package.
Like the previous revision, many of the IOs have RC (resistor-capacitor) low pass filters.
It is very similar to the previous version, please refer to the radiosonde_hardware repo.
For now, I didn't have time to reverse engineer it, apart from searching the pins for the SPST switches and pullup pins (available on the uC connection schematic above).
Only thing worth (for now) mentioning are the reference heating resistors on the cut-out part of the PCB. In my firmware, they are used to slightly heat up the board near it, which contains the 26MHz crystal. The fw contains some wild functions to control it, which is used to limit the radio losing PLL-lock at very low temperatures (this also occured on older boards, like here or here). The PLL-lock loss happens mainly on the RTTY modulation using faster than 45 baud rates (lock-loss on Horus TX mode was not observed) when the Si4032 and the crystal cool down below 0°C. The heating logic is described in the operation manual, but worth mentioning is that you could try to improve the heat transfer by mounting to the resistors something heat-conductive (warning - it must not conduct electricity or it will cause a short circuit), like a small thin insulated elastic copper plate.
The GPS module in the new revision is the component that differs the most - this time it is a uBlox M10050-KB in a QFN28 package. The circuit around it seems like a standard implementation, with RF filters and probably an LNA chip, and standard coupling capacitors and RC low pass.
The bad thing is that the only official document I could find about it was this leaflet which doesn't seem to be a complete datasheet.
Luckily, the circuit implementation isn't hard to understand, so the UART pins were obvious to find. They are located on the right side of the chip, with RC filters before plated holes to the other side of the PCB, connected to pin 16 of the GPS as RX and 17 as a GPX TX. This GPS UART interface is then connected to the PB6 and PB7 pins of the uC.
Nearly the same as in the previous rev.
It is a Silicon Labs Si4032. It uses the same 26MHz clock as the GPS module, so the RF frequency on the output is different then the one set in the registers.
desired_freq * 26/30 = output_freq
The register values can be calculated on this site, altough the frequency calulation algorithm is already implemented in the firmware.
The SOIC8 SPI flash memory footprint is also available here like in the previous version, probably to be used with a military RS41-SGM edition, that features a Radio Silence Mode. More about it in the radiosonde_hardware document.
The XDATA port is the same on all models, with the following pinout (schematic is a copy from the mentioned repo):
Pinout of XDATA connector Pinout of connected ribbon cable
-------
GND | o o | XDATA_RX(PB11) > 1 GND
| | 2 XDATA_RX(PB11)
XDATA_TX(PB10) | o o | +3V_MCU 3 XDATA_TX(PB10)
- | 4 +3V_MCU
V_Boost| o o | VBAT 5 V_Boost
- | 6 VBAT
MCU_RST | o o | SWCLK(PA14) 7 MCU_RST
| | 8 SWCLK(PA14)
SWDIO(PA13) | o o | GND 9 SWDIO(PA13)
------- 10 GND
I only have the RS41-SG version (without additional pressure sensor board), so the PCBs only contain a footprint for soldering one. I bet that the pinout is the same, desciription can be found in the radiosonde_hardware.
Besides the lack of the C604 capacitor (but the footprint is still there?) the circuit looks the same as the one before.
I had no luck for now to implement some decent protocol, because the board doesn't feature any NFC frontend (which means carrier OOK modulation for rx by the sonde and rfid load changing as a tx from sonde).
The only thing that it is capable for now (via the hardware side) is that the sonde can be instantly turned ON on detection of the 13.56MHz RFID carrier. You can try this with your NFC-capable smartphone - just turn on the NFC function (the one for the RFID cards) and stick the back of your phone to the back of the sonde or somewhere near the coil on the bottom. The lights should turn ON and the sonde should start working.
If you didn't find an answer for your question about the sonde hardware in this text, please visit the previously mentioned radiosonde_hardware, which provides more detailed descriptions of some components.
Feel free to ask in the issues here about any features :)