This is a very simple 433 MHz Temperature & Humidity transmitter. It has an STM32 MCU and an SHTC3 sensor onboard.
The 433 MHz transmitter is realized by using the WL4456 IC.
WL4456 is made by: “Shenzhen Welles Technology Co., LTD” and costs about US $0.14 at LCSC. Unfortunately their datasheet is sparse (and only in Chinese). There is no block diagram of how the chip works, but we can surmise it has an x32 PLL to generate 433.92 MHz from the attached 13.56 MHz crystal.
From there, we can assume there is basically some power management circuitry and an output buffer to gate the RF output depending on the ASK modulation pin. The rated output power is 12dBm. I haven’t yet attempted to confirm that.
The MCU
The MCU in this device is the STM32F042G6U6
The board has a vertical type-C USB connector. The intention is that you can plug this board into a wall-wart type C power supply and it will transmit the temperature and RH to your rtl_433 instance somewhere else. For example I have these going to Home Assistant.
I wrote source code for the MCU to modulate the carrier to emulate an “Oregon SL109H” sensor. This way, these sensors work with the RTL-433 receiver without modifying the receiver source code. They are auto-detected by rtl-433.
I have a limited quantity of these boards. Let me know if you want one and I’ll put them up for sale.
Antenna
One of the drawbacks of 433 MHz devices is that they definitely require an offboard antenna. The ideal wire antenna length is 6.8 inches. If you’re only going a very short distance you can probably get away with just a short piece of wire inside an enclosure, or a “spring” type antenna.