I recently found out about the SA818 and SA868 modules that are used inside of some cheap VHF and UHF HTs, so I bought a few from aliexpress along with some tiny surface mount low pass filters.
The datasheet shows that they are pretty easy to interface with. You can program it using the serial RX and TX pins. You can set the frequency, squelch, and volume of the module. Other than that, there are pins for audio in, audio out, ptt, power on/off , and an antenna connection.
Thats it. It is really simple to set up.
I used a raspberry pi 3B+ and the SA818 python library to program the module. I wired up the power and ptt pins to the PI and wrote an extremely simple python program to tranmit for five seconds and then turn off.
I reused the SMA connector that was soldered onto a junk board I had laying around and I connected it to the spectrum analyzer and ran my python script. The spectrum analyzer showed the transmitted carrier for five seconds so that proves that the module was working.
I’m not sure what I will do with this. I’ve seen others make repeaters using two of the modules. I’ve been playing around with APRS lately, so maybe I’ll make some sort of APRS thing. Maybe a digipeter using direwolf installed on the pi or something like that.