Continuing my experimentation with cheap radios to create svxLink nodes, my next victim is the BaoFeng BF888S. For about $25 including tax you can acquire two BF-888s radios with accessories. These radios are incredibly cheaply made but should be suitable to use for a low power analog node. They are UHF and cover 400-470 MHz with an output power of 1.5-2 watts, powered by a 3.7V lithium ion battery. There are 16 memory channels. Programming is accomplished using CHIRP.
There have been quite a few versions of the circuit board that’s used in these radios. The component placement has changed, but the various components seem to have stayed mostly the same.
Here’s the original BF-888S. There are a bunch of different model names for this radio but they are all pretty much the same radio.
The signals we’re interested in are TX Audio, RX Audio, PTT, and “COR” which indicates when a signal is present. The radios support CTCSS encode and decode. You can obtain access to three of the needed signals via the side panel speaker/mic jack.
The COR signal can be obtained from the enable line to the audio amplifier. That’s an IC located on the “component” side of the main circuit board. In order to attach a wire to it, you need to remove the battery, then remove the three nuts that fasten the antenna jack, power/volume pot and channel selector rotary encoder and then remove the outer case. Next you need to desolder the battery connector from the PCB and remove the two Torx screws that hold the PCB to the metal chassis.
I chose to ditch the case and design a 3D printed case to house the minimal radio board, a voltage regulator, and DB9 interface connector. If you do not use the side-mounted 2.5mm/3.5mm speaker/mic jack, you will need to desolder and remove the electret mic element to eliminate audio from entering there while transmitting. I also removed the antenna jack and side-mounted button board. I replaced the antenna jack with a male SMA oriented such that a rubber duck could be used with the enclosure in the horizontal position.
Since I was ditching the chassis and thus the shielding provided by it, I made a copper shield for the MCU and AT1846S RF transceiver chip.
If you want to mount the circuit board into another enclosure, you can obtain the signals you need by connecting directly to the circuit board.
I’m waiting on a small 3.7V voltage regulator to power the transceiver but here’s what the 3.5″ x 2.5″ (86x56mm) 3D enclosure I came up with looks like:
Here’s the transceiver with lid attached to my PiZeroW + RL-20 audio interface board.
Once I add the voltage regulator, I may ship 12 volts over to the transceiver through the 9 pin connector so that I don’t have to power both separately.
Leave a Reply