Baofeng UV-5R Serial Radio Interface Mk II
WHEN I received my new Baofeng UV-5R, I DECIDED to follow the time honored amateur radio tradition of homebrewing, and scoured the Internet to find a circuit. There were several, but they weren't specifically designed for this radio.
I finally decided that I'd be better off designing one from the ground up. (Pun not intended!) I started by identifying what the circuit had to do: convert the RS-232 +6v and -6v levels to the +3.5v and 0v CMOS logic levels the radio uses, and run under its own power.
Another design criterion I prefer to use is that all the parts should be available locally. After all, what's the point of building one if you can order a ready-made unit and receive it sooner —and cheaper—than your parts order from DigiKey or Mouser?
Radio Shack still stocks NPN switching transistors, small signal diodes, resistors and capacitors, perf-boards and DB-9 connectors, so there were almost all the parts waiting just a couple miles away. The only "exotic" part was the 4-terminal 1/8 inch audio plug. I dug up an old cellphone headset which used that plug, and after determining I had no further use for it (it wasn't wired correctly to use it as a speaker-mic for the radio), I cut the cord about a foot from the end and prepped it.
Now, it was simply a matter of laying out the parts on the perf-board, and soldering everything together.
My level converter uses the switching transistors as switches—when the input logic from the computer goes from 0 (+6 volts) to 1 (-6 volts) the transistor goes from saturation to cutoff, and goes from TTL 0 (0 v) to TTL 1 (>3.5v) and sends the signal to the radio at the proper voltage level. Similarly, the other
transistor receives TTL 0 (0v) and 1 (>3.5v) and switches between saturation and cutoff to switch the RS-232 RxD line from +6 to 0 volts which while not technically "correct" is an acceptable level transition for modern computers with UARTs.
Pullup voltage is obtained by rectifying the +6 volts from the pin 4 DTR signal from the computer and building up a charge in the 22uF capacitor. The voltage is stabilized and converted to the appropriate level by the 180-ohm and 2.2 K resistors.
The 10K resistor between DB-9 Pin 3 and the base of the transistor is optional. My unit works better with it, but you may not need it.
On the Job
This simple circuit works very reliably. It transfers the entire memory contents of the radio in seconds. After all that programming, you feel a sense of relief, knowing you can restore those settings in a few seconds, should your ever "misprogram" the radio.
The manufacturer's software as well as the open-source "CHIRP" program both work flawlessly with this interface.
The only time it didn't work for me was when the radio was receiving a signal. The received audio confuses the program, and it reports that the serial port is "not found." The cure is to remove the antenna, close the squelch and be careful not to transmit.