I've been a life long Star Trek original series fan. Yes, it is pretty nerdy I know. I was talking with my friend Ed KA3LJL about what it might take to create a working communicator. We tossed around a few ideas and I settled on the Chip Carter toy that came with a nice illustrated book. For the price I couldn't go wrong, or could I? When it finally arrived just before Christmas, I discovered that this toy is actually 2/3 the size of the original series prop from the show. Ok, so now I have a real challenge in miniaturization.
The heart of the working amateur radio 70cm band communicator is the SA818 module. This little wonder costs less than the communicator toy did and will put out 1 watt on high power and 1/2 watt on low power. To maximize battery life, low power is really all that is needed. The purpose of this build is to create a small HT that I can use with my AllStarLink node around the house. And I could also use it at hamfests to impress my fellow nerdy friends.
So the first thing to do was take the toy apart and see how much room was in there.
The SA818 runs easily from a single Lithium Ion battery. I had a very small battery from an old GPS that still held a nice charge. This fit PERFECTLY into the toy. The sound board and internal speaker would have to go. There just wasn't enough room to maintain the original sounds. (Maybe I can program AllStarLink to make the sounds?)
There were a lot of supports in the plastic case that had to go as well. I made a hot wire plastic cutter and went to town slicing up the internals of the case. The original coin cell battery door would be used to hold a pin header for frequency and tone programming.
The swirly "speaker" would also have to go and be replaced with a real speaker. The module doesn't produce enough audio for a loudspeaker by itself. I had to get a little amplifier board to get sufficient audio output. Despite the SA818 application datasheet showing that a mic could drive the input without a pre-amplifier, this was also not the case. I had to create a preamp board for an electret mic element. It was probably the toughest part of the build. I made it out of surface mount components except for the low gain transistor. the circuit works a little too well, and the gain had to be dialed back with a few resistor modifications. In the final version I removed the mic gain adjustment and just left it at a fixed value. The element was then soldered directly to the board.
The whole system was mocked up on a breadboard just to test the functionality and audio quality. I didn't want to start building and packaging the final product without a good understanding of how the whole device performed. The next part of the build took weeks of planning with only a few components added at a time.
One big design challenge was to come up with a functional antenna. I was just going to use a dummy load resistor to keep the module happy, but that just wasn't practical. I may want to take this down through the neighborhood on a walk, so I wanted a real antenna. I tried a
magnetic loop for 70cm. It worked, it was very narrow and it was efficient. The problem was that it would not fit in the case. Ed and I kicked around antenna ideas for a couple weeks. The flip-up grill was supposed to be the antenna, so we decided to make it the antenna. Some strips of adhesive-backed copper tape zig-zagged back and forth across the grill made a perfect antenna. I was able to use my NanoVNA to get everything trimmed just right. The antenna would eventually be fed by a small low pass filter board that I bought on eBay. It is actually pretty large, so I'm glad that it fit alright after I took off the SMA connectors. It was impractical to feed the antenna grill with a piece of wire or coax. It had to be able to move and twist as the grill was opened. I created a sliding joint using the copper tape that mated to the case and was fed by a small chunk of RG-174 coax. This design is frankly brilliant if I do say so myself. It functions perfectly and affords a wide enough bandwidth and is probably better SWR than most rubber ducky antennas.
With the antenna design proven, I moved back to the main construction. A main perf board would hold the components. The battery sits underneath everything else. The RF board and filter were joined together using pieces of copper strip material to provide a mechanical stiffness. The RF board was then secured to the perf board by just soldering the tabs.
The preamp and audio output boards would just mount to the main perf board. The mic would use the grill in the front part of the case. I glued the original rubber grommet to the case. This was salvaged from the old cordless phone that I stole the mic from.
One requirement was to have it charge from USB. I had a USB mini jack from the GPS unit that worked perfectly. The case was notched out to allow access to the USB port. The battery has an on-board charging circuit, so I just put a single diode to drop things down from 5 volts to be nice to the charging circuit.
Here we see how the board fits in with the USB charging jack mounted.
The programming port is tucked into the old battery compartment.
The header allows the communicator to connect to a 5vdc FTDI TTL adapter. Top pin is TX then RX and finally ground.
As things got closer parts were fitting exactly where they needed to be. A potentiometer with a tiny shaft would stick through one button hole to allow the audio to be adjusted. A Push On / Push Off switch would serve as the power switch.
Things are very tight in the case. All the interconnecting wires had to be in just the right spots for the case to close.
The most fiddly part was the PTT button (not pictured). I was going to cut a hole in the side of the case and just mount it off the main perf board. This would have been easier, but it wouldn't have been as cool as using the existing holes. I ended up mounting a tiny tactile switch under the leftmost jewel lamp. There had to be a lot of precision filing done to make everything sandwich together perfectly. The other fiddly part was the LED for receive audio. It put an ultra-blue LED under the rightmost jewel lamp. It was way too big and had to be filed down to fit as well. After filing an LED lens it will be cloudy. Just slather some petroleum jelly over it and it will turn clear again. (This is just one of those tricks I learned watching MacGyver.)
I'm genuinely please with how this turned out. It is really slick. I can stick it in my shirt pocket and flip it open when I get a call on AllStarLink. I bought enough parts to build two, but I won't do it again with the 2/3 size toy.
Beam Me Up.
73 DE N3FIX
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