So the bottom layer is the brains. As I discussed before, the larger board is actually on old repeater control board, all be it a flawed design, I could re-purpose it for this project by modifying the board. In your design, and when I get around to it, I have already designed a replacement for this board thats a lot smaller, and does the job. In my case, with the bad boards I had, I could go ahead and start on this and visit it later to replace the re-purposed board. You could even design something to do the same job. The cool thing with this is you can be the designer. The basic building blocks of this are simple to use, easily available, and not hard to modify, so have at it.......
Ill point out too that each layers base is built on 3/8 thick plexi-glass. I both love this medium and hate it. I love it in that its readily available, and with just a Dremel tool, I can cut and shape it, with not too much mess. The finished product looks clean, and cool. Its light weight as well. The cons are of course you cant solder to it, grounding things takes a little thought process since you cant use the layer as a ground, and drilling.........oh the drilling. I have broken several nearly completed layers using a brand new drill bit and a variable speed drill. I finally found out if you put painters tape on the area to be drilled, especially if its close to the edge, mark the holes and carefully at slow speed drill until your are almost through, reverse the drill and carefully push the bit the rest of the way through, actually melting the remaining material out of the way, you end up with perfect holes. I broke 5 of these layers before I found the combination that worked....I think perhaps in later designs I might switch to aluminium stock....this is a prototype after all
The bottom layer consists of the modified controller board, a MCU, a repeater module, a buffer board and a FOB interface board, all of which were originally designed for the repeater. Since I had extras, I just populated the boards, and left off what I didnt need.
Figure 1
Figure 2
The boards are visible here. You can see that the controller board is huge, in my case 4inchs x 4inches. This is total over kill, but remember, these are useless for the repeater, so why not use them here. You can see as well that there is a lot of unused ports. The port next to the Arduino ProMini328 interface board, is what would normally be the MCU display board running a touchscreen on the controller unit in the repeater. Likewise, there are two smaller header ports, one for a Control radio link transceiver, and one for a remote base. Of course both arent used here.
The MCU interface board is nothing more than allowing me to take a smaller Arduino ProMini, and turn it vertically making use of the vertical space instead of having to make a bigger board. Its also removable, and able to be programmed in place with a laptop if I want to upgrade the firmware. I have also broken out the SDA and SCL pins so that in the repeater, a I2C LCD screen could be attached for troubleshooting problems. Ill explain more of that when I write up the repeater build, for now its just there. The arduino works just like any other arduino, I just mounted it vertically is all so I can squeeze 2 of these MCUs on one smaller 4x4inch board.......saves on PCB costs.
The USBFOB board is located in the lower right hand corner of Figure 2. You can see the baby blue BAT43 diodes that stand out.
The USBFOB board is a simple design. Many that have ALLStarLink repeaters, nodes and other equipment have used the StarTech USB sound card to interface a computer with the Allstar network. These works great allowing for radio carrier detection, PTT, and audio interfacing. I designed a little board to allow nearly all of my connections to the USB soundcard, to be made external so that I could minimize the amount of soldering to the CM108 chip in the FOB. I can do SMD soldering, but this way I simply remove a LED, and make just 2 solder connections to the CM108 chip and Im done. I dont have to solder transistors, diodes and all that in the FOB. Conversion of the USB Sound card can be found on the web, Ill cover my mods later, for now here is the schematic of the board I made, and this is plugged directly into the repeater control board....or better in this case my node controller card. Note to I add notes to my schematics, its just for me, that way if I have to fix something, I dont have the ask myself...ummm " why is that transistor there again"..........just saves time.
Finally beside the radio module ( which I have covered in a previous blog) this is the buffer board. Now this board isnt totally necessary. I like to use a buffer when I am sending signals from one unit to the MCU. This cant be done several ways, some use ICs, some use pullups and pull downs, I like to channelize things. Its a little more work, but it gives me a clean 4.3V HIGH signal and a clean LOW signal to take in from a module, and send it to a MCU. I also like to plcae a diode on the output path. You can see several of these in Figure 1. This provides isolation as well between the buffer and the MCU. I like to keep the MCUs isolated, and with transistor output, everything is clean and loading between stages kept to nearly zero. Its an old habit from decades past. I relaize you can put IC buffers in there like a 74HC07, or similiar, but when you have 300 2N3904s and 3906s hanging around, you tend to use them when you can....The buffer board is noninverting, two stage board, what goes in is what comes you at a pure 5Vdc, the .7Vdc drop is the isolation diodes I mentioned earlier.
The last thing to mention is I did go with a commercial LDO voltage regulator, seen on the left in Figure 2), they are fully adjustable from 1.5 to 37Vdc. I love the LM2596 chip! Its also handy to have the 7 segment readouts, no meter needed. You simply feed these with 12Vdc, adjust down to 5V, done!
I havent covered programming ht MCU, Ill write more in a later blog about the firmeware. For now this covers the bottom layer! More later
Happy Coding!
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