In order to drive the relay modules, a set of control signals is needed. Since I only have 18/5 cabling at most, that means that I'll only have 3+VCC+ground to work with, but there are five relay modules plus a reset line. Boolean logic to the rescue!
My thinking was using binary number values from 0-4 to select which relay to enable, while I'd have a 7 (0b111) indicate a reset signal. After creating the truth table on paper, I converted it into AND and NOR gates and simulated it in Qucs-S.
Then I made footprints for the 74HC00A-series ICs that contain the gates I'd be using, and lastly created a schematic showing the routing for all of the lines I'd need in KiCad.
With that, I had what I needed to actually create the board on perf board. I also made a test fixture using buttons and LEDs to verify that it was correct.
The only bug I ran into was where the Relay4 line was high whenever I held all buttons down, which should only trigger Reset. Turns out I only forgot to connect a couple of traces. With that fixed, it worked 100%!
Man, it's been far too long since I last posted. I've been working hard on automating my 12-20m magloop antenna and already have the variable capacitor remote control and position sensor working. Next up: remote control for the transformer feed so I can have it automatically configure itself based on a requested frequency!
I spent a few days earlier this week struggling with transistors, but it ended up being worthwhile since I learned a lot. For example, an n-type MOSFET will only allow voltages that are up to the gate voltage across, which means that it really needs to be on the ground side of whatever component is being controlled. You can see the behavior here (with BJTs for comparison):
The bottom right circuit in the above diagram matches how I eventually successfully used the 2N7000 MOSFETs to control the RF relay in the video here:
This project also had me working on my first PCB design in KiCAD, since there was no way I was going to do all that fiddly soldering of these SMD relays to through-hole perf board multiple times!
