Back in 1986/1987, my friend, Jim Rothrock and I made a battery backed up real time clock called LittleBen which plugged into the second mouse port.
Jim wrote the software and I designed and built the hardware. I think we sold about 30 of them. They were all hand built. It was my first experience in running a business - IIRC, we broke even on the hardware but didn't make enough money to pay Jim for writing the software. I designed the circuit, printed circuit board, had them manufactured, called around and got quotes for all the parts, ordered the "best deal" - back then shipping was like $2.xx for a package! Then, I hand assembled them. This was the most challenging part because it was a lot of soldering and doing it at home without a great workbench was difficult. Then, I had to let them run for a little bit so that I could fine tune the frequency so that they didn't gain or lose time, make sure the battery was charging and holding charge, etc.
I did the hardware development on an Atari 800 using Action! (https://en.wikipedia.org/wiki/Action!_( ... _language)
) since I was more familiar with that setup than my Amiga. Jim looked at it and ignored it and wrote his own version.
Inside, it had a Signetics chip (now NXP), a NiCd battery which was charged from the port, a variable capacitor to trim the frequency and a couple of other resistors, capacitors and I think a diode (for charging).
I've attached a picture of the manual, hardware unit, and the chips that were in it.