That's a cool project.<p>Something I'd find irresistible is messing with the protocol to find the limits of your watch receiver. I mean sure the spec is 500 ms pulse for a one and 200 ms for a zero, and we can safely assume anything from 499 to 501 ms is close enough, but I wonder how far those limits can be pushed. Is the crossover from understanding 1/0 the arithmetic mean or geometric mean or some arbitrary limits or does the firmware crash or go into easter egg mode or ...<p>Another fun game to play is the AT44A is much weaker than the 8-bit home computers from the 80s that could synthesize live audio in real time up to a couple KHz. A somewhat more advanced chip in the 2010s could numerically synthesize live 60 KHz signals, which might be fun instead of altering hardware PWM. Then you could play with amplitude and keying signals. Its well trodden ground that square wave modulation of a morse code signal is extremely wide bandwidth. A fancy raised cosine would be very smooth. Perhaps you'd get better SNR at the receiver with less keyclicks, so you could reduce power, or it would be more reliable per watt transmitted. Or it might not matter. But live realtime synthesis would none the less be entertaining alternative to modifying PWM registers on the fly. A cleaner yet simple modulation method might be let the PWM fly free and use a digital pot with a I2C interface to smoothly AM modulate the output, probably only cost one chip and might provide some performance gain. Most digital pots are just FET with some buffers and drivers, they seem to be spec'd to 44 KHz sometimes 192 KHz so I assume 60 KHz would be no big deal.<p>Another funny thing about ferrites and reciprocity is how interesting their output is when they saturate, and how low the power level is when they saturate. That's why very little ham radio work has been done with ferrites as transmitting antennas, compared to the more conventional yet still rather obscure air core magnetic loop antennas. Ferrites are cool, strange chemical magic. Anyway a simple air core coil made resonant-ish might work better as an antenna than you'd think, maybe better than the ferrite. You get best performance with the correct ferrite core material for the frequency band selected. Probably the gutted radio clock you used, used the correct core material, or maybe it used the cheapest one that mostly worked (although not in your dorm room as you mentioned). Trying to use Amidon core material 17 would be quite futile at 60 khz, a bar of amidon material #3 would probably work at 60 KHz assuming you could find it. Manufacturing your own optimized antenna large enough core to handle some power and with the proper ferrite core material would be an interesting 3rd experiment.<p>Daydreaming about ways to continue iterating on a design is fun.