In a UHF passive RFID system, a reader transmits an unmodulated continuous wave (CW) to tags, which not only provides energy to tags but also serves as the carrier of tag backscatter signals.
And early home computers made for the hobbyist market used switches extensively, because most home users couldn't afford expensive peripherals like punch card readers.
Atlas' engineers doubled down on this idea, and outfitted their computer with 4 paper tape readers, 4 paper tape punches, and up to 8 magnetic tape drives.
So if the reader registers all $3 bills have been added, it tells that to a micro controller that activates a servo motor that opens this door, revealing a tool.