The objective is to create a nice looking business card that's not too fat and that will remind whoever it was given to of the existence of the 'ratcatcher' every 2 hours or so with an annoying sequence of beeps.

Obviously, most of the cards will be given out with no electronics to those considered to be on the 'B' list ie mates in the pub. But if there's a serious looking 'A' lister like a billionaire living on a Caribbean island, then they get the version fully populated with electronics and battery.

The electronics is not completely ridiculous compared to the overall concept and features a small solar cell and energy harvesting chip to keep the lithium coin cell battery charged up. During the 'rest' period, the ESP32 MCU is completely turned off to save power and gets turned on through a transistor when the timer sends a pulse. A nifty CMOS logic circuit keeps the MCU in the 'on' state until all the annoying beeps have been broadcasted at which point everything is turned off again.. The energy harvester works at very low voltage levels and will charge the battery even when sitting on the billionaire's desk, assuming it is the right way up !

The real question is: 'how long before the prospective billionaire either sends the ratcatcher an email (presumably to get his rat infestation sorted) or throw the whole thing into the Gulf of Mexico in disgust?'