We've got this cheap egg cooker that plays the loudest, most terrifying alarm you've ever heard when it's done cooking. My wife requested that I make it "less aggressive" so I hacked it into the most gentle egg cooker of ALL TIME

(warning: probably loud)

These are such beautifully simple machines. It's got a heating element, a fuse and an alarm. when the water evaporates, the plate gets hot enough to open the fuse. The power is then rerouted through the alarm. So you control cooking length simply by adding more/less water!
@gvy_dvpont This is similar to the way that simple rice cookers and electric kettles work.
I always thought it was a very clever solution that didn't involved complicated parts.