This design harvests heat energy to make electricity, and uses it to flash a warning light. Flashing LED hot-surface detector:
Using a thermoelectric generator (TEG) to sense the temperature of a surface and a step-up switching regulator to boost the TEG output to a useful level, this warning circuit flashes a high-brightness LED when the surface temperature exceeds a threshold.
We’ve slowed the app notes a bit to accommodate the haters, but we thought this one is really cool. Thanks rsdio!