uses cheep arduino 455 transmitter/receiver and modified cheep 1$ 120v ac usb charger cube and photo resistor to sense when flame is on and lights led bead-lamp to indicate water heater is on.

very simple to build.

injects 120v ac 60hz into input of 455mhz transmitter threw photo resistor threw 50k resistor strung in series with 2 20 foot strands of Kevlar insulated 30guage wire from a section of cat5 or any wire

also shows how to modify the voltage from a 5v charger circuit

and how to make a great 455mhz high gain antenna by wrapping cat5 around a tissue roll then removing it from the roll and stretching it till the signal peaks.