PURPOSE:To elongate the life of a battery by transmitting a radiowave only when the water level in a bathtub is higher than a specified value based on the signal from a liquid level switch with power control circuit. CONSTITUTION:A power control circuit 20 detects the conduction of a liquid level switch 18. When there is conduction, the power of a battery 21 is supplied to a transmitter 17. Hot-water feeding into a bathtub 1 is started from a hot water feeding machine 4. Until the hot water level reaches the liquid level switch 18, the power control circuit 20 is OFF, and a radiowave is not transmitted. When the hot water level rises up and the liquid level switch 18 is immersed into the hot water, the power control output of the power control circuit 20 becomes ON. The radiowave which is modulated with the water level signal is transmitted. The radiowave is received by a receiver 22. The frequency is detected, and the water level can be found. The hot-water feeding into the bathtub 1 is controlled in correspondence with a preset water level.
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