I have three water level sensors that will live outdoors, and three fancy-looking indicator lights that will be inside. I would like to continuously transmit the on/off state of the three sensors inside. It can be through a window, so infrared seems like a possibility. The simplest thing I can think of is for each sensor to activate an oscillator at a different frequency, sum those signals together and send them over infrared, then detect them with whatever the opposite of an oscillator is -- a bandpass filter plus something? Like DTMF. Does this sound like it would work? If so, what is the name of the component that would detect oscillations in a certain band? Are chips available with several oscillators or detectors on them?
Edits in response to comments: Both sides of the window can have continuous power from a power brick plugged into the wall. The sensors can all be close to each other and wired to each other. I can also send a wire right to the other side of the window glass for the IR emitter, although it would be cool if I didn't have to. Reliability is not a huge concern, the worst that happens is that someone goes outside to water plants when they don't have to. Relatedly, I don't care if it works in the rain, the water will always be high enough if it's raining.
Edit 2: I guess I said "continuous" because I was thinking of an analog solution, but if the simplest answer really is some sort of packet-based thing, then I guess I don't care if it takes as much as 10 seconds to update the indicator lights.