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.

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    \$\begingroup\$ Are the sensors close by eachother physically? If so they could have a single controller that handles transmission. \$\endgroup\$ – jramsay42 Dec 4 '17 at 2:39
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    \$\begingroup\$ Infrared might not work throuh the glass, maybe try to test with one channel first. \$\endgroup\$ – MarkU Dec 4 '17 at 2:58
  • \$\begingroup\$ What powers these water level sensors? \$\endgroup\$ – ThreePhaseEel Dec 4 '17 at 3:23
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    \$\begingroup\$ check IR transparency of the glass by using a TV remote control and a cell phone camera. point the TV remote at the camera and press a button. you should be able to see the IR LED light up. then try it through the glass window. \$\endgroup\$ – jsotola Dec 4 '17 at 8:20
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    \$\begingroup\$ here are some modules for passing power through glass ... robotshop.com/en/wireless-charger.html \$\endgroup\$ – jsotola Dec 4 '17 at 8:23

Serial encoding of the three signals would be appropriate. A cheap little microcontroller will easily drive a 38 kHz signal into a transistor switch, which in turn modulates an infra-red LED. Something like this.
A suitable protocol is SIRC (Sony infra-red remote).
The infra-red decoder is already done for you in a simple, inexpensive three-pin chip used by TV receivers.
infra-red receiver
Arduino and LINUX include software (LIRC) that can extract bits in the serial-encoded data stream on its "OUT" pin.
While this is not continuous transmission, it is very easy to implement. Hopefully, successive serial codes might be transmitted quickly enough to represent the state of your three LEDs.

  • \$\begingroup\$ Thanks for the pointer to the software! I'm slightly hoping to avoid programming a microcontroller myself, although this would be an easy program I'm sure. It does seem like something someone would have done already, though - make a chip that serializes the values of some of its pins when a trigger is sent? If it doesn't exist, though, I might go for the microcontroller as you say. \$\endgroup\$ – rescdsk Dec 4 '17 at 4:48

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