Hi all. It's been a while. Hopefully when life slows down I can get back into the hobby.
I the mean time, I had a question about sound detection possibilities.
I am envisioning some wireless on/off control without using RF or IR. Range need only be 50-75 feet. I'd want the signal to be an audio wave outside of the human hearing range, which I understand to be 50 to 20,000 hertz. I'd need probably 5 to 10 frequency ranges narrow enough and far enough from the others so as not to be "confused" with the others on the receiving end. I envision the receiver which would be encased in a small bit of material being comprised of a microphone to detect the sound and a bandpass filter to ignore irrelevant frequencies. The transmitter would be a small speaker outputting the desired frequency that is, again, outside of the human hearing range.
For example:
Consider a transmitter with 5 different buttons that cause 5 different frequencies to be emitted through one or more speakers. There would be 5 different receivers, each with a microphone (encased in a plastic container, perhaps) and a bandpass circuit that triggers an action only if that frequency is detected.
Would it be possible for one button to cause , say, a 25 hz audio signal that could be RELIABLY detected by microphone attached to a bandpass filter detecting 20-30 hz?
Then another centered around 45 hz, then another at say 20,500hz, another at 21,000hz, etc
Could this be reliably detected?
Could I ever expect a 50-75 foot range?
Are there cheap microphones and speakers capable of hearing and producing (reliably) say 20 hz to 25,000 hz?
EDIT:
Maybe I'm going about this all wrong. The ultimate goal is to be able to generate a single reliable signal without line of sight to an object up to 50 to 75 feet away for less than $1.50 in materials for all electronic components involved on both the receiving and transmitting end. I think rf is out as an option here due to cost, and IR while cheap requires (more or less) line of sight.
Thanks