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I want to have two ultrasonic transmitters emit signals to three receivers for measuring distances and relative position of the transmitters to the receivers. The distance between the transmitters is about 1ft and the distance between the transmitters and receivers is about 3ft. If there is more than on transmitter how can the receivers distinguish between the two transmitters? I heard things about encoding the signal or multiplex but I do not know what they mean or if and how they will work for this case.

Two transmitters and three receivers

Nikolaos Beratlis
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My solution would be to modulate each transmitter with a different audio frequency, and then at the receiver employ two audio filters to separate them. This way, even though the signals may combine in the shared air medium, you still will be able to separate them with filters, much the way a graphic equalizer can separate music into frequency bands. I've done similar things with LEDs and lasers. The only difference here is that the ultrasonic transmitters are likely around 40 kHz, so you'd want to keep your modulation frequencies in the low-middle audio band (I'd recommend between 250 hz to 800 hz). On the transmit side, I'd recommend a strong simple modulation, such as totally cutting the output at the modulation frequency. On the receive side (and maybe on the transmit side too), you might be able to simplify the circuit design by employing some DTMF (dual tone multi frequency) encoder/decoder ICs, as they have been around since the advent of touch tone telephones, and you'll have a lot of parts to choose from.

Randy
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  • You only need 2 audio filters, one for each transmitter, but yes. And DTMF frequencies should be just fine. –  Nov 02 '14 at 18:42
  • Right... I re-edited my post. Wrote it in haste on the way out... my bad! :-) – Randy Nov 02 '14 at 20:31
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You will probably want to design the system so that the ultrasonic signal from each transmitter is active only when the other one is OFF. The three receiver/detectors are not going to be smart enough to distinguish which of the transmitters is sending the signal.

You could do away with the need to specially modulate the transmitter signal, as others have suggested, by applying a simple protocol to how the two transmitters are activated. You could divide up real time into a series of time slots that were all equal duration in time. Then activate transmitter 1 in the first of each third time slot, transmitter 2 in the second time slot and then leave both off in the third time slots of each triplet. A relatively simple time duration measurement between successive detects at each ultrasonic receiver by the microcontroller will allow you to easily distinguish when transmitter 1 is active versus transmitter 2, as transmitter 1 will always come first after the longer duration between detect events.

Michael Karas
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