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I have a question about ultrasonic sensors.

By connecting the car parking sensor to the scope I always get the same wave. But by changing the distance from the sensor, the parking system respond as expected (according to my distance from the sensors)

From my understanding i should see the sent wave (which i do) and after some time the echo signal (which i don't). if sound travels at a speed of 340 m/s (2.9ms for each meter) and i am 0.5 meters from the sensor i expected to see the echo signal attenuated after about 2.9ms which i don't although the car parking sensor does show i am 0.5 meters from the sensors. i thought perhaps the signal is highly attenuated, going as low as 50mV but all i see is noise. could it be that the frequency is inside the noise? even the noise does not appear to change.

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    \$\begingroup\$ What sort of connections does the sensor have? I'm not familiar with the auto distance sensors but thought they'd probably transmit pulses continously and have a seperate line for the receiver so the distance would be the phase difference between them. \$\endgroup\$ – PeterJ Aug 21 '13 at 13:33
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    \$\begingroup\$ Perhaps it is like an ultrasonic motion detector. Those look for a 'beat note' between the returning signal and the sending signal. The beat note is down around 50Hz. So set your scope to a slower sweep, and look for the shape of the ultrasound signal's envelope. \$\endgroup\$ – Bobbi Bennett Aug 21 '13 at 13:56
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Doesn't the sensor have a separated receiver?

As you said it could be that the received signal is highly attenuated and the SNR is so low you cant see it. One way to find out if there is a copy of the pulse inside the data is using a math tool called auto-correlation, but I doubt that a simple ultrasonic parking sensor uses it.

To have a better idea you can try to track the PCB and find out if you can see a filtered signal.

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Here's a circuit of one: -

enter image description here

I've marked on the output signal and the input signal in red. They use seperate transducers and this seems sensible to me but, is it like this or does it indeed have a single transducer and the received signal is not big enough to be seen on a scope.

There is the possibility that it does have a Tx/Rx switch and when in Rx mode the signal is routed to a transimpedance amplifier - this would make it difficult to see any voltages reflected from the target but my money's on a seperate transducer.

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  • \$\begingroup\$ That's a bad schematic: none of the components, particularly the ICs, have values. \$\endgroup\$ – Johan.A Aug 21 '13 at 18:00
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    \$\begingroup\$ @Johan.A - true but it is to demonstrate that these devices are available with seperate Tx and Rx transducers. I'd take a guess that IC1 were a 4060 though and maybe IC2 was a 4040 or something similar. \$\endgroup\$ – Andy aka Aug 21 '13 at 18:02

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