Ultrasonic Transmitter/Reciever Question

[Integrate]

Hello. I am having a small problem with an ultrasonic transmitter/receiver I built for a proximity sensor. I am using pwm from a pic at 24k (0.5 duty cycle) to oscillate a 24k ultrasonic transmitter. The receiver's signals go into a high pass opamp filter which also amplifies the signal 100x. The signal then gets rectified, and finally goes into an LED.

The circuit is kind of doing what I wanted. When I point the transmitter at the receiver, I expect to see the led turn on, which I do. However, as I move the transmitter towards or away from the receiver, I get points everyfew cm where the led goes dim. I think that the sound is somehow causing points of destructive interference, creating dead spaces. The same phenomenon happens when I point the transmitter/receiver pair at a wall.

Now my question is if this is typical of an ultrasonic transmitter/receiver? Is there a way around this problem, like if I transmit at slightly different frequencies?

Thanks.

 

[Ross Craney]

You will get nulls in any enclosed area , this is how the old ultrsonic movement detectors for burglar alarms used to work. The Rx & Rx are usually in a fixed position with the Rx receiving a constant signal level. Any variation of this level would trigger an alarm. The trouble with ultrasonics is that they usually operate around 20Khz - 40Khz which contains the bulk of the frequencies generated by air leaks
( think wind leaking thru doors & windows ) & so were not reliable for burglar alarms. In fact ultrasonic detectors were used for finding air/gas leak from pipes not so long ago.

 

[Integrate]

Thank you for the response. Is there any way around the problem? I plan to send an ultrasonic pulse and measure how long it would take to get back to the receiver to measure a distance. How does a normal ultrasonic module, like the ones in lego, handle these null areas?

(by null areas, I'm assuming you mean the ones caused by the rx tx signal interfering with itself, not natural null areas found in the room from the air leaks)

Thanks.

 

[Ross Craney]

Thank you for the response. Is there any way around the problem? I plan to send an ultrasonic pulse and measure how long it would take to get back to the receiver to measure a distance. How does a normal ultrasonic module, like the ones in lego, handle these null areas?

(by null areas, I'm assuming you mean the ones caused by the rx tx signal interfering with itself, not natural null areas found in the room from the air leaks)

Thanks.

Yes , the additive & subtractive components of the wave propogation. As to the how to get around the problem - I'm not sure. Are you measuring between the Tx & Rx or are you co-locating the Tx & Rx & measuring a reflected signal off an object ?
Isn't there ultrasonic tape measures around ( or are they laser ) May be easier to just buy one of those.

 

[birdman0_o]

Send a short burst so that it does not have any time to create inference ! If you send a constant signal there will be destructive interference.

 

[Integrate]

Send a short burst so that it does not have any time to create inference ! If you send a constant signal there will be destructive interference.

Thanks for the answer. I was testing the circuit with continuous transmission, I forgot that I was supposed to do short bursts .

 

[birdman0_o]

Feel free to update if it works after the mod!