Piezo transmitters and receivers

[fastline]

I am looking to do some testing of an ultrasonic distance finder. I am looking for economical solutions for the piezo trans and receiver. I am not sure if one unit can function as both or if they are designed to either be a receiver or transmitter and if they are supposed to be "matched"? I am not sure why but worth asking.

I would operate in the 20-50khz range most likely. I would need to apply a pretty strong voltage to the trans for long distance but I doubt the receiver would need to be anything special.

From my understanding, the voltage returned on the receiver may be usable directly to a PIC? We would likely use a PIC 16F series IC. I am not sure if this would be worth trying to find an IC already designed for the task or not? Do they even make them?

 

[edeca]

Something like this product from Sparkfun might be useful. There is sample code and a datasheet.

What sort of distance do you need?

 

[fastline]

About 200ft. I figure I would need a reasonably powerful and focused transmitter, and sensitive receiver.

 

[MOR_AL]

You may use one tweeter as transmitter and receiver.
1 – Find if the tweeter has a resonance frequency. If you find one use that frequency to transmit. If you don’t find it use something like 20kHz.
2 – Transmit a square pulse with half period of the frequency used.
3 – The signal received will be a low level underdamped sinusoidal wave so you will have to amplify that AC signal and rectify it until a reasonable level to trigger next circuit; a Schmitt trigger. The necessary gain will be a function of your transmitted power, the distance, the reflective index of your target, its area and the pressure to voltage conversion of your tweeter. Your receiver may be a tunable one to increase signal to noise relation.
It may be necessary to learn some theory about ultrasound and sonar.
MOR_AL

 

[fastline]

Thanks and I do agree that I need to educate a bit on the subject. This seems a great project to do that. I am wondering if the receiver will be tuned or marketed at its resonant frequency thus help to reduce false triggers by only wanting to excite at an ultrasonic frequency?

In my shopping, I can find "buzzers" all day long and I already have a long list of them here but without specs on them (sample pack from someone). However, I cannot seem to find "receivers" anywhere. I suspect the buzzers I have may not even be true piezo elements anyway. They seem to have a metal disc and rated with ohms and watts. I have been running them with a function generator at low power around 20K. I can hear them around 16K.

I am not only trying to find the parts, but I am still a little confused on how the circuit will work. I know there are some premade toys out there but that sort of defeats the purpose of this exercise. Would this be easier with a sonar designed IC? It was my understanding we would simply transmit say a 20khz sine pulse for .50ms. The receiver would be setup to to be looking for 20khz signals and would ignore say anything before 18K and above 22K. Maybe I have this wrong. You are indicating transmission of square wave to return sine and not sure the logic on that. Is that to reduce signal degradation on the return?

 

[MOR_AL]

I am wondering if the receiver will be tuned or marketed at its resonant frequency thus help to reduce false triggers by only wanting to excite at an ultrasonic frequency?

Sorry. My English is not so good. I didn’t understand.

They seem to have a metal disc...

The function of metal disc is to decrease the Q of piezoelectric resonance. This increase bandwidth and reproduces a larger set of frequencies. A good piezo will have a high Q factor. It transmit and receive (resonates) at a narrower bandwidth as a crystal used on oscillators. With high Q factor the received signal will be tuned at piezo level. This increase signal to noise ratio.

Would this be easier with a sonar designed IC?

Yes! But I don’t know what IC to use.

It was my understanding we would simply transmit say a 20khz sine pulse for .50ms. The receiver would be setup to to be looking for 20khz signals and would ignore say anything before 18K and above 22K. Maybe I have this wrong. You are indicating transmission of square wave to return sine and not sure the logic on that. Is that to reduce signal degradation on the return?

Not 0.50ms. Transmit 1/(2*20e3) or 25us.
The equivalent circuit of a piezo is a series RLC in parallel with a capacitor. Without a metal disc its Q is very high. The voltage may be a square signal but the current will be a sinusoidal one. It is not necessary to generate a sinusoidal pulse to transmit. It is easier to generate a square wave. Transmitter and receiver will have a large amount of sinusoidal power related with other frequencies.
MOR_AL