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# Ultrasonic transducer question

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#### aruna1

##### Member level 1
Hi
I'm working on a ultrasonic transmitter and using transducer pair like in image I attached.

Currently I'm giving 9V (7809) to drive the transducer (TX) through a H-bridge.

My question is;
I have read in internet that these transmitter drums can operate voltages up to 20V and as the voltage increase the transmission distance increases.I don't have any datasheet on the module and it doesn't have any model number or reference.

So is it possible to use a charge pump system to boost 5V supply to 15V and use this 15V to drive the transducer? Since charge pumps cant deliver large current I wonder whether this is possible. But if it possible it will save me from making an inductor based boost converter.

If this is possible what is the maximum ultrasonic burst duration I can achieve with charge pump?

I wonder how they do it on ultrasonic range finder modules like SRF05, they work with 5V.

as I'm using 9V currently I was able to send ultrasonic signal to about 25m. but this is possible only If I'm transmitting continues 40KHz stream.
But if I send 40KHz burst , receiver sometimes failed to grab it even at distance of 8m and receiver is already using gain of 1360.

please note this system is not using the reflected ultrasonic signal. transmitter and receiver are in two different places and used to measure the distance between transmitter and receive.

I want to send a super power ultrasonic burst so my receiver wont miss it at 20m

I Googled "ultrasonic transducer" and soon came up with some interesting thoughts. The first is that an exponential horn, matches the transducers output to the air better and narrows its beam so more energy is squirted in the target direction, the same would apply to the receiver transducer. It would seem that their impedance at their operating frequency is about 500 ohms with an average power of 200mW. You say that you are operating it from a H bridge running of 9V. This means that the peak to peak voltage across the transducer will be 18 V. So is this 20V, RMS sine wave drive?
I would have thought a cercuit such as this :- ttp://www.reconnsworld.com/power_voltdoubler.html would supply enough current for your application.

Frank

I've been working with ultrasonic transducers for my last project with your exact application (separate transmit and receive modules). I used the SRF05 initially and then implemented my own ranger.

You can easily find transducers that operate up to 400V (from Mouser), so 20V isn't probably the actual limit. From my quick measurements, it seemed like the voltage at the receiver side increased linearly with the transmit voltage. Since you are not relying on receiving a reflected signal, you should receive a much larger voltage than these rangers work with. Theoretically you should be able to get more range.

A charge pump supplies low DC current, but you can always put a large capacitor to store the generated voltage. Since you transmit in bursts, the capacitor should be able to supply the current without the voltage drooping too much.

I'm not sure how you plan on driving the transducer, which behaves like a capacitor (mine was ~2000pF). I put a resistor in parallel with it to turn it into an RC-circuit. The trade-off in picking the resistance is in getting a clean, fast edge vs. current consumption.

Here's some good information about generating a higher supply voltage. They used a standard RS-232 chip, which is a pretty clever idea. You can get +/- 10V from 5V easily this way.

SRF04 Technical Documentation

Some more thoughts: In a single burst, you can send out more pulses which will increase the maximum range sensed. Also, increasing the gain on the receiver side will increase the maximum range, of course. This is why I ended up designing my own ranger, so I could customize these parameters.

I think it'd be worth trying the inductive driver if having 20Vp-p doesn't work out. You should be able to generate huge voltages but it's harder to design.

For reference, my application only needed 1m range, but needed to handle very wide beam angles. I drove the transducer using 0-12V. I had a gain of approximately 500 which was barely enough for 1m. For a narrower transducer beam angle, you probably wouldn't need as much gain.

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The sensors look like far east products that are on the market since 30 or 40 years. They are often selled by DIY electronics catalog distributors, you should be able to find a datasheet somewhere on the internet. Major manufacturers (e.g Murata) have similar products with a probably more exact specification. The transducers use rather thin piezo discs, so the 20V limitation sounds reasonable. By considerably exceeding it, you run at risk to either break or depolarize the piezo elements.

These sensors are generally small band, so the most simply method to increase the output voltage with low supply levels is an LC resonant circuit or a transformer. A bridge configuration is of course an option. The input impedance of the transducers is moderate and don't need a strong driver.

please note this system is not using the reflected ultrasonic signal. transmitter and receiver are in two different places and used to measure the distance between transmitter and receive.

Unless running two transmitter and receiver in a ping-pong operation, you won't be able to measure time-of-flight.

The first is that an exponential horn, matches the transducers output to the air better and narrows its beam so more energy is squirted in the target direction.
Basically yes, I would expect however, that these transducers designed for a certain application already achieve a (limited) acoustic impedance matching. Parabolic reflectors are always an option to narrow the beam width.

Unless running two transmitter and receiver in a ping-pong operation, you won't be able to measure time-of-flight.

Why not? Send transmit pulse, then time how long it takes to receive. Then some basic calibration removes systematic error. Or do you mean synchronizing the two systems?

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