to drive ultrasonic transducer at 40kHz

[Vibrant2020]

Hi,
Purpose : to drive ultrasonic transducer at 40kHz
Parts used:
1. Ultrasonic Transducer (taken out from general module from HC-SRO4)
2. PWM generator module with led display (micro usb 2 channel pwm generator with led)
3. LM386 amplifier module
4. +5v/500mA for PWM module
5. +9V/7AH (battery) for LM386 amplifier module

For testing purpose, The following parts used:
1. LED
2. Buzzer (working on 6 to 9 V range)

PWM Generator Module Testing Status:
Have connected the LED at the output and tested. It works fine.

LM386 amplifier module testing status +9V:
Have connected the Buzzer and tested. Works okay but there is not enough amplification. Voltage at output pin was around 3V. When i rotate the 10K pot to low, it correctly off the buzzer sound. But when i increase, not enough sound i obtained.

Now I connected the PWM generator module's PWM1 output to LM386 amplifier Module and connected the Ultrasonic transducer at the LM386 output pin. I set the PWM1 output as 40khz / 50% duty cycle (40.0 on led display) and switched on. I put my phone mic in front of the transducer and tested. I didn't get any audio signal on my phone's mic.

PWM module used:

Amazon.in : pwm module

LM386 Amplifier Module used::

Amazon.in : lm386 audio amplifier board

Your opinion & suggestion welcome.

Thank you,
Vibrant

--- Updated Sep 23, 2020 ---

Pictures attached For your referrence.

 

[danadakk]

Schematic of how the LM386 circuit was done ?

Any part number on the transducer ?

Typically these transducers a very large C load on a driver. So how
the LM386 is comped may affect output performance.


Regards, Dana.

 

[KlausST]

Hi,

mind that LM386 frequency range is experessed in sine waveform, but PWM is in square waveform.
Square waveform includes a lot of overtones, theoretically to inifinite (sine) frequency.. thus expect distorted waveform.

The more "square" waveform you want the higher bandwidth your amplifier needs.

Klaus

 

[FvM]

These transducers have a real impedance of several 100 ohm at series resonance frequency. If you want maximal sound pressure, the transducer can be driven with several 10 V, some types even more. In so far LM386 is no good choice.

 

[Audioguru]

Why do you think your phone will produce audio when its mic hears 40kHz??
3V RMS is 8.5V peak-to-peak which is more than the maximum output of an LM386 that has no load and is powered from 9V.
Modern amplifier ICs are bridged so the output voltage amplitude swing is doubled.

 

[dick_freebird]

Because these piezo ultrasound transducers are resonant,
why not just bang it with a power MOSFET driver and let it
eat or ignore the other harmonics? Not like somebody's
going to notice the 120kHz, 200kHz, ... overtones I reckon.

 

[Vibrant2020]

Schematic of how the LM386 circuit was done ?

I didn't design the circuit. Just i bought from the amazon.

Any part number on the transducer ?

As i already said, it was taken out from the commonly available ultrasonic module HC-SRO4. No details available about the transducer except it resonates at 40khz range.

To other seniors FVM, KlausST AUDIOGURU & dick_freebird:

The circuit is microphone jamming (nearby) circuit - legally allowed to use for personal/company privacy protection and available across the world on online stores (ebay, amazon, etc), security shops but costlier.

Any help like providing circuits, references, etc would be highly appreciated.

Thanks,
Vibrant

 

[Audioguru]

A mic jamming circuit?
You said, "I put my phone mic in front of the transducer and tested. I didn't get any audio signal on my phone's mic." But you did not say that is what you want, and it stopped the phone's mic from working.

Since there are many phone manufacturers using different mics then the circuit will work best with mics that have plenty of pickup at 40kHz (few).
The output level from an LM386 amplifier is probably too low and then you must use a very short distance between the 40kHz transducer and the phone's mic.

--- Updated Sep 24, 2020 ---

I looked it up in Google. The jammer is used to disable a nearby hidden recording or transmitting mic, not a phone. The articles say the jammer is very directional and must be close to and pointed directly at the hidden mic. One jammer is a huge bracelet with many 40kHz transducers pointing in many directions. The articles (written by the same guy) do not say that many mics do not pickup 40kHz then they are not jammed.

 

[FvM]

I imagine that the said jammer can work with microphones that have a residual sensitivity at 40 kHz, e.g. electrete microphones, if it produces a very high sound pressure > 120 dB and overloads the buffer amplifier.

 

[Audioguru]

But the LM386 few volts output will not drive the ultrasonic transducer loud enough.

 

[dick_freebird]

It might be more effective to use a high power piezo
tweeter and lower the frequency to just above human
audible range, to get into the passband of the mic
preamp and better saturate it.

 

[Vibrant2020]

Thank you all for your suggestions.
As AudioGuru and danadakk said, I could not able to jam the mic using LM386 module.

But I can jam the microphone of the mobile phone successfully using using CA-3110 audio amplifier module.

The distance between the microphone and the ultrasonic transducer should around only 1 to 2 CM.

The CA-3110 audio amplifier module output is 15W as said in the module and the website description.

I need the distance around 20 feet (normal room size). Can you suggest me to accomplish this task?

To dick_freebird:
I don't have much knowledge about the Ultrasonic transducer. As you said, frequency doesn't matter but it should be picked up the microphone and sufficient to jam the voice. I will test with the MOSFETs and let you update with the results. If you can provide any reference on the circuit please share with me.

To all:
Initially I had a trouble to find whether the Ultrasonic sonic transducer correctly resonates at the 40 kHz (Amplified PWM) input. So I used the commonly available HC-SRO4 module (Receiver) to test the transducer(Transmitter - which has been taken out from the same module).
I found the Receiver transducer correctly resonates at around 39.0 to 41.0 kHz of transmitter transducer.
I connected the LED at the module's "echo" pin. By default, the "echo" pin will be high. So the LED will be ON. If the module detects the ultrasonic sonic wave, it pulls down the "echo" low. So the LED will be OFF.

I have attached the video file for your reference.

Thanks to all for providing the help.

Waiting for your suggestions and help to accomplish the task.
Vibrant.

 

[KlausST]

Hi,

if we follow square formula you need about 15W * (20 feet / 2cm) ^2 = 1.4 MegaWatts.

Klaus

 

[Audioguru]

Electret mics are designed to receive audio, not ultrasonic air vibrations. At 40kHz their sensitivity is not shown in datasheets then it might be fairly low. They need an extremely loud level to be overloaded.

The Ultrasonic transmitter transducer has a fairly low maximum allowed driving voltage, therefore its maximum allowed output power is fairly low.
The low power LM386 amplifier might match the maximum allowed power of the ultrasonic transmitter transducer but the much higher power from a CA3110 amplifier might soon damage or destroy it.

Therefore I agree that you will need a massive amount of power in many ultrasonic transmitter transducers to jam a microphone at a distance of 20 feet.