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A microphone jammer is a device that broadcasts ultrasound waves and, due to the non-linearity of mic amplifiers in small gadgets (i.e. smartphones) it overwhelms the microphone so the only sound the mic hears is hissing/beeping. On the internet there is surprisingly not much about these devices. There is however this link and the github schematic is this one: enter image description here
So, I am a relative noob with electronics but the questions are:

  • Does the 555 here generate just a wave or some other noise?
  • What are scope and scopeground pins?
  • What is this transformer for and can I modify the circuit not to use a transformer, as I can't find the model?
  • Can I use the UST40T ultrasonic speaker on this schematic?
  • What else do I need to modify to make it work?
  • Can one or two more speakers be attached or will I need a more powerful amp?
    If I need to modify something, assume I can supply it with 5V, 9V and 12V.
    P.S. Are these ultrasonic speakers easily broken/blown/damaged?
    Thanks in advance.
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  • \$\begingroup\$ Lots of tweeters for dog bark inhibitors, look up GRC \$\endgroup\$ Aug 18 '20 at 15:31
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Markus, let's walk through the circuit, then talk about whether it "will work" to safely jam a microphone. Answering your questions:

  1. The 555 output at pin 3 is a square wave. It won't be perfectly symmetric because the capacitor C1 charges through R1+R2+R4 (178k ohms) and discharges through R4 (100k ohms), but close enough.
  2. The Scope and Scopeground pins are just a way for you to monitor the frequency of the 555 square wave output.
  3. The G13599 transformer is easily found by Googling. It's $3.99 at Electronic Goldmine. They even provide a datasheet from which you'll see it's a step-up transformer to convert 3V to 200VAC for strobe lights.
  4. The UST-40T transducer has a max input voltage of 20V. You would probably blow it out instantly from a 200V signal from the transformer output.
  5. How could you make it work? -- you could try connecting 10 ultrasonic transducers in series, so each one only sees a 20V signal. This is risky, because if they are not matched, the voltage won't divide equally, and you could blow them all. Instead, just leave out the transformer, connecting the transducerpower pin where pin 1 of the transformer is currently.
  6. The UST-40T is a piezo crystal and yes, they can be easily blown. If you want higher power, you could try a hi-fi tweeter (here's one for less than 10 bucks). Or, adding a horn to provide a smoother transition between the transducer and the air will make it more efficient, as well as directing the sound.

Now, let's talk about safety and efficacy. Even though you can't "hear" ultrasound, it can still damage your ears, and it's even worse for your pets who can sense high frequencies. Be careful!

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  • \$\begingroup\$ Thank you! I am pretty much a n00b in electronics, but this answer clarified that for me. I knew the transformer seemed sketchy, but maybe this is a high voltage transducer. \$\endgroup\$
    – user261126
    Aug 19 '20 at 16:17
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If your goal is to overwhelm a microphone using out of band nonlinear distortion, you're going to want two things:

  1. A lot of power so that you drive the target into saturation
  2. A mix of frequencies spaced apart by a few kilohertz so that you generate lots of intermodulation distortion that lands in the pass band of the microphone.

With that in mind, I see some problems with your approach:

First, you are using a tiny, low power speaker with a very narrow bandwidth. That conflicts with both points 1 and 2 above. Instead, I would get a large, high power tweeter and a cheap amplifier. These are both inexpensive and easy to get off the shelf items.

Second, the circuit you're showing only generates a single fundamental frequency and its harmonics. This is not going to generate any audio-band IMD, and so is not going to work well. Instead, I would use two timers with the frequency offset by 2 KHz, and a center frequency close to the edge of the audio band. 24 and 26 kHz for example. These will be strongly detected and (if you get enough power) intermodulate to a very annoying 2 KHz.

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