Surveillance Bug

audioguru said:

The pin 5 of the Cmos 555 is modulated to make FM, not AM. the receiver demodulates the FM with "slope-detection".

Most of the important high frequencies of speech are removed by the lowpass filter in the transmitter so speech will sound muffled. The filter should have a higher cutoff frequency and be sharper.

The ears of the headphones are connected in series which makes them out-of-phase which will sound weird.

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Electronman said:

I am wondering how many distance it is able to go due to ultrasonic absorbtion?

It depends on the power rating and power fed to the transmitter ultrasonic transducer. I think powerful ones are very expensive.

Why he wanted to make a such device while he could use RF?

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The article says that an RF one can be detected. An ultrasonic one cannot be detected unless they knowm it is ultrasonic.

why He has used a virtual ground using an Opamp? A resistor divider could be used instead I think?

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He used an LM833 dual opamp and used the second one as a virtual ground. A resistor divider and capacitor would be fine instead.

What is your idea about that output transformer?

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It is a good way to step-up the signal to the transmitter's transducer.

It would not be a remote hidden microphone finally due to nonlinerity behaviuor of the air right?

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The non-linearity of air occurs only when the ultrasonic beam strikes an object then the modulation and carrier frequencies intermodulate the air and make audible but very distorted sound.
This project uses a receiver to de-modulate the ultrasonic beam so there is no non-linearity.

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audioguru said:

The non-linearity of air occurs only when the ultrasonic beam strikes an object then the modulation and carrier frequencies intermodulate the air and make audible but very distorted sound.
This project uses a receiver to de-modulate the ultrasonic beam so there is no non-linearity.

Click to expand...

Electronman said:

If he uses th transmitter in a room then it would strikes the objects.

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