Check if someone is wearing a wire

[zanov]

I know this is very problem because scanning all frequencies is something can not be done with a simple RF hobby circuit.

But what if (help me here) if something can be sensed via some Hall Effect, Wiegan Effect IC, circuit of a presence of energy in close range to mean something might be going on?

 

[flatulent]

The standard aircraft RADAR early warning receiver is a diode detector followed by low frequency amplifiers. The problem with this on the ground is the large number of high powered broadcast transmitters located near population centers. It would be worth your effort to make a breadboard circuit and see if the broadcasting in your area dominates the output.

 

[zanov]

I made this circuit in picture. I found this circuit in book. This is my first attempt.

I was in basement and adjustested R1 and R3 resistor while staying 2meters near my monitor. It was detecting very good when monitor was turning on/off.

The moment I entered upstairs to show my folks my designed circuit it was sounding the alarm everywhere.

So this circuit was not good use to me. I dont want to use an antenna here because I dont know in what band remote is transmiting and I am eagered to go after these Hall effect devices but dont know where to start here.

I'll find it.
thanks

 

[radiohead]

This design has some flaws ...

The amplifying cascade will have a lot of gain for sure, but only at lower frequencies (depends on GBW product of opamp). To detect signals at higher freqs, you have to downconvert to low freq. E.g. with a simple hf diode just as stated before. This part is missing ... you will only hear LF hiss in the air, like switching transients from you monitor ...

 

[zanov]

opamps are BiMOS opamps from Intersil and have only 20MHz bandwidth theoretically I guess.

I got the circuit from a book from RadioShack you are absolutely right circuit observes switching transients of monitor being turned on/off.

Guys show me some benchmark from where else I can look, diode solution can be a burnden. What parts, how the circuit?

thank you

 

[johnyaya]

A couple years back I was getting CE certification at TUV. Part of the process is checking emmissions (over the air). Using high-end spectrum analyzers and an array of broadband antennas to monitor the emmissions from my device at various angles (fixed antenna while rotating my device). I was amazed at all the transmissions out there, and we were 10 miles outside of town! So, to have a device that detects ALL transmissions and somehow detects a weak signal on top of that doesn't seem feasable. With all of the advanced techniques for detection avoidance I would think that this type of detector would have limited applications. But that's just my gut feeling...

 

[radiohead]

That is absolutely correct. It's like a deaf guy hearing for the first time. He can distinguish something, but really doesn't know what.

If you live in a city nearby radio network / mobile phone network antenna, the detector could trigger all the time. So you would have to higher your threshold, with lower sensitivity resulting. Selective detection is the key. Why would we ever want a superhet/ZIF else after all?