So I was exploring the internets when I found this: http://www.bugplan.com/a15.htm , a very powerful amplifier, designed by an electronics engineer with experience designing circuits for law enforcement, I obviously couldn't resist building it and I have to say I'm very impressed with the performance of this circuit, it doesn't only allows you to hear even the faintest of sounds, like the swinging motion of an HDD arm inside your computer, when used with other sensors like a tape head it can be used to hear the different electromagnetic fields around, detect mains cables, power lines...

For me, this makes this device a must have, apart from the malicious intent to spy on your neighbor it can also be used as diagnostic and troubleshooting tool.

IMPORTANT:This kind of circuits or/and their use might be illegal in your area, so it might be wise to check your local regulations before using it, use it responsibly, invasion of privacy is illegal.

Step 1: The Circuit

This is a fairly complex circuit to implement on a perforated board without errors, so I thought it would be easier to make a PCB with Eagle and print the circuit using the toner transfer method. This also makes things easier for people who want to replicate the circuit.If you use the toner method make sure to check for continuity between separated traces or traces and ground, small defects can easily occur using this method.

Breadboarding is also an option, but the circuit performance might be affected due the capacitance and inductance of breadboards.

For more detailed info about the circuit I encourage you visit his website:

http://www.bugplan.com/a15.htm

I've attached the schematic and board files so you can print the circuit or modify it if you wish. The circuit is tightly packed, and although this doesn't seems to have any effect on it's performance (I made sure to use thick traces with a big clearance) you might need to modify it if your components are not small enough.

Step 2: Making the Microphone

For the microphone you can use a piezo, but many other components will work, like a simple coil of wire, I had no luck with an electret microphone I had lying around.

I soldered a couple of wires to the piezo disk, the positive goes onto the gray area, while the negative get's soldered to the disk, I placed the disk over a wet napkin to avoid overheating it, then I glued a knob with epoxy. I used some wire with tubing I had laying around, you really want the wire to be in a tube of plastic or fabric, since the vibrations on the wire get transmitted to the piezo and affects the quality of the sound received.

I also experimented using a VHS tape head, works quite well at detecting magnetic fields that fall under the audible range.

It seems using coaxial wire or even twisting the two wires together help to reduce hum, this is because the wires act like a capacitor, filtering out some noise.

Step 3: Enclosure

Once the circuit is finished and it's working properly, it can be placed in an enclosure to make it more convenient to use. I used a 8.5cm long, 5.5cm wide and 4cm tall box, it's slightly longer than what I needed, but it will do.

I've chosen to use a pushbutton instead of a switch because the sound can get pretty loud if you bump or scratch the piezo when relocating it, pushing a button seems more convenient than toggling the switch each time.

The circuit is tightly packed into the box and the switches, potentiometer and jack get screwed to the sides of the board.

Step 4: Tests

This is what it sounds like after placing the piezo on my laptop, you can hear the HDD making it's usual noises. The audio quality is bad since I had to place my earphones next to the microphone.