I wanted to know how small a FM spy bug could be build when manually assembled.
This is what I came up with, it measures about 0.05 square inches and is powered by a single 1.55V silver oxide battery.
Frankly, this is just a fun object, I don`t have a practical use for it.
I`m sure professionnally made spy bugs could even be smaller and work at higher frequencies which allows the antenna to be made smaller.
Step 1: Specifications
Watch the video to see the spy bug in action!
FREE: You will also find a link in the video description to download the complete PDF manual and Gerber files (needed for PCB manufacturing).
1.55V silver oxide button cell or any other voltage source from 1.1V to 1.55V
less than 200uA
at least 48h with the smallest currently commercialized battery
Depends on the antenna, with a quarter wave length antenna about 160ft
Best suited for indoor room to room surveillance
FM Radio frequencies, tested at about 85MHz
Step 2: The Circuit
All the components here are the smallest that could be obtained for a reasonable price. All the components together cost about $6 US.
To the left we see a microphone which works down to a voltage of 0.9V. Its maximum voltage is 1.45V though, which is why we need to decrease the battery voltage a bit. This is done with the schottky diode.
The oscillator to the right is then modulated with the microphone output signal.
The frequency can be adjusted with C4 and/or L1. Once everything is assembled it`s easier to replace L1 or put a capacitor in parallel with it. If you watched the video on the previous page you will understand why. :-)
Step 3: The PCB
The PCB has to made with a layout editor. One can use a software like EAGLE for that (which is free for personal use) or any of the other many software solutions on the market. Some examples are Altium Designer, OrCad, Cadence etc.
The PCB could possibly be made at home but I didn`t have the courage/patience/money to try it out. This PCB was made using a pooling service in the US for a very low price.
Step 4: Solder Paste
First we grind off some of the material from the borders.
Then we need to deposit some solder paste on each pad. Solder paste is a powder metal solder suspended in flux.
To deposit the paste on to the pads we will need a microscope (otherwise we couldn`t distinguish the pads). The solder paste could be put into a syringe with a very small diameter needle or we just use a needle for that. I found it to be easier to use the needle.
The right amount of paste on each pad is important, if there is too much it should be removed before proceeding.
Step 5: The Components
Now the components are put on the board with the help of fine tweezers.
Step 6: Soldering
The board now needs to be heated. This can be done in an oven or on a hotplate. I found it easiest to glue the board on to a hotplate with superglue before depositing the solder paste and the components.
When assembled the hotplate can then just be turned on without moving the PCB, lowering the risk of accidently shifting the components.
The temperature is adjusted to about 250C (480F). As soon as the solder paste melts it will connect all the components to the PCB traces just like normal solder wire. The board should then be removed (careful not to shake it too much) and left to cool down.
Step 7: Battery
With the layout I used we could then solder a wire into two vias on the back that serves as a battery holder.
Alternatively two wires can be connected to + and - in order to connect a different type of battery.
Step 8: Antenna
The last step is to connect the antenna wire. Ideally it should be at least 1/4 wavelength long.
The wavelength is calculated like this:
Wavelength = speed of light / frequency. That means for about 100MHz we would get 75cm (about 2.5ft).
I agree this is pretty long, but the wire can be very thin which can make it hardly visible.
Have fun! :-)