Bugs & Cockroach.
So what is the purpose of this project?
This is strictly an educational project explaining
how bugs works and can be built. According to the
law it is legal to build them, but not to use them.
All contribution to this page are most welcome!

What is a bug?
A bug is a transmitter which can transmit audio or video. A bug should be small and easy to hide.
The purpose of a bug is to hear conversation that you are not suppose to hear.
It is not a big deal, half of the American population has been bugged during the last year by several well known intelligence's and also by unknown intelligence's. To make a RF signal the bug needs power. This is the weak point of the bug. If you want to transmit a long distance you need lot of power. The more power you need the larger battery size.

The bug I present has low power consumption and can transmit a few hundred meters. Another important part of the bug is the antenna.
A long (1/4 wave length)antenna will transmit the RF signal over a larger distance than a short antenna.
As you understand there is lot of factor working against each other. In this project I try to combine all factors for optimal use of the bug. You will also be able the set the transmitting power yourself. Another disadvantage of transmitting to much power is that other people might tune in your signal and hear the information, which is not good.

Use enough power and save battery power. This bug will consume about 22mA at 4.2V
It will work down to 3V but the power will be about 13mA then.
It will also work up to 9V but if you push it to much it will get hot and break.
The frequency can be set from 80 to 120MHz.


The audio is picked up by a capacitor-microphone. The signal is amplified in a transistor and put to a DC bias of 1V. The AC voltage of the audio signal will change the capacitance in the varicap and together with L1 and T1, it will form an oscillating circuit. The signal is then boosted in another transistor T2.
The coil L1 is air wounded and has a diameter of 7.2mm and is 3 or 4 turns.
3 turns will set the frequency from 108MHz and up.
4 turns will set the frequency from 80MHz and up.

The resistor Re will set the current in the booster. If you decrease this resistor you will increase the current and thereby get more power out from the transmitter. This is not good because you will consume the battery then. I have chosen to set the Re to 100 ohm.
The resistor R1 and R2 will set the amplification of the transmitter. To get best power performance I advice you to use potentiometer and test the transmitter, then you can change the pot to resistors when you have found a good working point.

In my test I set Re to 100 ohm and R1 and R2 to 18k, which gave me good performance with 4.2V.

If you use a proper antenna you should have about 70-80 cm wire.
This might not be practical to hide so you can use shorter wire, but the transmitting distance will decrease.
I use a 10cm piece of wire, which is horrible, but it worked.

Field Test
In the filed test I put the bug in a building and went out into a park 100m away and could still hear the signal. The antenna I used was a 10cm piece of wire. The power consumption was 22mA at 4.2V and I find this good enough for me.
With my digital wattmeter I could measure the output power to 4mW into 50 ohm load at 4.2V supply.
At 9V power supply I had 10mW into 50 ohm.

Final word
Build this but don't use it, okay!
You can always mail me if there is anything unclear.
I wish you good luck with your projects and thanks for visit my page.

Back to main Page  |  Contact Me  |  Cheap components

Copyright © Last modified on 26 april 2003.