Making a wireless microphone at home can be real fun; here we present one such simple project which can be used as a spy gadget. The wireless microphone can broadcast audio picked from a room you leave that microphone in, to a nearby FM Radio and enables you to listen what is going on in the room. The presented wireless microphone is actually a miniature radio bug which has an effective range of about 150 meters.
FM Wireless Microphone Circuit
The heart of the circuit is a radio frequency (RF) oscillator. The oscillator can be tuned at a specific carrier frequency, at any value between 87.5 -108MHz (FM radio band in Europe). The oscillator is built around T2 according to Colpitts topology and uses an LC tank circuit. The name 'tank' circuit comes from the ability of an LC circuit to store energy for oscillations. The LC tank circuit is made from a small inductor (L1) which is connected in parallel to capacitor C3.
Audio is picked up by an electret microphone, and then is fed into an audio amplifier stage built around the first transistor (T1). The amplification stage is a standard self-biased common emitter amplifier. C1 is a 5n6 capacitor, which is used to isolate the microphone from the base voltage of the transistor and only allows alternating current (AC) signals to pass.
Output from T1’s collector is fed into the base of T2 in order to modulate the resonant frequency of the oscillator. This is done by varying the junction capacitance of transistor T2. The junction capacitance of a transistor is a function of the potential applied to the base of the transistor. So, by varying the potential (voltage), we are able to FM modulate the carrier and broadcast audio to a common FM receiver.
The oscillator circuit is actually a resonant high frequency common – base amplifier, based on T2 and an LC tank. C4 is used to provide positive feedback in order to convert the amplifier into an oscillator. In such arrangement, better known as Colpitts oscillator topology, an input signal is not needed to sustain the oscillation. The feedback signal makes the base-emitter current of the transistor to vary at the resonant frequency of the LC tank. This causes the emitter-collector current to vary at the same frequency. This high frequency signal is fed to the antenna and produces a high frequency (oscillating) current. The oscillating current produces a varying electromagnetic field which is actually the source of the radiated electromagnetic waves. C5 is used as a coupling capacitor for AC coupling the antenna and minimizing the effect of the aerial capacitance on the LC circuit.
How to assemble the Miniature Radio Bug
The circuit must be assembled on a general purpose electronic board according to the directions provided on the image below. The general purpose electronic board is a widely used type of electronics prototyping board, characterized by a 0.1 inch (2.54 mm) regular (rectangular) grid of holes. Like a normal PCB, it provides a means to hold all of the components together in one place as a single unit. But it does not provide the connection between components. So, it is up to you to make the connections required by using small wires or solder joints. Using as a guide the image provided below, you will be able to assemble the unit without significant effort.
The image shows components placement as viewed from the front side (not the side of connections) and can be used as an assembly guide. Black patches represent individual holes copper patches and connections between them. The use of coordinates makes this assembly guide to be much more instructive and accurate.
All resistors used in this project are all ¼ watt, 5% tolerance, or better. The capacitors are all common low voltage ceramics. The circuit is powered from an ordinary 9V battery. This type off battery is usually designated as NEDA 1604, IEC 6F22 and "Ever Ready" type PP3 (zinc-carbon) or MN1604 6LR61 (alkaline). The battery is connected to the PCB through an 9V-battery clip.
Making the inductor
You must make the inductor by yourself, by wrapping 4 turns of AWG 19 enameled wire around a 6mm drill bit. The coil turns must be wrapped as close together as possible. In alternative, you can wrap the wire around the drill bit loosely first and then squeeze the coil turns to be placed as close together as possible. Enameled wire is a copper or aluminium wire coated with a very thin layer of insulation. It is used in the construction of transformers, inductors, motors, speakers, hard disk head actuators, electromagnets, and other applications which require tight coils of wire.
Making the antenna
The antenna can be made from the same enameled wire used for making the coil. In the prototype, we use 40 cm of AWG 19 wire, wrapped around a 6mm drill bit. That kind of antenna is extremely convenient due to its small size and will be adequate for your spy gadget. In order to maximize the effective range of the transmitter, you could also use a classic half or quarter wavelength length antenna, like a Hertz dipole or a Marconi- monopole. Those antennas are best suited for outdoor use and can possible extend the effective transmitting range up to 1Km.
Calibration of the Wireless Microphone
This should be done with the unit placed at least 5m from the receiver, preferably in another room. This precaution has to been taken to avoid audio feedback due to sound looping between receiver and transmitter. Another way to avoid audio feedback is by using headphones. By using headphones you will be able to perform calibration even next to the unit.
The process of calibration would be much easier if you place the unit near to some source of sound, like a TV, ticking clock or just people talking. If you choose to use headphones, the source of sound could even be yourself. Remember that the wireless microphone is designed to be enough sensitive to monitor voices even in a large room.
Plug in the battery to the clip and try to pick-up reception on an ordinary FM radio by moving the tuning dial at around 102-106 MHz. Somewhere there the transmission should be picked up. If you cannot get the frequency you desire, you can squeeze the turns of the coil in the tank circuit closer together, to lower the frequency, or pull them apart just a little bit to raise it. This way the coil would be able to act as a variable tuning element. You can also modify the tuning frequency by altering the value of C3. The use of a larger or of a smaller value for C3 will lower or increase the tuning band, respectively.
The circuit works best when powered from a 9V battery. It can be also powered form a low ripple power supply unit. The ripple should be extremely low, otherwise you will get hum in the receiver. Adding and removing the battery from the battery clip acts as a switch for the circuit. Or, you may add your own switch.
We wish you to have a lot of fun with this miniature FM transmitter!