The article explains a very simple circuit of a wireless speaker system which can be used for playing hi quality music wirelessly from your TV set, DVD player, Ipod, cell phone or from any music system.The speaker thus can be placed in any corner of the house within a radial distance of 50 meters and high quality music can be enjoyed without the hassles of long connecting wires.
For implementing the entire wireless speaker system, we actually need to make two sets of circuits, a transmitter circuit for transmitting the music signal from the source input as discussed above and a receiver circuit for receiving the transmitted music signal and for playing it in the attached speaker.
As shown in the figure, the configuration looks a little different from the usual single transistor transmitter circuits where a single stage is used for the audio amplification and for the generation of the modulated carrier waves.
The usual single transistor transmitter circuit has the advantage of smaller and compact size and minimal power consumption, but is not suitable for long and strong signal transmissions.
However since such circuits are commonly used for wireless, stage-microphone applications, the distance is not a factor but compactness and low power consumption is definitely a must, and therefore becomes quite suitable for the intended application.
The present design is not intended for the above application so the first two features are not important, however the proposed idea of a wireless speaker system surely requires a long range and a distortion free power transmission, so that the reception can be heard at any corner of a particular premise or even across an apartment.
Therefore a stronger signaling or transmission of stronger carries RF signals becomes a necessity.
That’s exactly why we have incorporated a couple of extra stages in addition to the central carrier wave generator stage.
The first transistor and its associated components form a neat little audio amplifier stage and also a buffer between the audio source and the transmitter circuit.
This stage amplifies the received signal to stronger levels, and this stage also allows keeping the volume of the source signal to minimum levels.
The amplified signal is passed on to the next stage, which is the actual RF signal generator stage.
This stage is basically a simple feedback type of oscillator, wired to produce RF signals in the range of around 90 to 100 Mhz.
The amplified signal from the collector T1 start forces T2 to modulate the generated RF with the injected audio signals.
The modulated signal from the collector of T2 can also be directly used for the intended wireless music receiving, however since we are interested in making it more powerful, we introduce another stage which becomes responsible for amplifying the modulated signals to much stronger levels so that it may be heard across many 10s of meters away and even in cell phone radios.
The inductor L1 is the most critical part of the circuit. Its dimensions are: 1mm, super enameled copper wire, having 5 turns of 6mm diameter. The tap to C6 is taken by scratching the second last turn of the coil toward the positive side end.
The whole transmitter circuit can be built over a small piece of veroboard and housed inside a suitable sized metal box along with the required power supply section enclosed.
This concludes the transmitter circuit.
The Receiver Circuit
Ideally you won’t need to build this as the receptions can be heard crystal clear over an ordinary FM radio set. Therefore you might just want to use the FM radio itself as the wireless loudspeaker, or probably add an ampli-speaker box in conjunction to you FM receiver.
That’s it, your wireless speaker box system is ready and may be used for listening to any audio transmission without connecting wires across a radial distance of more than 50 meters, if the antenna is made large enough, the range may be well increased to beyond 90 meters.
R2 = 2K2,
R3 = 470 Ohms,
R4 = 39K,
R5 = 470 Ohms,
C1 = 0.1 uF,
C2 = 4.7 uF,
C3, C6 = 0.001uF,
C4 = 3.3pF,
C5 = 10pF,
C7 = 100uF/16V
D1----D4 = 1N4007
L1 = See Text
T1, T2 = BC547B,
T3 = BC557B
TR1 = transformer, 0-9V, 100mA