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Wireless Audio Transmitter

C. Tavernier


Sitting peacefully under a tree at the bottom of your garden, or stretched out beside your swimming pool, you may feel like listening to your favourite music from your hifi. Rather than turning the volume up beyond reasonable limits and risking upsetting all your neighbours or attracting the wrong audience, we suggest building this little wireless audio transmitter/ receiver combination. Using the UHF ISM (industrial, scientific, medical) band and quality FM (frequency modulation), it won’t impair the sound quality and will let you listen nice and discreetly. The transmitter uses a well-known module manufactured for some years now by Aurel as their ‘FM audio transmitter’. It works in the licence-free 433.92 MHz band and so allows our project to operate completely legally as the transmitter is type-approved to quite strict technical specifications. Note however the frequency you’re using is not exclusive as it is shared by many other wireless devices such as headphones and key fobs for garage doors and so on. The equipment is low-power however and should




have a short range. The Aurel module is a complete FM audio transmitter designed for powering from12 V. The only external components required, R5, R6, and C5, form the preemphasis (high-boost) network specifi c to frequency modulation. Used alone, this module offers a typical audio input sensitivity of 100 mV rms. So we are driving it from an opamp with gain adjustable between 0.5 and 5, extending the voltage range from 50 to 500 mV, to make it compatible with any audio device line output. Note in passing that, if you reduce resistor R1 to 2.2 kΩ, you can increase the sensitivity to 2.5 mV so that the transmitter could then be used as a UHF radio mic for use in shows and events, for example. The power supply can be obtained from a 12 V battery or a ‘plug-top’ power supply; diode D1 protects the circuit from reversed polarity by. The receiver is just as simple, since it uses the complementary module to the previous one, again from Aurel, and naturally called their ‘FM audio receiver’. This receiver has a squelch (FM noise silencing) adjustment, set by the voltage applied to pin 15.


Potentiometer P1 connected to this makes it possible to adjust the squelch threshold so as to have a receiver that won’t output noise in the absence of a signal, using the information provided on pin 18. This is High when a signal is present and Low when absent. Here it drives an 8-into-1 CMOS analogue multiplexer, of which only input 8 is used. This solution employs a very cheap, good-quality analogue switch that is easy to use. Its output passing via the volume control P2 and is applied to the well-known small integrated power amplifier LM386. The transmitter’s RF output power of a few hundred milli watts is more than adequate for such an application, and its quality likewise, especially if you combine it witha loudspeaker worthy of the name, with a pair of headphones as the next bestalternative. The Aurel receiver module and CMOS multiplexer both require a 5 V supply; this is stabilized by a standard 3-terminal regulator. The circuit as a whole is powered from 9 V, and is also protected against possible reverse polarity by diode D1. Given the relatively high current consumption of the amplifi er, especially if you use it for longer periods, rechargeable NiMH batteries will obviously be preferable to primary cells, which wouldn’t last very long and will turn out rather expensive in the long term, as well as bad for the environment. As far as the antennas are concerned, for both transmission and reception, simple quarter-wave whips ensure a range of a hundred metres or so – even more if line-of-sight. You can of course buy such antennas ready-made, but a simple piece of stiff wire around 17 cm long (i.e. a quarter wavelength at 433.92 MHz) will do the job just as well, and cost a lot less. Equipped with these two modules you can make the most of your music wherever you like. Don’t forget, though, that outdoors, the best music of all is that of birds, that is, the feathered variety.