Theory of loud-speaker and Head-phone ::
Loudspeaker and Headphone works in the same principle of converting electrical
signal to sound waves. We now discuss the basic construction of these
transducers. It has a permanent magnet inside and some cupper coil around the
magnet. The coil is attached to a diaphragm. The diaphragm is like a dish like
shape and supported by a metal frame. Now when alternating audio signal is
flowing through the coil, makes a alternating magnetic field. This alternating
field, which is created in coil, interacts with the field of permanent magnet.
Thus a force is created which makes vibration in the coil which intern
vibrates the diaphragm. This vibration falls in the audio frequency. Thus
sound is produced.
Theory of Microphone ::
Microphone is a transducer, which converts the sound energy to electrical currents. Microphone may be of two type 1) Magnetic type and 2) Carbon type.
Magnetic type microphones are similar structure to that of a headphone or loudspeaker. It has a permanent magnet inside and some cupper coil around the magnet. The coil is attached to a diaphragm. The diaphragm is like a dish like shape and supported by a metal frame. Here sound waves make vibration in the diaphragm. As coil is attached to it, it also vibrates. We know a moving coil in a magnetic field produces a current in it according to the law of generator. Thus a alternating audio current is produced in the microphone.
Another type of microphone is the carbon type. It consists of a diaphragm and some carbon granules. The diaphragm is attached with the carbon. The two electrical terminals are connected in the carbon. A sound wave makes vibrations in the diaphragm and the diaphragm either makes some pressure or release pressure in the carbon according to the intensity of sound. The resistance of the carbon is dependent on the pressure on it. Application of pressure in carbon decreases its resistance and releasing in pressure increases the resistance. If microphone is connected with a resistance an some bias is applied, and sound is applied to it, there will produce some alternating current
The audio signal generated by a microphone is very weak. It must be preamplified to give it sufficient gain so that it can be fed to a transmitter. The unit has a operational amplifier in inverting mode.
Beep tone generator has three 555 Timer in Astable mode. Beeps are generated in 1s of time interval and stays for 350ms. In this 350ms two beeps are generated in 50ms interval and stays for 150ms each. Thus a twin tone just like a Mobile Telephone ring. The RC value in the timer is selected such that these three timers do the job for us. The first timer gives a on time of 350ms and off time of 1s. then in 350ms the second timer gives 150ms on then 50ms off then 150ms on. Thus the third timer which makes tone delivers two beeps in short time of 350ms and kept silent for 1s.