The range of this FM transmitter is around 500 meters at 3V DC supply and receiver is able to capture signals from 70~100 meters. The circuit consists 3 stages. The first stage is a MIC preamplifier built around BC548 transistor by varying 2K2 with higher values increase the sensitivity of mic. The next stage is the heart of this device an astable multivibrator changes output high and low, at VHF Frequency, Here we use BC548 General purpose NPN transistor, capable of handling 250mW of current & Oscillates upto 300Mhz This VHF oscillator is fixed to oscillates in the middle of FM band, ie 100MHz with the help of 10pF capacitor and 2K2 and 4turns coil. The preamplified audio from 1st stage is coupled to VHF oscillator with the help of 0.1uf or 104capacitor, The audio signal vary the oscillator frequency and the new frequency ie. audio freq+oscillator freq (signals cancel each other or add up here, ie 100MHz vary from 99.001 to 101.001) is fed to the 3rd stage. This mixed signal is normalized with the help of 10K and 470Pf capacitors attached to +ve and -ve rails respectively. The 3rd part of the circuit is a class-A tuned amplifier that consists of 10pf capacitor & 10 turns coil boosts signals from the oscillator. Use of the additional RF amplifier increases the range of the transmitter. This part is optional if you want a simple voice transmitter, If you need High Power Transmission you must use high rated transistors with, BC series can hadle upto 250mW of power, ie enough for 200 meter communication.
Now the receiver part is also same as transmitter, first
When electricity pass through a conductor it produces electromagnetic waves around it, if the electricity/frequency is high then it'll propagate to long distance, similarly when electromagenetic waves hit conductor it'll produce electricity, that is very time difficult to measure with any instrument.
From AIR the antenna pick up whatever electromagnetic waves hitting the wire, that mixed signal fed to the 1st part BC548 Oscillator , here also oscillator is oscillating at 100Mhz the incoming mixed signal vary the oscillator frequency or cancel each other and produces required signals. That is Detection, only a allowed part of signal pass rest cancel each other. that output is very low cannot hear, that why we implement a 2nd part preamplifier, that'll convert low level signal to identifiable high and low signals for our amplifier.
The Transmitter & Receiver are very simple to understand. The core of both TX & RX is astable multivibrator that oscillates at 100MHz, At TX section the incoming audio signal vary the oscillator signal,At RX section mixed signal fed up with oscillator and cancel the oscillator frequency the final result is usefull signal.
When electricity pass through a conductor it produces electromagnetic waves around it, if the electricity/frequency is high then it'll propagate to long distance, similarly when electromagenetic waves hit conductor it'll produce electricity, that is very time difficult to measure with any instrument.
From AIR the antenna pick up whatever electromagnetic waves hitting the wire, that mixed signal fed to the 1st part BC548 Oscillator , here also oscillator is oscillating at 100Mhz the incoming mixed signal vary the oscillator frequency or cancel each other and produces required signals. That is Detection, only a allowed part of signal pass rest cancel each other. that output is very low cannot hear, that why we implement a 2nd part preamplifier, that'll convert low level signal to identifiable high and low signals for our amplifier.
The output from free preamp is pretty pure, can be fed into our LM386 general purpose amplifier that'll convert it audible sound for a speaker. The sensitivity of the receiver is poor according to the conditions. The receiver cannot be used to receive local FM stations, if you attach the antenna wire to home TV antenna/Dish or Analog Cable plug-point you may get some channels. Here is the Best All purpose amplifier LM386
the receiver part does'nt work, post a signal receiver circit
ReplyDeleteYh i'll chk that
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