The name ‘Tank’ circuit comes from the ability of the LC circuit to store energy for oscillations. Where f is the frequency in hertz, x is the coil length, C is the capacitance of trimmer in Farads, and L is the inductance of coil in Hendry.Įvery FM transmitter needs an oscillator to generate the radio Frequency (RF) carrier waves. The resonant frequency can be calculated using the formula In the tank circuit, the capacitor stores electrical energy between its plates while the inductor stores magnetic energy induced by the windings of the coil. The variable capacitor and the inductor coil form the Tank circuit (LC circuit) that resonates in the 88-108 MHz. The resulting value is in Micro Henry.Ī small button type variable capacitor with a value of 22 pF can be used to adjust the resonant frequency of the tank circuit. Where r is the inner radius of the coil, x is the length of the coil and n, number of turns. The inductance of the coil can be calculated using the formula Remove the enamel from the tips and solder close to the transistor. Remove the coil from the screw driver and use the 5 turn Air core coil. Coil can be wound around a screw driver (with same diameter) to get a 5 turn coil of 0.2 inch long. For this circuit, the coil radius was selected as 0.26 inches (outer diameter) and 0.13 inner diameter. Then only the inductor resonates in the 88-108 band FM frequency. The length, inner diameter, number of turns etc are the important parameters to be considered while making the inductor. The inductor used in the circuit is a hand made coil using 22 SWG (Standard Wire Gauge) enameled copper wire. (Flat side on which the number is printed). Pin assignment of 2N 2222 A is 1 Emitter – 2 Base – 3 Collector (EBC) from the front side So maximum power output should be around 125 milli watt. Over powering of 2N 2222A can generate heat and destroy the device. It has maximum power rating of 0.5 Watts. The given values are necessary for the 2N 2222A transistor.ĢN 2222A is the common NPN transmitter used in general purpose amplifications. The voltage divider R1 and R2 limits the base current of T1 and R3 forms the emitter current limiter. Variable resistor VR1 restricts the current through the MIC. A capacitor between 4 and 10 PF is necessary. Presence of the capacitor C3 prevents this decay. When the tank circuit vibrates for long time, the frequency decays due to heating. As long as the current exists across the inductor coil L1 and the Trimmer capacitor, the tank circuit (Coil-Trimmer) will vibrate at the resonant frequency. Capacitor C3 across the transistor T1 keeps the tank circuit vibrating. Capacitor C2 (0.01) act as the decoupling capacitor. The 4.7 uF capacitor isolates the microphone from the base voltage of the transistor and only allows alternating current (AC) signals to pass.Ī large value capacitor induces bass (low frequencies) while a low value one gives treble (high frequencies). C1 modulates the current to the base of transistor. Sleeving the MIC inside plastic tubing can increase its sensitivity enormously.Ĭ1 is the first decoupling capacitor impedes the different frequencies of speech signals. The condenser MIC should be directly soldered on the PCB to get maximum sensitivity. Variable resistor VR1 adjusts the current through the MIC and thus determines the sensitivity of MIC. The diaphragm inside the MIC vibrates according to the air pressure changes and generates AC signals. The condenser MIC is used to pick up the sound signals. The design details of each component are as follows. The circuit diagram shown below is that of a Single transistor FM transmitter with a range of 30-50 feets and 100 – 125 milli watt output. Coil diameter, length, number of turns and gauge of the wire used for coil winding. Transmitter frequency, output power and range of transmission.ģ. The important parameters for the optimum performance of an FM transmitter are :ġ. Length of the Antenna used to transmit the frequency. Even a slight change in the coil specification or slight change in the variable capacitor value can shift the harmonic frequency instead of the 88-108 MHz FM band.Ģ. Tuning of the FM transmitter to the desired frequency. The performance of an FM transmitter depends on two important aspects.ġ. Get a Long range ( 100 Meters) FM Transmitter circuit in this link The Note given here explains how a simple FM transmitter is designed and assembled. Designing and assembling an FM transmitter is a difficult task. Usually 87.5 – 108.0 MHz is used to transmit and receive the FM signals. Throughout the world, the broadcast band falls within the VHF part of the radio spectrum. FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech. In Telecommunications, Frequency Modulation (FM) conveys information over a carrier wave by varying its frequency.
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