![]() ![]() 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. A large value capacitor induces bass (low frequencies) while a low value one gives treble (high frequencies). The 4.7 uF capacitor isolates the microphone from the base voltage of the transistor and only allows alternating current (AC) signals to pass. 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.Ĭoil Specification - 5 Turn 22 SWG Enameled Copper WireĬoil Length 0.25 Inch - Diameter 0.265 Inch Coil diameter, length, number of turns and gauge of the wire used for coil winding. Transmitter frequency, output power and range of transmission. 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. Design Considerations The performance of an FM transmitter depends on two important aspects.ġ. 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. When connected, the received signals can be distinctly heard over the headphones with this little “audio marvel”Plugging in the headphone initiates the circuit and the circuit starts operating with its functions and the switches OFF itself when the headphone is removed from the circuit.In telecommunications, frequency modulation (FM) conveys information over a carrier wave by varying its frequency. Headphone Becomes the Load and the SwitchĪ 64 Ohm headphone becomes the collector load of the transistor, where the demodulated and amplified signal is applied. The coupling capacitor at the base of the transistor makes sure only the radio information passes to the transistor while the DC component from the supply is appropriately blocked. ![]() The concentrated but very low in power signal from the above LC tuned stage is fed to the base of the transistor which as performs the function of a demodulator as well as a amplifier stage. The coil is tuned using a GANG condenser or a variable capacitor which is connected in parallel to the antenna coil.The coil and the GANG together form a resonant tank circuit, which lock on to the received or the resonant frequency at a particular setting. The figure below shows how the single transistor radio can be made, we can clearly see that it just involves a single transistor as the main active component.A regular type of MW antenna coil has been used for collecting or sensing the MW receptions. However simplicity will always involve some drawbacks also, here the present design will be capable of receiving only strong stations and also selectivity might not be very pleasing, typically if there are a couple of strong stations mingling around the band. The circuit of a single transistor radio shown here though looks pretty ordinary, yet incorporates all the above stages and becomes just suitable for receiving the nearby radio stations. What are the fundamental criteria associated with radio reception? An antenna stage, a band selector stage, a demodulator stage and a receiving element.When all of these come together radio reception becomes as simple as a piece of cake. ![]()
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