separationīetween the radio and FM transmitter is that the FM transmitter emits The reason that there must be at least 10 ft. So tuning takes a little patience but is not difficult. So it is best to adjust it in steps of 5 to 10 degrees at each The normal FM band tunes in over about one tenth of the full range of the tuning cap. One full turn of the trim cap will cover its full range ofĬapacitance from 6pF to 45pF. If you have difficulty finding the transmittingįrequency then have a second person tune up and down Remove the screwdriverįrom the trim screw after every adjustment so the LCĬircuit is not affected by stray capicitance. Winding should be touching another winding. Spread the winding of the coilĪpart by approximately 1mm from each other. The radio to somewhere about 89 - 90 MHz. Place the transmitter about 10 feet from a FM radio. (Look up Hartley and Colpitts oscillators in a Note that the tank circuit does not oscillate just by havingĪ DC potential put across it. Simply store energy to be returned to the system later.) The purely reactive elements, the C and the L Network only the resistive elements will dissipate electricalĮnergy. (one with no resistance) energy cannot be lost. The name 'tank' circuit comes from the ability of the LCĬircuit to store energy for oscillations. The 27pF couplingĬapacitor on the aerial is to minimise the effect of the aerial This causes the emitter-collectorĬurrent to vary at the same frequency. The feedback signal makes theīase-emitter current of the transistor vary at the resonantįrequency. (LC) circuit, the BC547 and the feedback 5pF capacitor are Generate the radio Frequency (RF) carrier waves. The tank (LC) circuit: every Tx needs an oscillator to Only allows alternating current (AC) signals to pass. Microphone from the base voltage of the transistor and Microphones are small, have excellent sensitivity, a wideįirst amplification stage: this is a standard self-biasingĬommon emitter amplifier. TheĮlectret capacitor is connected to an FET amplifier. Movement of the plate changes the capacitance. Of this material is used as part of the dielectric of aĬapacitor in which the diaphram of the microphone forms Placing it in a magnetic field then allowing it to cool while It is made by heating a ceramic material, The electret microphone: an electret is a permanentlyĬharged dielectric. Let us look at the individual blocks of the circuit The tank circuit isĬonnected in a Colpitts oscillatorcircuit. JunctionĬapacitance is a function of the potential differenceĪpplied to the base of the transistor. The junction capacitance of the transistor. The tank circuit (the 5 turn coil and the trimcap) by varying Transistor where it modulates the resonant frequency of Outputįrom the collector is fed into the base of the second Audio picked up andĪmplified by the electret microphone is fed into the audioĪmplifier stage built around the first transistor. The circuit is basically a radio frequency (RF) oscillator These lengths are 150 cm and 75 cm respectively. Varying L of the LC circuit rather than C.) Adding andĬonnect a half or quarter wavelength antenna (length of Simply adjusting the spacing of the coils. Trimcapacitor - say 47pF - and to vary the Tx frequency by It is quite possible, but not asĬonvenient, to use a fixed value capacitor in place of the Spacing in not critical since tuning of FM transmitter will be doneīy the trim capacitor. The wire is tinned copper wire, 0.61 mm in diameter.Īfter the coil in soldered into place spread the coils apartĪbout 0.5 to 1mm so that they are not touching. The coil should be about 3mm in diameter and 5 Rail (that is, to the ground or zero voltage side of theĬircuit). Pin connected to the metal case connected to the negative That the electret microphone should be inserted with the Components may be added to the PCB in any order.
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