In this project, let’s learn how to make a simple “Long range FM transmitter” at low cost using minimal components. As you can see in the diagram, the circuit is simple, easy to build and test. Let’s begin to build our long range FM transmitter.
How To Build a Long Range FMÂ Transmitter
The use of transmitters which have a more powerful output than theÂ â€˜flea-powerâ€™ are sometimes required when there are many obstacles in the path of theÂ surveillance transmitter and monitoring station receiver, or theÂ distance between them is too far so as to make a low powered deviceÂ feasible. Whereas a typical micro transmitter will produce an RFÂ power in the order of just a few milliwatts, i.e. a few thousandths of aÂ watt, the VHF-FM transmitter described has a powerÂ output of between around a half and 2 watts, depending on the powerÂ source, which may be anywhere between 6 volts and 30 volts d.c. TheÂ battery or batteries should be of the alkaline high power type, sinceÂ the current drain will be found to be relatively higher when comparedÂ to microtransmitter current drain. The power output of this device isÂ somewhat proportional to the current drain and so therefore bothÂ may be decreased by altering the value of R6 to a higher resistance, orÂ a variable resistor with a value of around 1k may be introduced inÂ series with the existing R6, so as to give a variable power output. TheÂ variable resistormust not be a wirewound device because this wouldÂ act as an inductor which will cause feedback problems.The audio input to the power oscillator, which incidentally isÂ formed by TR2 and associated components, is derived from aÂ piezoelectric microphone which drives the simple audio frequencyÂ amplifier TR1. The input of the audio amplifier is controlled by theÂ gain pot R1, which selects the correct amount of voltage that isÂ generated by the piezoelectric microphone, then connects this signalÂ to the base of audio amplifier TR1 via C7.Â It may be found that there is insufficient housing space for a bulkyÂ piezoelectric microphone, so with a slight modification to the circuit,Â it is possible to employ an electret microphone insert as shown.Â Since the RF field that is generated by this transmitter is relativelyÂ large, the problem of RF feedback may very well be encountered. ThisÂ may be overcome by placing the transmitter inside a metal enclosure,Â keeping all internal wiring as short as possible and the aerial wire.
Component listing for 1 watt transmitter
Resistors Semiconductors R1 = 27k TR1 = BC547 R2 = 330k TR2 = 2N2219 fitted with heat sink R3 = 5k6 MIC = piezoelectric microphone R4, 5 = 10k R6 = 100R
L = 6 turns 22 gauge enameled wire wound on 3â„16â€³ former
C1, 2, 3 = 330 pF C4 = 2â€“10 pF trimmer C5 = 4p7 C6 = 1 nF
C7,C8 = 40uF/25V Electrolytic
Circuit diagram for Long range FM Transmitter
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