![]() If you want the microphone to be less sensitive, you can replace the R1 by a higher resistor, try 10k or 22k, and this might overcome the feedback problem if you use this wireless microphone FM transmitter for a public address system. The antenna use 1/16 wave length to compromize between the efficiency and the size. ![]() The core of this FM transmitter circuit is Q2, a modified Collpits oscillator that the frequency is determined by L1, C4, C6, and the transistor’s internal base-emitter capacitance. The first transistor (Q1) is the pre-amplifier for the microphone, and you can ommit this circuit if you don’t want to transmit the sound picked up by the mic, for example you can can connect your mp3 player directly to C1. Here is the schematic diagram of the circuit: This wireless microphone is very sensitive, pick up every sound in the 20m radius, and transmit the radio signal up to 2 kilometers in open air. With a matching antenna, the FM transmitter circuit shown here can transmit signals up to a range of 2 kilo meters. ![]() This is as shown below.Please be warned if operating this circuit might violate the regulation of your country, because this FM transmitter circuit radiate strong radio frequency to the environment. In the calculator we enter the desired RF frequency which is 88MHz and also enter the value of the two capacitors which is 47pF and 5pF. For this we use the Oscillator Online Calculator and select the Colpitts oscillator. No license is required for this transmitter according to FCC regulations regarding wireless microphones. To calculate this number of turns required for given coil diameter first we need to calculate the inductor inductance required value for given frequency and two capacitor values. This circuit is a simple two transistor (2N2222) FM transmitter. The coil inductor is made of 10 turns with 4mm diameter. Here the value of the L1, C4 and C3 are such that the oscillator oscillates at 88MHz. The value of L1, C4 and C3 are chosen such that it oscillates in the FM frequency range which is 86 to 106MHz. The oscillator circuit is made up the second BC547 transistor configured with inductor L1, C4 and C3 in colpitts oscillator configuration. The amplified signal is then coupled into the oscillator circuit using the electrolytic capacitor C2. ![]() The pre-amplifier amplifies the small audio signal of around 10-15mV amplitude from the microphone to higher amplitude. The tutorial Testing of self biased BJT amplifier on breadboard with PC oscilloscope shows how to design and test self biased amplifier. Figure 1 is a schematic diagram This circuit use the varicap diode number: SD111 or will use number 1S553, 1S85 instead. But this circuit is special that we use a varicap diode. This circuit is a 1.5V FM transistor circuit that use also two transistors like others circuit before. The first transistor Q1, the BC547 transistor, which is biased using self biasing method with resistors R2 and R3 is the pre-amplifier. 1.5V FM transmitter using a varicap diode and two transistors. The audio enters the pre-amplifier via the coupling capacitor C1. Audio enters the transmitter through this microphone. R1 is required to power up the microphone because the electret microphone has a FET(Field Effect Transistor) inside it which requires power. In the above circuit diagram, MIC1 is the electret condenser microphone.
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