FM transmitters are used to reduce noise in certain places.FM transmitter circuits are used in wireless sound systems in vehicles and offices.Then, the modulated signal is transmitted or radiated as radio waves at a frequency in the FM frequency range. This circuit is a natural extension from the crystal radio and one-transistor radio circuits shown in the Radio section of this site. The main purpose of the variable capacitor is to change the resonance frequency to obtain the best FM signal band. This two transistor AM radio receiver uses the cheap and commonly available BC549B transistors to demodulate and amplify an AM radio signal. The audio signal obtained from the microphone passes through the base of the transistor to modulate the output signal of the LC circuit in the form of FM. The transmitter circuit stores energy for oscillation as it is obtained from LC circuits. When current is passed through L1 Inductor and the variable capacitor, the FM Transmitter Circuit will start oscillating with the resonant frequency of the carrier signal. The oscillation circuit consists of the transistor (2N3904), inductor, and variable capacitor. The oscillator is required to generate carrier waves in the FM transmitter. On the circuit, the trimmer capacitor To control the frequency from 87MHz to 108 MHz. Both circuits is a very small size and cheap. The air pressure around the mic produces such capacitance. We have 2 circuit are: using 6 transistors and IC. The microphone is assumed to catch the sound signals and there is the presence of a sensor with a capacitance value inside it. For what it's worth, here's a little 6-transistor HX108-2 that I built.The circuit diagram is shown for the FM transmitter. The former is to adjust the dial calibration at the top end, the latter to optimise the sensitivity. RI is a simple but very effective input attenuator that serves as an RF GAIN control. This allows one end of Ll to be grounded, permitting the use of a five-pin plug-in coil form for 1.1, 1.2 and 1.3. Likewise, at the high frequency end, the article doesn't really clarify the functions of the oscillator trimmer (CB, associated with the T2) and the RF trimmer (CA, associated with the ferrite rod). In this circuit, the RF stage uses a PNP transistor for Q 1. The oscillator coil (T2 in this set, and almost inevitably red) is for adjusting the dial calibration at the LF end, but doesn't affect the sensitivity much (if at all). For optimum sensitivity, it's necessary to adjust the position of the coil on the ferrite rod - the easiest way is to tune to a weak signal near the lower end of the band, slide the coil for best results and secure with a little wax or hot-melt glue. There is nothing unusual with this design. The general rule - "adjust coils at the lower frequency end, and capacitors at the top" - applies here, as usual. I designed and made a PCB, and constructed a small aluminium case to keep the receiver as comact as possible. This can be done by ear on a weak, stable radio station - there's not really any need to use test equipment. So here's a few comments: All the kits I've had featured pre-aligned IF transformers: there really wasn't any need to adjust them except perhaps a tiny tweak once you've done everything else. I've not built this particular model, but I've made several other Chinese 6 / 7 transistor kits - all of which (once you've deciphered the instructions!) have worked very well. Thank you for the patience when reading this work. We repeat this operation until the radio catches both frequencies at both tuning wheel end positions. We rotate the coil T2 core until we have the best quality audio signal. After that we turn the variable capacitor at the lowest frequency and apply with the signal generator an AM signal with frequency 535 kHz. We rotate the small capacitors placed in the variable capacitor until we start to hear the audio signal. We rotate the tuning capacitor to the end pointing that frequency. First we apply an AM frequency corresponding to the highest input frequency (1605 kHz). It is successive approximation and must be performed few times. After that the coil T4 was tuned applying signal at the base of VT2. The coil T5 was tuned in the way to hear the best and strongest audio signal from the speaker. The FM transistor consists of the oscillator to produce the RF signal. The signal was injected first at the base of VT3. The FM signal is obtained by differing the carrier frequency by allowing the AF. ![]() The tuning I started again from the back end in direction the front end. I set to AM with frequency 455 kHz and low signal level. For precise tuning I used a RF signal generator described here. It is better for that purpose to use some nonmagnetic screwdriver. The tuning is performed by rotating the ferromagnetic coil cores.
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