FM RADIO RECIEVER

This simple FM radio receiver circuit consists of a regenerative rf stage, TR1, followed by a two of three-stage audio amplifier, TR2 to TR4. In some areas 3 stages of audio amplification may not be necessary, in which case TR3 and its associated components can be omitted and the free end of capacitor C5 connected to the collector of TR2.

Circuit Diagram

The critical part of the fm radio receiver is the first stage, TR1/VC1, where the wirings must be kept as short as possible. Coil L1 is formed by winding 8 turns of 1mm (20 swg) enamelled copper wire on a 6 mm diameter former, which is then removed. After that L1 should be stretched carefully and evenly to a length of about 13mm.

Transistors List

TR1 = BF199

TR2 = TR3 = TR4 = BC547

The tunning capacitor VC1 is one of the two fm sections of a miniature fm transistor radio with built-in trimmers (VC2). The “earthy” end (moving vanes and spindle) is connected to the 22pF capacitor C1. The value of the rf choke L2 is not critical, anything from 1µH to 10µH being suitable.

The output is suitable for ordinary earphones connected in series to provide an impedance of 64Ω.

Tuning in the FM radio receiver

To operate the radio receiver, potentiometer VR1 must first be advanced slowly (towards the end of the track connected to battery positive) until, at about the half-way point, a sudden slight increase in background noise will be heard, indicating the onset of oscillation. It then should be backed off, very slowly, until oscillation just stops; it then should be possible to tune in some stations.

The correct frequency range of 87 MHz to 108 MHz can be obtained by adjusting VC2 at the high frequency (108 MHz) and slightly stretching or squeezing together the turns of coil L1 at the end (87 MHz).

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LONG RANGE FM TRANSMITTER ELECTRONIC DIAGRAM

LONG RANGE FM TRANSMITTER ELECTRONIC DIAGRAM

This circuit works optimally by adding RF amplifier and antenna. Here is the schematic diagram :

Parts list :

•     Diode D1 : BB109
•     Resistor R1 : 10k ohm
•     Resistor R2 : 100k ohm
•     Resistor R3 : 180k ohm
•     Resistor R4 : 4K7
•     Resistor R5 : 15k ohm
•     Resistor R6 : 68 ohm
•     Resistor R7 : 470 ohm
•     Resistor R8 : 39k ohm
•     Resistor R9 : 10 ohm
•     VR1 : 47k ohm
•     VR2 : 22 ohm
•     Capacitor C1-C3, C8 : 0.1 uF
•     Capacitor C4 : 4.7 pF
•     Capacitor C6 : 0.01 uF
•     Capacitor C7 : 5.6 nF
•     Capacitor C9 : 100 pF
•     Transistor T1: BF494
•     Transistor T2:2N3866
•     Trimmer VC1-VC2 : 50p
•     L1 : 4 round 20 cables SWG in plastic with 8mm diameter
•     L2 : 2 round 24 cables SWG
•     L3 : 7 round 24 cables SWG in plactic with 4mm diameter
•     L4 : 7 round 24 cables SWG in ferrid bead

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One Transistor FM Radio Receiver Circuit

Here’s simple FM receiver circuit for a simple superregenerative FM radio. It is sensitive, selective, and has enough audio drive for an earphone. These designs generally have low component counts, however the design or my construction have been far from simple.

FM Receiver Schematic

FM Radio Receiver Circuit Layout
Because this is a superregenerative design, component layout can be very important. The tuning capacitor, C3, has three leads. Only the outer two leads are used; the middle lead of C3 is not connected. Arrange L1 fairly close to C3, but keep it away from where your hand will be. If your hand is too close to L1 while you tune the radio, it will make tuning very difficult.

Winding L1
L1 sets the frequency of the radio, acts as the antenna, and is the primary adjustment for super-regeneration. Although it has many important jobs, it is easy to construct. Get any cylindrical object that is just under 1/2 inch (13 mm) in diameter. I used a thick pencil from my son’s grade school class, but a magic marker or large drill bit work just fine. #20 bare solid wire works the best, but any wire that holds its shape will do. Wind 6 turns tightly, side-by-side, on the cylinder, then slip the wire off. Spread the windings apart from each other so the whole coil is just under an inch (2.5 cm) long. Find the midpoint and solder a small wire for C2 there. Mount the ends of the wire on your circuit board keeping some clearance between the coil and the circuit board.

A tuning knob for C3

C3 does not come with a knob and I have not found a source. A knob is important to keep your hand away from the capacitor and coil when you tune in stations. The solution is to use a #4 nylon screw. Twist the nylon screw into the threads of the C3 tuning handle. The #4 screw is the wrong thread pitch and will jam (bind) in the threads. This is what you want to happen. Tighten the screw just enough so it stays put as you tune the capacitor. The resulting arrangement works quite well.

FM Radio Receiver Circuit Adjustment
If the radio is wired correctly, there are three possible things you can hear when you turn it on: 1) a radio station, 2) a rushing noise, 3) a squeal, and 4) nothing. If you got a radio station, you are in good shape. Use another FM radio to see where you are on the FM band. You can change the tuning range of C3 by squeezing L1 or change C1. If you hear a rushing noise, you will probably be able to tune in a station.

Try the tuning control and see what you get. If you hear a squeal or hear nothing, then the circuit is oscillating too little or too much. Try spreading or compressing L1. Double check your connections. If you don’t make any progress, then you need to change R4. Replace R4 with a 20K or larger potentiometer (up to 50K). A trimmer potentiometer is best. Adjust R4 until you can reliably tune in stations. Once the circuit is working, you can remove the potentiometer, measure its value, and replace it with a fixed resistor. Some people might want to build the set from the start with a trimmer potentiometer in place (e.g., Mouser 569-72PM-25K).

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BA1404 HI FI Stereo FM Transmitter Kit circuit

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Be "On Air" with your own radio station! BA1404 based transmitter is an exciting product that will broadcast high quality stereo signal in 88MHz - 108MHz FM band. It can be connected to any type of stereo audio source such as iPod, Computer, Laptop, CD Player, Walkman, Television, Satellite Receiver, Tape Deck or other stereo system to transmit stereo sound with excellent clarity throughout your home, office, yard or camp ground. Add our 500mW FM / VHF Transmitter Amplifier / Booster for even longer range.

The circuit is based on the popular BA1404 stereo broadcaster IC which contains all the complex circuitry for generating the stereo FM signal. 38 KHz crystal provides a rock solid stability of sub-carrier for stereo signal. PCB includes green solder mask layer for easier soldering and protects wires that do not need soldering.

Kit Components
1x Printed Circuit Board [5 cm x 3.2 cm]
1x BA1404 Stereo FM Transmitter IC
1x 38KHz Crystal Oscillator
1x 3.5T Variable Precision RF Coil
1x 10uH Inductor
4x 10uF/35V Capacitor
4x 1nF Ceramic Capacitor
2x 1nF Mylar Capacitor
1x 220pF Ceramic Capacitor
5x 10pF Ceramic Capacitor
2x 47K Resistor
2x 27K Resistor
1x 150K Resistor
1x 5.6K Resistor
1x 270 Resistor
1x Instructions
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80 MHz 108 MHz FM transmitter circuit

FM transmitter or often called fm transmitter uses 2 transistors in this article uses 2 transistors 2n2222. If the fm transmitter is in use voltage supply of 9 volt battery and use an antenna whose length is less than 12 inches, then this fm transmitter will be within FCC limits.

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SMD FM Transmitter

Let’s construct a low-power FM transmitter using surface-mount devices (SMD) that will be received with a standard FM radio. Soldering surface mounted devices is not so hard and actually is quite easy. There are many designs for small FM transmitters but they have some problems. First, you need an audio amplifier to get enough modulation. Second, the antenna is attached directly to the collector.

Third, the coil L must be wound by hand and adjusted by stretching. It all ads with a weak signal that tends to drift in frequency. In contrastm the transmitter schematic we present here eliminates some of those problems, using varactor diode for tuning and modulation, givind great sensitivity without an audio amplifier.

FM Transmitter – How it works
The figure below shows the schematic of the transmitter which consists of two stages: an oscillator and an output amplifier. Modulation is from an electret microphone but you can use a low power audio source.

Oscillator stage
Transistor Q1 is a Colpitts oscillator where the frequency is determined by the parallel resonant schema formed by inductor L, varactor V1 and capacitors C7 and C8. Q1 is a common-collector amplifier where the power gain counts. V1 is actually a dual varactor that eliminate the possibility of forward conduction at the sinewave peaks.

The frequency of oscillation is set by adjusting the DC voltage on V1 with potentiometer R2. R4 and C3 form a low-pass filter to prevent RF from feeding back onto the DC.
Capacitors C7 and C8 form an AC voltage divider to provide feedback at the emitter of Q1 to sustain oscillation. A necessary condition for oscillation to start is for the radio (C7+C8)/C7 to be sufficiently bigger than 1.

SMD transmitter schema schematic

Frequency Modulation
Modulation is done by superimposing an audio signal from the electret mic onto the DC bias applied to V1. R3 and C1 form a low-pass filter to prevent RF from feeding back to the mic. R3, R4 and R2 form a votage divider for the audio.

Transmitter output stage
The output of the oscillator is fed through C9 to the Q2 emitter-follower. The output of Q2 drives the antenna through C11. The Q2 emitter-follower it ensures that the oscillator is not loaded down by the impedance of the antenna and it provides power gain to drive the antenna.

SMD Transmitter layout
The figure below shows the layout of the PCB and it uses surface-mounted devices like resistors and capacitors (non-polar devices). All the caps are size 0805 and all resistors are size 1206. use through-hole components for Q1, Q2, IC1 and V1. You can use an SOT-89 device for IC1 and an SOT-23 device for V1. Use MPSH10 or a transistor equivalent. Here you can learn how to solder smd chips

The inductor
A coil would consist of two or three turns of wire but for this schematic we will use an inductor with loops of copper on the PCB. Such flat spiral inductor are common at these frequencies.
One formula for flat spiral inductors is:
flat spiral inductors formula where
L = inductance in uH
r = radius of coil (outer radius + inner radius divided by 2 ) inches
N = number of turns
d = depth of coil (outer radius minus inner radius) inches

Tuning range
While commercial FM band goes from about 88 MHz to 108 MHz, the L and C values used in this design allow tuning up to 100 MHz.

Transmitter testing
You will need a portable FM radio and an assistant. First, find an empty spot on the FM dial and set your radio about 30 feet away (9 meters). The radio’s volume control should not be set too high to prevend feedback. Next, power-up your transmitter and talk to yourself as you adjust the frequency with the trim-pot. When your assintant hears you, your transmitter is tuned. You might have to adjust the radio’s tuner slightly for best reception.

Have fun with it but remember that using the transmitter as a bugging device may not be legal in your country. To use the schema as a wireless microphone, increase the value of R3. The transmitter range is about 100 feet (30 meters) inside a building.
Parts list

 

Source by :www.diagramproject.com

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