Simple FM Receiver

 

 

Frequency modulation is used in radio broadcast in the 88-108MHz VHF band. This bandwidth range is marked as FM on the band scales of radio receivers, and the devices that are able to receive such signals are called FM receivers. The FM radio transmitter has a 200kHz wide channel. The maximum audio frequency transmitted in FM is 15 kHz as compared to 4.5 kHz in AM. This allows much larger range of frequencies to be transferred in FM and thus the quality of FM transmission is significantly higher than of AM transmission.

Here’s a simple FM receiver with minimum components for local FM reception. Transistor BF495 (T2), together with a 10k resistor (R1), coil L, 22pF variable capacitor (VC), and internal capacitances of transistor BF494 (T1), comprises the Colpitts oscillator. The resonance frequency of this oscillator is set by trimmer VC to the frequency of the transmitting station that we wish to listen. That is, it has to be tuned between 88 and 108 MHz. The information signal used in the transmitter to perform the modulation is extracted on resistor R1 and fed to the audio amplifier over a 220nF coupling capacitor (C1).

You should be able to change the capacitance of the variable capacitor from a couple of picofarads to about 20 pF. So, a 22pF trimmer is a good choice to be used as VC in the circuit. It is readily available in the market. If you are using some other capacitor that has a larger capacitance and are unable to receive the full FM bandwidth (88-108 MHz), try changing the value of VC. Its capacitance is to be determined experimentally.

The self-supporting coil L has four turns of 22 SWG enamelled copper wire, with air core having 4mm internal diameter. It can be constructed on any cylindrical object, such as pencil or pen, having a diameter of 4 mm. When the required number of turns of the coil has reached, the coil is taken off the cylinder and stretched a little so that the turns don’t touch each other.

Capacitors C3 (100nF) and C10 (100µF, 25V), together with R3 (1k), comprise a band-pass filter for very low frequencies, which is used to separate the low-frequency signal from the high-frequency signal in the receiver.You can use the telescopic antenna of any unused device. A good reception can also be obtained with a piece of isolated copper wire about 60 cm long. The optimum length of copper wire can be found experimentally.

The performance of this tiny receiver depends on several factors such as quality and turns of coil L, aerial type, and distance from FM transmitter. IC LM386 is an audio power amplifier designed for use in low-voltage consumer applications. It provides 1 to 2 watts, which is enough to drive any small-size speaker. The 22k volume control (VR) is a logarithmic potentiometer that is connected to pin 3 and the amplified output is obtained at pin 5 of IC LM386. The receiver can be operated off a 6V-9V battery.

Crystal AM Transmitter

 

 

Here is the circuit of a medium-power AM transmitter that delivers 100-150 mW of radio frequency (RF) power. At the heart of the circuit is a crystal oscillator. A 10MHz crystal is used to generate highly stable carrier frequency. Audio signal from the condenser mic is amplified by the amplifier built around transistors T1, T2 and T3. The amplified audio signal modulates the RF carrier generated by the crystal oscillator built around transistor T4. Here modulation is done via the power supply line. The amplitude-modulated (AM) signal is obtained at the collector of oscillator transistor T4.

Fig. 1: Circuit of crystal AM transmitter

Fig. 2: Oscillator coil

Fig. 3: Modulation transformer

By using matching dipole antenna and co-axial cable, the range of signal transmission can be increased. For maximum range, use a sensitive radio with external wire antenna. The circuit works off a 9V-12V battery. For oscillator coil L1, wind 14 turns of 30SWG wire round an 8mm diameter radio oscillator coil former with a ferrite bead (see Fig. 2). For modulation transformer X1, you can use the audio output transformer of your old transistor radio set. Alternatively, you can make it from E/I section transformer lamination with inner winding having 40 turns of 26SWG wire and the outer winding having 200 turns of 30SWG as shown in Fig 3.

Low-Range AM Radio Transmitter

 

 

Here is a simple radio transmitter for transmission up to 25 metres. It is basically an AM modulator whose signal can be received on the normal AM radio. It can also be used as an AM radio tester.IC 555 (IC1) is used as a free running multivibrator whose frequency is set above 540 kHz. Here the circuit is designed for a frequency of around 600 kHz. The frequency of the multivibrator can be calculated as follows:
f=1.443(R1+2R2)C1

where resistors R1 and R2 are in ohms, capacitor C1 is in microfarads, and frequency f is in hertz. This frequency can be changed by simply replacing R2 with a variable resistor or C1 with gang capacitors. But it may increase the complexity of the circuit. A condenser microphone is used for speaking.

The IC 555 multivibrator is used as a voltage-to-frequency converter. The output of the condenser microphone is given to pin 5 of IC1, which converts the input voltage or voice signal into its appropriate frequency at output pin 3. This frequency produces an electromagnetic wave, which can be detected by a nearby radio receiver, and you can hear your own voice in that radio. Note that the receiver should be AM type. If there is no noise in receiver, tune it to 600 kHz.The circuit operates off a 9V regulated power supply or a 9V battery. For antenna, connect 2-3m long wire at pin 3.

CRYSTAL AM TRANSMITTER

 

 

The LM386 is a common integrated circuit that acts as an audio amplifier.  These little things can actually make some pretty big sound.  I've used them in a few projects, like my Arduino doorbell.

Pin diagram for the LM386 IC

The most important pins are 3 (your audio input), 6 (your voltage input), and 5 (your audio output).  Pins 2 and 4 go to ground.  A capacitor between 1 and 8 will increase your audio gain (a.k.a. louder volume).  Pin 7 with a capacitor can be used to clean up the audio output.  Checkout the datasheet for more information.

The transmitter circuit feeds the signal output of the LM386 into the crystal oscillator to modulate the RF signal.  A few capacitors and a resistor are added to the LM386 as per its datasheet specs and to clean up the audio signal.

If you don't have all the parts for the lm386 portion of the circuit, don't worry.  The only part here that really matters is the capacitor on pin 3, which helps increase the audio quality.  The capacitor and resistor on pin 5 don't seem to make much of a difference, but I left them in because they are part of the standard lm386 build out from the data sheet.  The capacitor on pin 6 where the voltage is input, smooths power fluctuations, but with a 9v battery input you don't need to worry about this too much.

After you power up your circuit and plug it into an audio source (like your phone), tune your radio to 1000 kHz and take a listen.  If all went well you should hear music playing, and it should sound a lot better than the circuit in part 2.  If the audio is distorted, try lowering the input volume on your phone / mp3 player.  The distortion is overmodulation.