FM Radio circuits

Hi guys,

         As u all know, the circuit we are going to build this week and next is an FM radio. I will be posting here possible circuits which I believe will work without any glitches. This time around, all of u will have to solder the components onto a PCB. We will be guiding u on how to play with soldering irons..:p. Everyone plz submit the FM radio on Sept 1st, which is next Thursday. I have chosen this date coz of many upcoming holidays. So make use of this forum very much to clear all ur doubts regarding practical electronics. We will find expert answers on all the doubts.

SMALL FM RADIO CIRCUIT

Perhaps this is one of the simplest and smallest FM radio receiver that can receive the FM stations available locally. Its simple design makes it ideal for a pocket sized FM receiver. The output of the receiver drives a head phone.

The circuit works off a small 4.5 volt battery or two 3.6 volt Lithium button cells.
The fm receiver section has two RF transistors T1 and T2 to detect the Frequency Modulated signals. Coil L1 and the trimmer capacito form the tuned tank circuit to tune the receiver to the best FM station with strong signals. The signals are capacitor coupled through C2.

10K preset VR controls the volume to the input of the amplifier. IC1 is the micro power audio amplifier that works between 4.5 to 12 volts DC. The amplified sound can be heard through the low impedance head phone or small Mylar speakers.

FM radio coil

Coil is the important part of the FM tuner. It is made up of 18 SWG enameled copper wire. Wind 4 turns on a ball pen to get 0.5 cm inner diameter. Remove the enamel from the tip of the wire and solder tightly in the PCB. Trimmer and coil should be soldered very closely. Adjust the spacing of the coil winding if necessary to get the station clearly. Assemble all the receiver components as close as possible to get better result.

Pinout of BF494 and LM386

FM RADIO RECEIVER CIRCUIT - a cOMPLEX CIRCUIT

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.

Transistors List
TR1 = BF199
TR2 = TR3 = TR4 = BC547

       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.

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 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).

 So, these are two circuits which I found to be practically possible. If anyone else have any other suggestions, plz leave a comment or SMS me. 

                     Good luck people...