FM Transmitter (88 MHz - 108 MHz)

This transmitter picks up the sounds around him with his microphone, and sends them on the FM band (88 MHz - 108 MHz). This transmitter is obviously mono, but is very easy. It can serve as a wireless microphone (the same principle used by those sold commercially), such as micro spy (note the right to privacy!) If you are a tad biologist, it can serve you for "spying" the songs of birds without disturbing them by your presence, and, of course, train yourself to become a radio host.

Normally, such devices are prohibited in France as its maximum range is a few hundred meters, so if you do not shuffle a broadcast, its use will be tolerated.

Its implementation will cost between 20 and 100 francs, according to the components you have to buy.

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1.The electronic diagram

This scheme, some of you probably know in broad terms and is in fact widely used, either in the "KIT" or in magazines. This variant uses a single variable capacitor. The quality of editing is not great, but largely only in the intended applications, the assembly still benefiting from the superb sound quality that allow transmission on FM









The assembly works as follows: T1 is responsible to amplify the sound from the microphone; T2 makes the carrier whose frequency is set by the oscillator "common base Colpitts" (L1, C5, C6) - C6 being adjustable, so we can adjust the transmission frequency - and modulates the signal amplitude. This signal is then applied to the antenna that propagates in the form of electromagnetic waves.

Note: The carrier frequency is:

2.La list of components

R1 = 10K, Brown, Black, Orange
R2 = 4.7 K, Yellow, Purple, Red
R3 = 10K, Brown, Black, Orange
R4 = 10K, Brown, Black, Orange
R5 = 100 Ohms, Brown, Black, Brown
R6 = 220K, Red, Red, Yellow
C1 = 4.7 uF - 25 ... 63 V - Implementation Radial
C2 = 470 pf - Model type ceramic
C3 = 4.7 uF - 25 ... 63 V - Implementation Radial
C4 = 470 pf - Model type ceramic
C5 = 2.2 pf - Model type ceramic
C6: Capacitor adjustable, 22 or 25 mp
C7 = 10 nf - Model type ceramic
T1: BC 548 B, C or BC 548 or BC 549 B BC 549 C or
T2: 2 N 2222 A
P1: Potentiometer adjustable carbon, 22K, implementing horizontal
L1: 1μH - Reel 'home' made ​​with enameled wire 1 mm in diameter, see text
Microphone: Microphone elec, 2 son

I will tell you how to make L1:
Get a cylinder 6 mm in diameter, such as a pen or a Shank. On this cylinder, coil 4.5 turns of enameled wire indicated in the nomenclature. Remember that the coil must have legs to be welded, so do not cut the wire too tight! Then remove the cylinder and then gently stretch the coil so that the resulting total length is about 13mm. The wire used can not be welded directly, you must remove the enamel on the legs with a piece of abrasive cloth (paper used in sanding). The coil is ready!

Note: Because the coil is not coupled with another, the sense with which you wound has absolutely no importance.

3.Schéma realization of veroboard

The assembly has many closely spaced solder, so I advise you to get a soldering iron to endow a very fine point.
Of course, we will first remove the copper pellets colored in green, with a drill about 5 mm in diameter.

Then proceed with the installation of five straps, not to forget them and the rest of the components.
We solder the transistors last, these components being the most fragile, they are the ones who "love" the least heat of the soldering iron.
Finally, weld the coupler 9 V battery (red wire is the greater, the black at least), then the microphone, taking care to its polarity.

I do not recommend the deportation of the components (to put on the front, for example), because we work at high frequency, and any connection too long will act as a significant roll and this can prevent the assembly work. Thus, it is best to solder the capacitors as close to the piece of veroboard (leaving them only 1 or 2 mm in length of legs in surplus).

The installation consumes little, so the 9 V battery is used because of its small size, allowing you to stay around the assembly. If you want to feed the assembly by the industry, we need the power supply is pretty well filtered, the assembly consuming little, take a miniature transformer, and do not put it too close to the plate.

For the antenna, take a simple piece of wire. The best results are obtained with a wire 74cm long, in fact, this corresponds to the wavelength of the FM band This is of almost anything to take a longer wire, but it may be useful to link the mass assembly (the least the battery / power supply) to the ground. Of course you can, and I advise you use a telescopic antenna.
But if a range of a few tens of meters for you, you can use the assembly without antenna or ground connection.

4.Essais and settings

Before the tests that follow, carefully inspect each card to verify that no short circuit between two tracks and no track is cut. Also check that you have soldered the components in the right place in the right direction.

Connect a 9 V battery assembly. Turn the cursor in the middle of P1 (P1 set to adjust the microphone sensitivity). Position the mounting near an FM radio. So, try to find the frequency with which issues the bracket for this, ask an accomplice to whistle next to the microphone (not to shout into the microphone used is very sensitive). If you can not, can be mounting it transmits on a frequency already used by a radio, and then sends it outside the band 88 ... 108 MHz.

Indeed, the elements of the assembly are planned to cover extensively the FM band. (By the way, if you add turns to the L1, you shift the emission band down to 88MHz above, if you would remove the turns, you stagger up, above 108Mhz). Then you simply turn the cursor C6 with a screwdriver to adjust the frequency.

Note: C6 is very sensitive, the simple act of touch (with the screwdriver, for example) changes the transmission frequency, so do not start looking for the new frequency only after removing the screwdriver.

If you hear a hissing sound violent, is the famous acoustic feedback, the sound from the loudspeaker of the radio coming on mic, the result is a loop that results in this unpleasant noise.
The transmission frequency of the assembly is not stabilized, so during the first 10 minutes after the start, this frequency will vary slightly.

One final note: If you lock your installation in a metal case, connect it to the mass of the issuer (at least the battery / power supply), so it forces you to connect an antenna to your assembly, antenna that is located outside the metal case, it is obvious.