LAYOUT AND COMPONENTS MICRO FM TRANSMITTER

The schematic diagram of the micro FM transmitter is amazingly simple as it is formed of two floors. Yet it works no problem, the results are of amazing quality. Sounds are picked up by an electret microphone. It is supplied with an RC filter cell consists of resistors R1, R2 and capacitor C2. The signals are transmitted to the amplifier transistor T1 via capacitor C5 for blocking the DC component.

Once amplified, they are taken from the collector of T1 using a capacitor (C4) to attack the base of transistor T2 which is the issuer itself. It is a transistor connected as an oscillator whose operating frequency is set by the LC network placed in its collector circuit. C9 10pF capacitor maintains the oscillations of the circuit. BF signals injected into the base of transistor modify slightly the oscillation frequency, which produces a modulation of the emission frequency. HF signals can be taken directly to the collector of the transistor with a capacitor (C8) of very low value and then be directed to the transmitting antenna. Adjusting the adjustable capacitor C7 can stall the transmit frequency on the FM broadcast.

PCB layout (to scale for the self engraved, at worst it can make one with the formula for resonance):

Board layout:

Practical realization

The practical realization of the radio microphone was reduced to its simplest expression. Indeed, the self L1 was carried out in copper track on the circuit board. Suffice it to implement the single strap to connect it to the adjustable capacitor. It will start by wiring the establishment of the strap and all resistance. We then solder the capacitors and transistors. One of them, T2, may be selected from various models. It will preferably BF199. But we found that its supply is could cause problems, we conducted tests with other models.

Thus, the 2N2222 works fine, as well as the BC547 and BC550. But the pinouts are different, it was provided on the turntable an extra hole that will orient the transistor chosen.

Settings and testing

It will set the FM receiver on the bottom of the band at around 89MHz. The transmitter is powered by a 9V battery, we operate the adjustable capacitor in order to receive the broadcast on the receiver. To do this, use a plastic screwdriver. Attention, because the setting is very sharp. After receipt obtained, we can tweak this setting using the receiver. It may be that during the first minutes of issue, the frequency drift. This is normal, the frequency of the transmitter is not stabilized by a quartz. When we conducted the tests without an antenna on the transmitter, we obtained a range of more than ten meters through walls of a room, with a battery power fairly low. A sound is heard as the microphone sensitivity is high. It should also not placing it too close to the sound source because there is a risk of saturation. The assembly is completed, it will be placed in a small box designed to hold a 9V battery. It will be necessary to provide a switch for turning on or off. A hole should be checked for the passage of the microphone.

Nomenclature of micro FM transmitter

Resistors
* R1: 4.7 kOhms (yellow, purple, red)
* R2, R4: 47 kOhms (yellow, purple, orange)
* R3: 2.2 MΩ (red, red, green)
* R5: 1 k (brown, black, red)
* R6, R7: 10 k (brown, black, orange)
* R8: 100Ω (brown, black, brown)

Capacitors
* C1: 10 μF/16V
* C2: 47 μF/16V
* C3: 1 nF
* C4, C5: 100 nF
* C6, C10: 330 pF
* C7: adjustable capacitor 3 / 30 pF
* C8: 1.5 pF

Semiconductors
* T1: BC548 or BC 547C
* T2: BF199 2N2222 or

Various
* 1 Microphone Electret 2 son
1 * 9V battery connector
* A single pole switch
* 1 plastic case