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HomeElectronic Circuits3V FM Transmitter

3V FM Transmitter39 Comments

3V FM Transmitter Circuit

This project provides the schematic and the parts list needed to construct a 3V FM Transmitter. This FM

transmitter is about the simplest and most basic transmitter to build and have a useful transmitting range. It is

surprisingly powerful despite its small component count and 3V operating voltage. It will easily penetrate over

three floors of an apartment building and go over 300 meters in the open air.

It may be tuned anywhere in the FM band. Or it may be tuned outside the commercial M band for greater

privacy. (Of course this means you must modify your FM radio to be able to receive the transmission or have abroad-band FM receiver.) The output power of this FM transmitter is below the legal limits of many countries

(eg, USA and Australia). However, some countries may ban ALL wireless transmissions without a licence. It

is the responsibility of the constructor to check the legal requirements for the operation of this kit and to obey

them.

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PARTS LIST

R1 22K

R2 10K

R3 47K

R4 1M

R5 470

C1

22n (223)

ceramic

C2 100n monoblock

C3 1n (102) ceramic

C4 5.6p ceramic

C5 27p ceramic

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C6 22n (223)

VC1 Trim Cap 6-45 pF

Q1 BC547

Q1 BC547

L1 5 turn coil

MIC

Electret

microphone

AERIALAerial Wire165cm

BT1

3-5V (2 AA

Battery holder)

FM Transmitter Circuit Description

The circuit is basically a radio frequency (RF) oscillator that operates around 100 MHz. Audio picked up and

amplified by the electret microphone is fed into the audio amplifier stage built around the first transistor.

Output from the collector is fed into the base of the second transistor where it modulates the resonant

frequency of the tank circuit (the 5 turn coil and the trimcap) by varying the junction capacitance of the

transistor. Junction capacitance is a function of the potential difference applied to the base of the transistor.

The tank circuit is connected in a Colpitts oscillator circuit.

The electret microphone: an electret is a permanently charged dielectric. It is made by heating a ceramic

material, placing it in a magnetic field then allowing it to cool while still in the magnetic field. It is the

electrostatic equivalent of a permanent magnet. In the electret microphone a slice of this material is used as

part of the dielectric of a capacitor in which the diaphram of the microphone formsone plate. Sound pressure

moves one of its plates. The movement of the plate changes the capacitance. The electret capacitor isconnected to an FET amplifier. These microphones are small, have excellent sensitivity, a wide frequency

response and a very low cost.

First amplification stage: this is a standard self-biasing common emitter amplifier. The 22nF capacitor

isolates the microphone from the base voltage of the transistor and only allows alternating current (AC) signals

to pass.

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The tank (LC) circuit: every FM transmitter needs an oscillator to generate the radio Frequency (RF) carrier

waves. The tank (LC) circuit, the BC547 and the feedback 5pF capacitor are the oscillator in the Cadre. An

input signal is not needed to sustain the oscillation. The feedback signal makes the base-emitter current of the

transistor vary at the resonant frequency. This causes the emitter-collector current to vary at the same

frequency. This signal fed to the aerial and radiated as radio waves. The 27pF coupling capacitor on the aerial

is to minimise the effect of the aerial capacitance on the LC circuit. The name tank circuit comes from the

ability of the LC circuit to store energy for oscillations. In a pure LC circuit (one with no resistance) energy

cannot be lost. (In an AC network only the resistive elements will dissipate electrical energy. The purely

reactive elements, the C and the L simply store energy to be returned to the system later.) Note that the tank

circuit does not oscillate just by having a DC potential put across it. Positive feedback must be provided. (Look

up Hartley and Colpitts oscillators in a reference book for more details.)

BC547 NPN Transistor Pin Configuration And Top View

ASSEMBLY INSTRUCTION

Components may be added to the PCB in any order. Note that the electret microphone should be inserted with

the pin connected to the metal case connected to the negative rail (that is, to the ground or zero voltage side of

the circuit). The coil should be about 3mm in diameter and 5 turns. The wire is tinned copper wire, 0.61 mm in

diameter. After the coil in soldered into place spread the coils apart about 0.5 to 1mm so that they are not

touching. (The spacing in not critical since tuning of the Tx will be done by the trim capacitor. It is quite

possible, but not as convenient, to use a fixed value capacitor in place of the trimcapacitorsay 47pFand tovary the Tx frequency by simply adjusting the spacing of the coils. That is by varying L of the LC circuit

rather than C.) Adding and removing the batteries acts as a switch.Connect a half or quarter wavelength

antenna (length of wire) to the aerial point. At an FM frequency of 100 MHz these lengths are 150 cm and 75

cm respectively.

CIRCUIT CALIBRATION

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Place the transmitter about 10 feet from a FM radio. Set the radio to somewhere about 8990 MHz. Walk

back to the FM transmitter and turn it on. Spread the winding of the coil apart by approximately 1mm from

each other. No coil winding should be touching another winding. Use a small screw driver to tune the trim cap.

Remove the screwdriver from the trim screw after every adjustment so the LC circuit is not affected by stray

capicitance. Or use a plastic screwdriver. If you have difficulty finding the transmitting frequency then have a

second person tune up and down the FM dial after every adjustment. One full turn of the trim cap will cover its

full range of capacitance from 6pF to 45pF. The normal FM band tunes in over about one tenth of the full

range of the tuning cap.

So it is best to adjust it in steps of 5 to 10 degrees at each turn. So tuning takes a little patience but is not

difficult. The reason that there must be at least 10 ft. separation between the radio and the FM transmitter is

that the FM transmitter emits harmonics; it does not only emit on one frequency but on several different

frequencies close to each other. You should have little difficulty in finding the Tx frequency when you follow

this procedure.

LEARNING EXPERIENCE

It should already be clear from the above circuit description that there is a surprising amount of electronics

which may be learnt from this deceptively simple kit. Here is a list of some advanced topics in electronics

which can be demonstrated or have their beginnings in this project:

Class C amplifiers; FM transmission; VHF antennas; positive and negative feedback; stray capacitance;

crystal-locked oscillators; signal attenuation The simple halfwave antenna used in the project is not the most

efficient. Greater efficiency may be gained by connecting a dipole antenna using 50 ohm coaxial cable.Connect one lead to the Antenna point and the other to the earth line.

You may experiment using 6V or 9V with the circuit to see how this increases the range of the transmitter. The

sensitivity may be increased by lowering the 22K resistor to 10K. Try it and see. Note that this FM transmitter

is not suitable for use on your body, for example, in your pocket. This is because it is affected by external

capacitance and the transmitting frequency drifts depending how close you are to it. Stray capacitance is

automatically incorporated into the capacitance of the tank circuit which will shift the transmitting frequency.

Please send your ideas, which are very important for our success

Aboutadmin

Ravindra, Hi...

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Zener Tester

MPLAB Integrated Development Environment

Tags:RF Schematics,Transmitter Circuits39 comments on 3V FM Transmitter

Newer comments

1. theguymathsays:May 23, 2010 at 1:59 pm

what is C6

Reply

o Ravindrasays:May 24, 2010 at 6:09 am

C6 = 22nF (223)

thanks

Reply

2. mahnooshsays:May 20, 2010 at 2:41 am

i need to shematic of negative ion generator . would u help me?

Reply

3. seetharamansays:May 1, 2010 at 2:55 pm

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Hi to avoid antenna damping the oscillator shift capacitor C5 to a tap at 1 turn from the supply end of the tank

coil L1 (I have used a 75cms length whip antenna. For 3Volt operation reduce Condencer mic bias resistance

to 3.3K. C6 is a must use 47uF with 0.1 Ceramic disc in parallel. A good circuit. Nice quality.

Reply

4. rajusays:March 8, 2010 at 2:29 pm

thanks for new

Reply

5. Silviasays:February 17, 2010 at 11:01 pm

HLQVOH I am always excited to visit this blog in the evenings.Please churning hold the contents. It is very

entertaining.

Reply

6. Ben Kamwendosays:November 18, 2009 at 7:44 pm

Hello,

I wanna thaks for the good works, I want a transmitter circuit lower than 20cm range.

Thanks.

Reply

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7. makhnovskayasays:November 18, 2009 at 9:24 am

Dear Authorhttp://www.electronicecircuits.com!

What charming message

Reply

8. Polpravsays:October 15, 2009 at 2:48 am

Hello from Russia)

Reply

9. lilikindslisays:October 5, 2009 at 1:24 am

5dFRIO I want to saythank you for this!

Reply

10. Davidesays:October 3, 2009 at 11:33 pm

needed here

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