This document describes the design and operation of a simple wireless FM microphone transmitter. The transmitter uses an electret microphone, two transistors, and a tuned coil to modulate an FM carrier signal between 87-108 MHz. The audio signal from the microphone is amplified and used to vary the resonant frequency of the tuned coil, transmitting the audio over FM radio frequencies. The transmitted signal can be received on any standard FM radio within a range of over 300 meters. The wireless microphone allows for freedom of movement and avoids issues with cables. Potential applications include entertainment, broadcasting, and public speaking.

1. Wireless FM Microphone
EFFORTS BY:
DEEPESH
KHANEJA

2. Introduction
This wireless FM microphone is simple to build and it has a useful
transmitting range (over 300 meters in the open air).
 Despite its small component count and a 9V operating voltage it
will easily penetrate over three floors of an apartment building.
 It may be tuned anywhere in the FM band (87-108MHz) and its
transmissions can be picked up on any standard FM receiver.
 It can be used as a simple cordless microphone.
 No license is required for this transmitter according FCC
regulation regarding wireless microphones

3. Project Description
Wireless FM transmitter is a device
that will enable audio playback
through any FM stereo.
Device will transmit any audio
output to any FM stereo located
within the operating range.
Allows the user to listen to music
through higher quality speakers vs
standard computer speakers.

4. General Diagram
3.5mm Stereo Cable FM Transmission
MP3 Audio MP3 Tx Receiver
Source
Music
User
(Doing the worm)

5. Block Diagram
DC Power
Pre-emphasis MPX Modulator FM Modulator
Audio Input
Buffer Amp Carrier Osc
Freq. Multiplier
RF Amp
Antenna Receiver

6. WIRELESS TECHNOLOGY
The concept of having no wires connecting devices to each other is appealing
for several reasons:
 Wires can be complicated and untidy, having no wires is pleasing.
 Wireless operations permits services, such as long range communications,
that are impossible or impractical to implement with the use of wires.
 The term is commonly used in the telecommunications industry to refer to
telecommunications systems (e.g. radio transmitters and receivers,
computer networks, ) which use some form of energy (e.g. radio frequency
(RF) etc.) to transfer information without the use of wires.

7. Principles of Wireless Radio
Communication
The simplest scheme of wireless communication would be convert
the speech or music to be transmitted to electric signals using a
microphone, boost up the power of the signal using amplifiers and
radiate the signal in space with the air of an antenna. This would
constitute the transmitter using modulator. At the receiver end, one
could have a pick-up antenna feeding the speech or music signal to
an amplifier and then to a loud speaker using demodulator. (below
figure)

8. Specifications
Variable frequency range: 88 – 108 MHz
Supply Voltage: 9V DC adapter
Weight: Less than 1 lbs.
Transmission Range: 30 m (unobstructed)

9. CIRCUIT DIAGRAM

10. Parts Used
le
T1,T2,: 2N2222 transistor L1: 3mm in diameter with 5 turns 0.61
R1,R6:1k R2: 15k . mm copper wire
R5,R7: 4.7k R3: 6.8k Electret Microphone
C1,C3: 10uF R4: 10k:
C4,C7: .1pF R8:2.2k
C5:5-60 pf R9:220 ohms
C2:2.2uf C6:8.2pf

11. Circuit Description
The circuit is basically a radio frequency (RF) oscillator that operates
around 100 MHz.
 Audio picked up & 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
transistor.
 Junction capacitance is a function of the potential difference applied
to the base of the transistor.

12. ELECTRET MICROPHONE
An electret is a permanently charged dielectric.
 Ceramic material is used as part of the dielectric of a capacitor in
which the diaphram of the microphone forms one plate.
Sound pressure moves one of its plates. The movement of the plate
changes the capacitance.
The electret capacitor is connected to an FET amplifier.
 These microphones are small, have excellent sensitivity, a wide
frequency response and a very low cost.

13. RESONANT FREQUENCY

14. ASSEMBLY INSTRUCTION
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.
.After the coil is soldered into place spread the coils apart about 0.5 to 1mm so that
they don't touch.
 Tx frequency is adjusted by varying trim capacitor.
 Connect a half or quarter wavelength antenna (length of wire) to the aerial point..

15. CIRCUIT CALIBRATION
Place the transmitter about 10 feet from a FM radio. Set the radio to
somewhere about 89 - 90 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 screw driver from the
trim screw after every adjustment so the LC circuit is not affected by
stray capacitance.

16. APPLICATIONS
Wireless FM microphones are widely used in the
entertainment industry,
television broadcasting,
and public speaking to allow public speakers,
interviewers, performers, and entertainers to move about freely
while using a microphone to amplify their voices.

17. Types of Modulation
(Methods to generate the carrier frequency)
AM FM PCM
Simple Better Range (Pulse-Coded
Least Less Modulation)
expensive Interference Greater
Susceptible to Popular Freq. Range
interference Range Not widely
used
More costly

18. ADVANTAGES &
DISADVANTAGES
The advantages are:
Greater freedom of movement for the artist or speaker.
Avoidance of cabling problems common with wired microphones
Reduction of cable "trip hazards" in the performance space
This wireless FM transmitter to connect wirelessly to your car or home stereo, portable
music player quickly and easily
The disadvantages are:
Sometimes limited range Some wireless systems have a shorter range, while more
expensive models can exceed that distance.
Possible interference with or, more often, from other radio equipment or other radio
microphones
Operation time is limited relative to battery life; it is shorter than a normal condenser
mic
Noise or dead spots (places where it doesn't work, especially in non-diversity systems)

19. Initial Prototype
Top View

20. Initial Prototype
Bottom View

21. Complications
Weak signal of the receiver input
Variable inductor difficult to tune
Several components very sensitive to external
interference
Excess noise on the transmitted signal

22. Solution to FM Noise
Current Solutions
Re-adjust the variable inductor
Re-adjust the variable resistor
Check proper voltage input
Additional Solution
Mount the components on a PCB

23. THANK YOU