Wiring Diagram circuit

The photo shows a wireless FM transmitter, pocket radio and yellow disk
for size comparisons. Speak into the transmitter and others hear you
on any FM radio. The transmitter can be built in an afternoon with
simple, affordable and widely available parts. Construction is fun and
much can be learned although performance is modest; for example, your
voice gets difficult to hear at distances greater than 25 feet.

Motivation and Audience

Our
long-term goals with such circuits are in remote control and data
acquisition. Touch tones could be transmitted to wirelessly turn on/off
a robots actuators. Sensor data could be converted into morse-code
like sounds and deciphered by a microcontroller to wirelessly monitor
environments. This tutorials audience would be also motivated by such
applications. Building a simple FM transmitter would be a first-step
towards such goals.
Not to give you false expectations, this FM
transmitter is far from perfect offering only modest performance.
First, tuning the transmitter can be frustrating. Even slight turns in
the variable capacitor can result in large frequency changes. Second,
transmitter tuning often resulted in a harmonic frequency. Instead of
the intended 108 MHz for example, capacitor tuning yielded a 216 MHz
transmitter frequency. In addition to hearing your voice one could
slightly hear radio station broadcasts.

One
answer is that much can be learned and this tutorial is is appendixed
with the underlying mathematics to calculate parameters like (1)
tranmitter frequency, power output and range (2) antenna length and (3)
required coil winding. Often on the web, one just finds a schematic.
By adding the analysis (with high school level math), one can conceive
improvements on transmitter performance.
This tutorials audience is thus electronics enthusiasts who:

• dont have much FM or radio-frequency (RF) experience
• want to build an FM transmitter
• want to learn how such transmitters work
• want to learn how components are selected e.g. calculating the number of turns when building an air coil, antenna length etc.

Again
this tutorial emphasizes that the transmitters performance is modest,
but is learned in its construction. The tutorial breakdown is as
follows:

• Parts List and Sources
• Construction/Schematic
• Theory
• Operation
• Where To Go From Here
• Author Information

Parts List and Sources

US-based
vendors include Jameco, Digikey, JDR and Radio Shack. Note: Boondog
has no association with these vendors. Attempts were acquire all parts
from a single vendor. Part numbers for common resistors are not given.

An
effort was made to find a single source supplier of all parts. Jameco
has almost every part cited in the tables. Details construction your
air core inductor using a McDonalds soda straw will be described in the
next section.

Construction

A combination of
wirewrapping and soldering was used to construct the FM transmitter.
Jamecos prototyping card provides enough room for (non-critical) part
placement. You should try to keep all parts close together and keep
wire leads short. The photos below illustrate possible part placement
(left) and the solder side (right).

Schematic

fmTx031402a.pdf is the Acrobat file of the same schematic. You will need Adobes free Acrobat reader to view it.

The schematic and constructing the circuit are relatively
straight-forward. Some highlights and clarifications towards circuit
construction are given next.

Electret Microphone

An
electret microphone has two pins which connect to the positive and
negative leads of a battery. As shown in the drawing below, one looks
at the bottom of the electret microphone. The pad that physically
touches the microphones casing connects to the batterys negative lead.

Batteries

You
can replace the coin cells, typically found in calculators and
watches, with regular 1.5V AA, C or D-cells. The coin cells however
take less room and can solder onto the protoboard.

2N2222A Transistor

The
2N2222A is a very common NPN transistor. The one used here (Jameco
#38236) is the metal can type (TO-18 casing). Its three pins are for
the transistors base (B), collector (C) and emitter (E). There is no
standard pinout for transistors. As such, request the transistors spec
sheet when ordering it to identify the pinout, or if you own a
multimeter with a transistor tester, use it.
The 2N2222A also
comes in a black plastic casing (TO-92 style) which you can use if you
want. The T0-18 is preferred because the can has a small tab that typically represents the emitter pin.

Make sure you correctly identify the 2N2222As pinout and correctly
wire the base, collector and emitter in the schematic. Often, circuit
malfunctions because the pins were mis-wired.

Variable Capacitor

The
leads for the variable capacitor do not fit in normal 0.1 inch
protoboards. You can dremel-drill into the protoboard to make the leads
fit. Alternatively you can solder wire to the leads, but if you do,
keep wires as short as possible in order to avoid stray capacitances.

Inductor

An
inductor is just a coil of wire and you need to wind one for this
circuit. An inductor is characterized by its length, radius and the
number of turns of wire in the coil. Magnet wire (Radio Shack part
278-1345) was used to build the inductor but you can use standard solid
strand 22 AWG gauge copper wire.
Some on-line and printed
articles describe winding the wire around a pencil. Unfortunately,
pencils come in different diameters and hence a McDonalds soda straw
was used; the yellow-red-white striped straw, found in every McDonalds
in the world, is the same size. The straws radius is exactly 0.1325
inches (diameter = 0.2650 inches) and 1/4 inches was snipped off the
straw.

Next,
a straight piece wire was wound around this 1/4 inch snippet six times
and then soldered on the prototyping board. The end result is an
inductor (also known as an air core coil) with an
0.1325 inch radius. If you wish, you can apply some womens clear
fingernail polish to permanently keep the wire on the straw snippet.

Antenna

A
30 inch long piece of 22 gauge solid stramd copper wire is a suitable
antenna. However when carrying the transmitter, you risk tangling the
wire. As such you can screw a telescopic antenna, like ones found a
radios, into the prototyping board.

Theory: How does the FM Transmitter Work?

The
variable capacitor and your self-made inductor will vibrate at
frequencies in the FM radio band (88 to 108 MHz). The electret
microphone has a resistance that depends on how loudly you speak into
it. This microphone is battery powered and according to the V=IR
Ohms Law, changes in resistance for fixed voltage will result in
proportional changes in current. This current feeds into the base of the
2N2222 NPN transistor which is connected to your variable capacitor,
inductor and antenna. The net effect is that depending on your variable
capacitors value, your voice will be modulated to transmit at a
frequency between 88 and 108 MHz. If a nearby pocket FM radio is tuned
to this frequency, youll be heard when speaking into your transmitter.

The component values in the circuit are derived to better
understand how this FM transmitter will work. The underlying math is
rather simple and can be found in most undergraduate university physics
textbooks.

Inductance of an Air Core Coil

Your self-made inductor has a value determined by its radius r, length x and number of wire turns n.

For your McDonalds soda straw inductor, r = 0.1325 inches, x = 0.25 inches and n = 6 turns and results in L = 0.171 microHenry or 0.000000171 Henry.
The specific frequency f generated is now determined by the capacitance C and inductance L measured in Farads and Henry respectively:

Resonant Frequency of a Parallel LC Circuit

FM
radio stations operate on frequencies between 88 and 108 MHz. The
variable capacitor and your self-made inductor constitute a parallel LC
circuit. It is also called a tank circuit and will vibrate at a resonant frequency which will be picked up your pocket FM radio.

In tank circuits, the underlying physics is that a capacitor
stores electrical energy in the electric field between its plates and an
inductor stores energy in the magnetic field induced by the coil
winding. The mechanical equivalent is the energy balance in a flywheel;
angular momentum (kinetic energy) is balanced by the spring (potential
energy). Another example is a pendulum where theres a kinetic versus
potential energy balance that dictates the period (or frequency) of
oscillations.
Given your variable capacitor ranges from 4 to 34
pF, your tank circuit will resonant between 66 and 192 MHz, well within
the FM radio range. To compute these values for different values of C, n, r and x
a simple Excel spreadsheet, called calcFreq.xls was created. Simply
enter the values and the inductance and frequency are automatically
calculated.

Antenna Length

You built your
antenna either with a piece of solid strand 22 gauge wire 30 inches
long or used a telescopically extendable antenna. Its length should be
approximately 1/4 the FM wavelength; recall that multiplying frequency
and wavelength equals the speed of light. Youll most probably be
operating your transmitter near 108 MHz, as such:

Fixed Capacitors

Referring to the schematic, C2 and C4 act as decoupling capacitors and typically 0.01 uF (or 0.1 uF) are used. C4 attempts to maintain a constant voltage across the entire circuit despite voltage fluctuations as the battery dies.

A capacitor can be thought of as a frequency-dependent resistor
(called reactance). Speech consists of different frequencies and the
capacitor C1 impedes them. The net effect is that C1 modulates the current going into the transistor. Using a large value for C1 reinforces bass (low frequencies) while smaller values boost treble (high frequencies).
The C3
capacitor across the 2N2222A transistor serves to keep the tank
circuit vibrating. In theory, as long as there is a supply voltage
across the parallel inductor and variable capacitor, it should vibrate
at the resonant frequency indefinetely. In reality however, the
frequency decays due to heating losses. C3 is used to prevent decay and the 2N2222A spec sheet suggests a capacitance between 4 to 10 pF.

Resistor for Electret Mic

The spec sheet for the Jameco #136573 electret microphone says the maximum current is 0.5 mA. When battery powered at 6V, then the voltage drop across R1 is V1 = 1.92V. The resulting current through the microphone is below the rated maximum since

Resistors and the 2N2222A

The 2N2222A transistor has rated maximums thus demanding a voltage divider made with R2 and R3 and emitter current limiting with R4.
The 2N2222As maximum rated power is Pmax = 0.5 W.
This power ultimately affects the distance you can transmit.
Overpowering the transistor will heat and destroy it. To avoid this,
one can calculate that the FM transmitter outputs approximately 124 mW and is well below the rated maximum. The mathematical details are given in rfMath.pdf.
The power is intimately related to the transmission range. At 124 mW
and 30% radiation efficiencies, the maximum distance between your FM
transmitter and a battery-powered radio will range betweem 35 to 112
feet. The calculations are given in rfDistance.pdf.

Operation

First, use a battery-powered pocket radio as a receiver. AC powered boom-boxes and home stereos (110 or 220 V) are not recommended; battery-powered radios are much better at receiving transmissions than AC-powered units.

1. Tune your radio to dead air, i.e. frequencies within the FM radio band that are silent or only have some hiss. Frequencies near 108 MHz
   are typically dead air. The Radio-Locator web page lists local radio
   stations in your area. This can help you identify dead air frequencies.
2. Turn
   on your FM transmitter, extend its antenna and keep the transmitter
   approximately 2 feet away from your FM radio. Speak into the mic while slowly
   adjusting the variable cap. Use your fingernail or non-metallic
   screwdriver until you hear yourself over the radio. This process is
   frustratingly tedious, requiring careful capacitor tuning. You are tuned
   once you hear howling (also known as a hot mic) which indicates transmitter-receiver feedback.
3. Increase the transmitter-to-radio distance. Congratulations - you have a wireless microphone!

Where To Go From Here

As
stated earlier, performance is modest. The authors experience
operating in a major city (Philadelphia, USA) with the battery-powered
radio tuned at 108 MHz yielded approximately 25 feet indoors and 50 feet
outdoors. Also, in addition to the authors voice, radio station
broadcasts could be slightly heard.
To tweak performance, a
spectrum analyzer can be used. Its a device that visually displays
frequencies are most predominant. The author discovered the circuit was
transmitting at approximately 200 to 220 MHz, rather than the desired
108 MHz! 216 MHz is a harmonic, being twice the desired 108 MHz.
Transmission range is thus reduced and susceptible to noice (radio
station broadcasts).
To transmit at the desired 108 MHz, the author considered the following:

• A hand-made coil thats 0.1 uH is difficult to test. Inductance meters to measure at the micro Henry range are expensive.
• Capacitance meters measuring picoFarads are also expensive.
• A 0.111 uH air core coil was purchased from Coilcraft, an inductor manufacturer.
• Spectrum
  analyzers costs thousands of dollars. A viable alternative is an $130
  USD Elenco F-2800 frequency counter. This is a nice unit to acquire if
  you plan on building FM transmitters more seriously.

For the
most part, the frequency counter displayed approximated 200 MHz even
with the Coilcraft inductor! Thus most probably the variable capacitor
is not truly giving a 4-to-34 pF range. Since the transmission
frequency stayed closed to 200 MHz, calcFreq.xls reveal that variable
capacitor actually stays close 4 pF rather than going up to the rated
34 pF. This should be expected since tolerances in capacitance are
rarely precise. The net effect is that without picoFarad resolution
capacitance youll be transmitting at a 216 MHz harmonic yielding
reduced range and susceptible to noise.

Final Words

This
tutorial along with appendixes detail fully a single transistor FM
transmitter construction and underlying math. The circuit can be built
in an afternoon with less than $10 USD of common parts, resulting in a
25 to 50 foot transmission range.
Like the author, readers might
be excited about the prospects of building FM transmitters. Many
circuit designs and schematics exist on-line and in print but dont
often provide much analysis. This tutorial attempts to fill this gap,
especially for first-time FM transmitter builders. The analysis allows
one to learn what roles and their values play in the circuit. Such
analysis provides a reader a stepping point towards improving or
customizing the circuit.
Illustrating the math and real-world
operation is the tutorials value. Some material towards learning more
might be acquired from the references below. Happy building!