1. BATTERY AND BATTERY CLIP
The battery is made up of a carbon rod core with zinc or similar body and a
chemical (electrolyte) which causes a chemical reaction which in turn allows
a current to flow. On the battery there is normally a clip or set of buttons which
prevent incorrect connection. (Batteries are not supplied with MTN-SUNSTEP
kits). The battery clip for MTN-SUNSTEP is for a 9V PP3 battery and has a red
or positive hook-up wire and a black or negative hook-up wire.
© 2003 SUNSTEP, Stellenbosch University
All rights reserved + -
SYMBOL
RESISTORS
Resistance is measured in ohms. Resistors are used to limit current. The lower
the resistance the more current can flow and the higher the resistance the less
current can flow. Resistors come in many colours, shapes and sizes, but the most
common are quarter or half watt resistors which are manufactured from high
resistance wire or consist of a small ceramic tube onto which a small spiral shape
layer of carbon is applied. The thickness and length of the layer and the fineness
of the carbon particles, determine the resistance. Resistors are also manufactured
in different power ratings for different electronic applications (see table below to
determine the specification of a resistor from the colour coding from its body.)
SYMBOLS
HOOK- UP WIRE
Hook-up wire, although not strictly an electronic component is nevertheless
used in most electronic circuits. Two basic types of hook-up wire are used.
For LINKS, tinned copper wire, saved from the excess component legs which
are cut off, can be used e.g. resistor legs. INSULATED HOOK-UP WIRE is
stranded wire with plastic insulation where a small piece of insulation is removed
on either side to connect components such as loudspeakers to the circuit board.
HOOK- UP WIRE YOU WILL NEED FOR THIS KIT
One 75 cm length of insulated wire for the antenna
Two 1 cm lengths of wire to connect the microphone to the p.c. board.
Five ± 1.5 cm lengths of insulated wire to form the inductor around the toroid.
TABLE OF RESISTOR COLOUR CODES
COLOUR VALUE THE BANDS OF A RESISTOR
BLACK
BROWN
RED
ORANGE
YELLOW
GREEN
BLUE
VIOLET
GREY
WHITE
Band One - 1st figure of value
Band Two - 2nd figure of value
Band three - Number of zeros/multiplier
Band four - Tolerance (±%)
Tolerance of colours: Red 2%; Gold 5%;
Silver 10%;No band 20%
HOW TO READ THE COLOUR BANDS OF A RESISTOR
To read the value of a resistor, start with the band of colour furthest away from the gold or silver (tolerance) band. According to the table, if this first band is orange, then the first number would be 3. If the second band is red, then the number would be 2. If the third band was yellow, this would determine the amount of zeros,( in this case 0000). If the fourth band (tolerance band) is silver, the tolerance is 10% and if it is gold, it is 5% therefore this resistor’s value would be 320,000ohms or 320k. There is however a small catch. The colour black is equal to nought (or zero) and depending on where this band occurs, it can be a little confusing. If the band furthest from the tolerance band is brown and the number is one, if the second band is black, the number is 0 and if the third band is red, this would determine the amount of zeros, except in this case the previous band was equivalent to a zero, therefore the value would be 1000ohms or 1k 
SYMBOLS
TOROID
A toroid is an element used in electronic circuits to form an inductor. A toroid
is a doughnut-looking ring made of powdered iron. These are manufactured
in different sizes according to the specification of the inductor required.
Toroid types are identified by colour and size. You’ll find toriods used in power
supplies for computers and other sensitive electronic equipment and in the
case of this kit we use the toroid to form an inductor. An inductor is a number of turns
or wire wound in air or around a metal core. When a DC current flows through
an inductor it looks like a short circuit wire with no resistance, but when alternating
current AC flows through an inductor, it behaves differently. Once the electrons start
flowing, a magnetic field is created around the inductor.
This magnetic field in turn generates a current in an opposite direction to that which is
applied to the inductor. This causes a sort of resistance to flow ( of alternating current )
that is called reactance. It is proportional to frequency. The magnetic field in the inductor
represents energy that is stored and together with a capacitor, which is a storage device,
a resonant circuit is formed.
© 2003 SUNSTEP, Stellenbosch University
All rights reserved
TRANSISTORS
Transistors are available in many shapes and sizes and manufactured for different
applications. Most transistors have 3 legs protruding from the housing although
some transistors have 2 legs where the casing serves as the third junction and some
transistors have a 4th leg where the 4th leg is a shield. Normally the 3 legs are
called base, collector and emitter. Damage could occur if transistors are fitted to
the pc board incorrectly. Transistors can be used as amplifiers or as a very fast
switch - if a certain amount of current is fed to the base of a transistor a larger current
is made to flow out into the collector and out of the emitter. If a varying current is
applied to the base then a larger version of this current can be obtained through
the collector. This would be called an amplifier. There are 2 basic types of
transistors, these are NPN or PNP. (See symbols for PNP and NPN types)
SYMBOL
PNP
NPN
SYMBOL

2. CORDLESS MICROPHONE SUNSTEP KIT No.11
Miranda Myburgh (Programme Manager)
Tel:
Fax:
Contact person:
Wouterien Matthew (Project Developer)
Tel:
Fax:
CAPACITORS (NON POLARIZED)
A Capacitor stores an electrical charge for a limited period. Capacitance is
measured in microfarads (uF) and picafarads (pF). Polyester and ceramic
capacitors are non polarised and are used mainly in audio circuits. Polyester
range in value from a few microfarads to ,001 microfarad and ceramic range
from 0,47 microfarads down to 1 pF.
The higher the capacity of the capacitor, the more the electrical charge they
can store. Capacitors are normally marked with a capacitance value and a
voltage rating. They are available in many colours, shapes and sizes depending
on their specific characteristics.
Capacitance is always measured in Farads (this is the storage ability).
SYMBOL
CORDLESS MICROPHONE SUNSTEP KIT No.11
DESCRIPTION:
This is an intriguing as well as educational kit. When successfully constructed, it will transmit a voice signal, from a small microphone through the FM transmitter circuit. This signal can be picked up by an ordinary FM radio receiver which is tuned to the same frequency as the transmitter kit. This signal can in turn be amplified through a public address system. A multitude of possible applications can be achieved using this very interesting kit.
MICROPHONE
A microphone is a device which converts sound waves into variations of an electric
current. Microphones are constructed in many different ways, for instance moving
coil, carbon diaphragm etc. The microphone used in this kit is called an electret
microphone.This is a small circuit element that detects sound waves and turns them
into electric energy, using a capacitor element which is permanently polarised.
Because the element is such a high resistance device, a transistor is built into the
case. This transistor requires power, and that is the reason for the voltage supplied
through a resistor to the microphone, The microphone has a connection from one of
the terminals to the case. This is the negative terminal and should be connected to
the terminal on the p.c. board marked negative.
COMPONENTS:
RESISTORS CAPACITORS TRANSISTORS
R1 - 6k8 OHMS C1,C2,C3 & C6 - 0,1MF POLYESTER Q1 - BC547B
R K OHMS C4 - 2n2 CERAMIC Q2 - BC547B
R K OHMS C5 - 1nF CERAMIC R4 - 1K 5 OHMS C7 - 15p 5%CERAMIC TOROID
R K OHMS C8 &C9 - 4p7 5% CERAMIC T50-10 BLACK
R6 - 47K OHMS
R OHMS MICROPHONE BATTERY CLIP
ELECTRET MICROPHONE V FOR PP3 BATTERY
ANTENNA
Because the radio signals should be radiated, we use an antenna, this helps the electric waves to
go into the air. The equation for wavelength is as follows:
wavelength = speed of light /frequency.
Assume we are transmitting a signal at 100MHz, this means that the wavelength is almost three meters. It is good practice to make the antenna a quarter of a wavelength thus 75cm would be
the correct length.
In this transmitter kit we use a shorter antenna, as a longer antenna would be an inconvenience, but the shorter antenna does however mean that some transmitting distance is lost.
© 2003 SUNSTEP, Stellenbosch University
All rights reserved
CIRCUIT DIAGRAM:
These boards are protected by copyright, SUNSTEP

3. PC BOARD LAY OUT ( ENLARGED )
KIT CONSTRUCTION
Step One: Select all seven resistors. Sort the resistors according to their values by using the colour code system shown on this pamphlet. Now solder them into the positions indicated on the p.c. board. Resistors are not polarity sensitive, so you may mount them either way round.
Step Two: Select all nine capacitors. Sort the capacitors according to their values. These values are indicated on the bodies of the capacitors. If you are not sure, do not hesitate to ask your educator. These capacitors are not polarity sensitive, so you may mount them either way round.
Step Three: Select both the transistors. Solder them onto the p.c. board, making sure that the collector, emitter and base legs are mounted according to the positions indicated on the p.c. board.
Step Four: Select the battery clip and solder the red wire to the position marked Positive + and the black wire to the position Negative - on the p.c. board.
Step Five: Solder the 75cm length of wire to the solder side of the p.c. board marked Antenna. This antenna can be shortened, but you will loose some transmitting distance.
Step Six: Select the small black toroid ring ( for making the inductor ) and place it in the position on the p.c. board marked L1. Now solder the five short insulated wires to connect the holes to the inside and outside of the toroid, using corresponding holes for each wire. It should look like five spokes of a wheel.
Step Seven: Solder the two 1cm lengths of wire to the position on the p.c.board marked MIC. Select the microphone and solder it to these two wires making sure that the positive terminal is soldered to the contact which is not connected to the casing of the microphone.
The extending piece of p.c. board is for mounting the battery. This can be done using a rubber band.
TESTING
Before testing the circuit, make sure that all your components are in their right places. You can ask someone to check your p.c.board for you.
Switch on your transmitter by attaching the battery.
Switch on the radio.
Turn up the volume slightly so you can hear the noise (oscillation ).
Hold your transmitter in front of the radio’s speaker.
Tune the radio until you hear a loud whistling noise. This is caused by noise from the radio being picked up by the microphone and transmitted back into the radio again. Move the microphone away from the radio.
If the wireless microphone happens to transmit on top of an existing station, the frequency can be changed slightly by shifting one or more wires on the toroid. A more drastic change can be made by changing C7 to a value slightly more or less than 15pf.
The circuit is sensitive to touch, so try not to touch the solder side when you transmit, if you do touch it, you might be changing the frequency of the carrier wave and then your radio would not be tuned in any longer.
PC BOARD LAY OUT ( ENLARGED )
These boards are protected by copyright, SUNSTEP

4. “MAKE ELECTRONICS WORK FOR YOU”
HOW DOES THIS KIT WORK?
In the communications world we send information from one location to another, this information is called the signal. The information can be a voice, radio or video as in a television broadcast. In order to achieve this, the original signal is multiplied by a sinusoidal waveform called the carrier. This process is called modulation.
There are different types of modulation and these are called amplitude or frequency modulation. This transmitter kit uses frequency modulation.
Can you see that the
frequency of the signal
indicated varies ?
This signal is called
a modulated signal.
One of the components used in this transmitter kits is the capacitor C7. This capacitor together with the inductor L1, determines the transmission frequency.
The circuit is basically an oscillator, oscillating at a very high frequency (the carrier wave) in the FM range ( MHz). The modulated signal is then transmitted
carrying the voice information that was added by the signal coming form the microphone.
The frequency range that is possible for the carrier wave can be estimated. The frequency of the oscillation is the resonant frequency of the inductor and capacitor in parallel. The inductor L1 is approximately 0.18uH and C7 is 15pf. Using the formula frequency = 1/2pi[ (L1 x C1) ] we get the frequency.
To recap what we have just learnt. FM means frequency modulation, a carrier’s signal frequency is varied by the sound that the microphone detects. This signal is then received by your FM radio where it is demodulated to reproduce the original sound.
© 2003 SUNSTEP, Stellenbosch University
All rights reserved
APPPLICATIONS, USES AND THINGS TO TRY WITH THIS KIT
Try reducing the length of your antenna and find out what range you can obtain with different lengths. Using more than 75cm will probably not give you more range.
Build your transmitter into a plastic or better still, a metal tube and place some soft foam over the microphone. This will reduce wind noise and the metal tube will reduce hand proximity.
Add a switch to your transmitter by connecting a switch in series with the positive lead from the battery.
Most FM radios have an earphone connection. By connecting this with a public address system, the microphone can be used as a roving microphone at sports meetings, etc.
The power output of the transmitter is deliberately kept low so that it does not cause interference on other radios, which would make it illegal.
© 2003 SUNSTEP, Stellenbosch University
All rights reserved
SUNSTEP’S THEME FOR THE MILLENIUM
“MAKE ELECTRONICS WORK FOR YOU”
SUNSTEP AIMS TO :
Enrich learners lives through the disciplines of science and technology with an emphasis on electronics.
Help you to use electronics to work for you by applying principles in basic applications to improve the quality of your life.
Help you to make electronics pay your way through entrepreneurial projects.
Circuit by Leon Korkie and Henry Chamberlain
Manual by Henry Chamberlain and Kevin Matthew