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Introduction
About the project kit
This project kit has been carefully designed for use by teachers in KS3 / KS4 design and technology.
They are designed such that even teachers with a limited knowledge of electronics should have no
trouble using it.
Using the booklet
This booklet is intended as an aid for teachers when planning and implementing their scheme of
work.
Please feel free to print any pages of this booklet to use as student handouts in conjunction with
Kitronik project kits. There are no page numbers in this booklet. This means you can feel free to pick
and choose which sheets you use whilst still retaining a feeling of continuity.
Support and resources
You can also find resources at www.kitronik.co.uk. There are component fact sheets, information on
calculating resistor and capacitor values, puzzles and much more.
Kitronik provide a next day response technical assistance service via e-mail.
If you have any questions regarding this kit or even suggestions for improvements
Soldering In Ten Steps
1. Start with the smallest components working up to
the taller components, soldering any
interconnecting wires last.
2. Place the component into the board, making
sure it goes in the right way around and the part
sits flush against the board.
3. Bend the leads slightly to secure
the part.
4. Make sure the soldering iron has warmed up and
if necessary use the damp sponge to clean the
tip.
5. Place the soldering iron on the pad.
6. Using your free hand feed the end of the solder
onto the pad (top picture).
7. Remove the solder, then the soldering iron.
8. Leave the joint to cool for a few
seconds.
9. Using a pair of cutters trim the excess
component lead (middle picture).
10. If you make a mistake heat up the join with the
soldering iron, whilst the solder is molten, place
the tip of your solder extractor by the solder and
push the button (bottom picture).
LDR (Light Dependent Resistor)
An LDR is a component that has a resistance that changes with the
light intensity that falls upon it. They have a resistance that falls with
an increase in the light intensity falling upon the device.
The resistance of an LDR may typically
have the following resistances
Daylight = 5000Ω
Dark = 20000000 Ω
You can therefore see that there is a
large variation between these figures. If
you plotted this variation on a graph you
would get something similar to that
shown by the graph to the right.
Applications
There are many applications for Light Dependent Resistors. These include.
Lighting switch
The most obvious application for an LDR is to automatically turn on a light at certain light level. An
example of this could be a street light.
Camera shutter control
LDRs can be used to control the shutter speed on a camera. The LDR would be used the measure
the light intensity and the set the camera shutter speed to the appropriate level.
Example
The circuit shown right shows a simple way of
constructing a circuit that turns on when it
goes dark. The increase in resistance of the
LDR in relation to the other resistor which is
fixed as the light intensity drops will cause the
transistor to turn on. The value of the fixed
resistor will depend on the LDR used, the
transistor used and the supply voltage.
Transistors
Functionality
A transistor in its simplest form is an electronic switch. It allows a small amount of current to switch on
or off a much larger amount of current. There are two types of transistor NPN and PNP, the different
order of the letters relate to the order of the N and P type material used to make the transistor. Both
types are available in different power ratings from signal transistors through to power transistors. The
NPN transistor is the more common of the two and the one examined in this sheet.
The transistor has three legs, these are the base, collector and the emitter. The emitter is always
connected to 0v and the electronics that is to be switch on is connected between the collector
and the positive power supply. The base of the transistor is used to switch current through the
collector and emitter. When the base is between 0V and 0.7V it is switched off and above 0.7V it is
switched on allowing the current to flow from the collector to the emitter. A resistor is normally
placed between the output of the integrated circuit (IC) and the base of the transistor to limit the
current drawn through the IC output pin.
Schematic symbol
The symbol for an NPN type transistor is shown to the right
along with the pins being labeled.
Values
Transistors don’t have values, but they do have different current ratings. The style of the package
also changes as the current rating goes up. Low current transistors come in a D shaped plastic
package, whilst the higher current transistors are produced in metal cans that can be bolted on to
heat sinks so they don’t over heat. The D shape or a tag on the metal can is used to work out which
pin does what. All transistors are wired differently so they have to be looked up in a catalogue to
find out which pin connects where.