Kitronik 2112 User manual

LIGHT ACTIVATED SWITCH
CONTROL ELECTRONIC CIRCUITS WITH THE OUTPUT OF THIS
TEACHING RESOURCES
INTRODUCTION
TECHNICAL SPECIFICATION
COMPONENT FACTSHEETS
HOW TO SOLDER GUIDE
Version 3.0

Light Activated Switch Teaching Resources
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Index of Sheets
TEACHING RESOURCES
Index of Sheets
Introduction
Technical Specification
Soldering in 8 Steps
Resistor Values
Sensing Light – Photodetectors
Using a Transistor as a Switch
Darlington Pair
ESSENTIAL INFORMATION
Build Instructions – Light Activated
Build Instructions – Dark Activated
Checking Your Circuit
Testing the PCB
How the Light Switch Works – Dark Activated
How the Light Switch Works – Light Activated
Applications
Online Information

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Introduction
About the project kit
Both the project kit and the supporting material have been carefully designed for use in KS3 Design and Technology
lessons. The project kit has been designed so that even teachers with a limited knowledge of electronics should have
no trouble using it as a basis from which they can form a scheme of work.
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.
Support and resources
You can also find additional 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, please e-mail us at:
Alternatively, phone us on 0845 8380781.
Technical Specification
Supply Voltage
Minimum = 3V
Maximum = 12V
A supply voltage of 3V to 5V allows for better adjustment.
Output voltage
Vout = Supply voltage less 0.9V
Output current
Maximum = 0.5A
Guidance note
You should ensure that you have a stable power source when using the output to switch on high output loads. This is
because if the power source is unable to provide enough power, this may result in a supply voltage dip and cause
output to switch off. At this point the voltage is likely to recover and turns the output on again. The output would
then be in a state where it is rapidly switching on and off.
Board dimensions (in mm)

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Soldering in 8 Steps
Place soldering iron tip on the pad.
Make sure the soldering iron has warmed up. If necessary use a
brass soldering iron cleaner or damp sponge to clean the tip.
Pick up the Soldering Iron in one hand, and the
solder in the other hand.
CLEAN SOLDERING IRON
2
PICKUP IRON AND SOLDER
3
HEAT PAD
4
Place the component into the board, making sure that it goes in the
correct way around, and the part sits closely against the board.
Bend the legs slightly to secure the part. Place the board so you can
access the pads with a soldering iron.
INSERT COMPONENT
1

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Feed a small amount of solder into the joint. The solder
should melt on the pad and flow around the component leg.
Remove the
solder, and then remove the soldering
iron.
Leave the joint to cool for a few seconds, then using a
pair of cutters trim the excess component lead.
APPLY SOLDER
5
STOP SOLDERING
6
TRIM EXCESS
7
REPEAT
8
Repeat this process for each solder joint required.

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Resistor Values
A resistor is a device that opposes the flow of electrical current. The bigger the value of a resistor, the more it
opposes the current flow. The value of a resistor is given in Ω (ohms) and is often referred to as its ‘resistance’.
Identifying resistor values
Band Colour 1st Band 2nd Band Multiplier x Tolerance
Silver  100 10%
Gold  10 5%
Black 0 0 1
Brown 1 1 10 1%
Red 2 2 100 2%
Orange 3 3 1000
Yellow 4 4 10,000
Green 5 5 100,000
Blue 6 6 1,000,000
Violet 7 7
Grey 8 8
White 9 9
Example: Band 1 = Red, Band 2 = Violet, Band 3 = Orange, Band 4 = Gold
The value of this resistor would be:
2 (Red) 7 (Violet) x 1,000 (Orange) = 27 x 1,000
= 27,000 with a 5% tolerance (gold)
= 27KΩ
Resistor identification task
Calculate the resistor values given by the bands shown below. The tolerance band has been ignored.
1st Band 2nd Band Multiplier x Value
Brown Black Yellow
Green Blue Brown
Brown Grey Yellow
Orange White Black
Too many zeros?
Kilo ohms and mega
ohms can be used:
1,000Ω = 1K
1,000K = 1M

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Calculating resistor markings
Calculate what the colour bands would be for the following resistor values.
Value 1st Band 2nd Band Multiplier x
180 Ω
3,900 Ω
47,000 (47K) Ω
1,000,000 (1M) Ω
What does tolerance mean?
Resistors always have a tolerance but what does this mean? It refers to the accuracy to which it has been
manufactured. For example if you were to measure the resistance of a gold tolerance resistor you can guarantee
that the value measured will be within 5% of its stated value. Tolerances are important if the accuracy of a resistors
value is critical to a design’s performance.
Preferred values
There are a number of different ranges of values for resistors. Two of the most popular are the E12 and E24. They
take into account the manufacturing tolerance and are chosen such that there is a minimum overlap between the
upper possible value of the first value in the series and the lowest possible value of the next. Hence there are fewer
values in the 10% tolerance range.
E-12 resistance tolerance (± 10%)
10
12
15
18
22
27
33
39
47
56
68
82
E-24 resistance tolerance (± 5 %)
10
11
12
13
15
16
18
20
22
24
27
30
33
36
39
43
47
51
56
62
68
75
82
91

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LDR and Phototransistor symbols are
similar, with the Phototransistor symbol
also being similar to a normal transistor
symbol.
An LDR
A Phototransistor
Sensing Light – Photodetectors
To sense light levels in electronics requires a component which is
sensitive to light. 2 types of photodetector capable of doing this are
Light Dependent Resistors (LDR) and Phototransistors.
An LDR is a component that has a resistance that falls with an increase
in the light intensity falling upon the device.
A Phototransistor is a transistor whose base is exposed to light, rather
than being wired to a pin.
As the light level increases this activates the transistor, in a similar
manner to increasing the base current of a regular transistor.
The resistance of an LDR may typically change by 4000x between
Daylight and darkness.
A Phototransistor’s gain varies with the amount of light it is exposed
to, typically from 100 to 1500
You can see that there is a large variation between these figures
depending on the light level. With appropriate circuits these changes
can be used to control other electronics.
Applications
There are many applications for photodetectors. These include:
Lightingswitch
The most obvious application is to automatically turn on a light at certain light level. An example of this could be a
street light.
Camerashuttercontrol
Photodetectors can be used to control the shutter speed on a camera. The photodetector would be used the
measure the light intensity and then 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.
Load
5v
0v
Load
5v
0v

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Using a Transistor as a Switch
Overview
A transistor in its simplest form is an electronic switch. It allows a small amount of current to switch a much larger
amount of current either on or off. There are two types of transistors: 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.
Schematic symbol
The symbol for an NPN type transistor is shown to the right along with the
labelled pins.
Operation
The transistor has three legs: the base, collector and the emitter. The emitter is usually connected to 0V and the
electronics that is to be switched on is connected between the collector and the positive power supply (Fig A). 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.
The base of the transistor is used to switch the transistor on and off. When the voltage on the base is less than 0.7V,
it is switched off. If you imagine the transistor as a push to make switch, when the voltage on the base is less than
0.7V there is not enough force to close the switch and therefore no electricity can flow through it and the load (Fig
B). When the voltage on the base is greater than 0.7V, this generates enough force to close the switch and turn it on.
Electricity can now flow through it and the load (Fig C).
Current rating
Different transistors 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 onto heat sinks so that 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 datasheet to find out which pin connects where.
IC
output
Load
5V
0V
Fig A – Basic transistor circuit
LOAD
<0.7V
Fig B – Transistor turned off
LOAD
>0.7V
Fig C – Transistor turned on
Emitter
Base
Collector

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Darlington Pair
What is a Darlington Pair?
A Darlington Pair is two transistors that act as a
single transistor but with a much higher
current gain.
What is current gain?
Transistors have a characteristic called ‘current
gain’. This is referred to as its hFE.
The amount of current that can pass through
the load when connected to a transistor that is
turned on equals the input current x the gain
of the transistor (hFE).
The current gain varies for different transistor and can be looked up in the datasheet for the device. Typically, it may
be 100. This would mean that the current available to drive the load would be 100 times larger than the input to the
transistor.
Why use a Darlington Pair?
In some applications, the amount of input current available to switch on a transistor is very low. This may mean that
a single transistor may not be able to pass sufficient current required by the load.
As stated earlier, this equals the input current x the gain of the transistor (hFE). If it is not possible to increase the
input current, then we need to increase the gain of the transistor. This can be achieved by using a Darlington Pair.
A Darlington Pair acts as one transistor but with a current gain that equals:
Total current gain (hFE total) = current gain of transistor 1 (hFE t1) x current gain of transistor 2 (hFE t2)
So, for example, if you had two transistors with a current gain (hFE) = 100:
(hFE total) = 100 x 100
(hFE total) = 10,000
You can see that this gives a vastly increased current gain when compared to a single transistor. Therefore, this will
allow a very low input current to switch a much larger load current.
Base activation voltage
In order to turn on a transistor, the base input voltage of the transistor will (normally) need to be greater than 0.7V.
As two transistors are used in a Darlington Pair, this value is doubled. Therefore, the base voltage will need to be
greater than 0.7V x 2 = 1.4V.
It is also worth noting that the voltage drop across the collector and emitter pins of the Darlington Pair when they
turn on will be around 0.9V. Therefore if the supply voltage is 5V (as above) the voltage across the load will be will be
around 4.1V (5V – 0.9V).
Load
5v
0v
Darlington
pair
Input
Load
5v
0v
Darlington
pair
Input