Download `Alarm Systems` teachers notes - the Sheffield Hallam University

DESIGN AGAINST CRIME
Alarm Systems - Teachers notes
Designing and The design and manufacture of a quality product is vital to
making the alarm the success of this design and technology activity. It has
been our intention in devising these projects that we support
teachers and pupils working towards this end.
The pupil workbook should be used to guide and support
pupils through their designing and making tasks but teachers
are encouraged to develop the activities according to their
own particular strengths and to the needs of their pupils.
The following notes are intended to examine more closely
the content of the alarm systems workbook, explain the
philosophy that underpins the structure, and provide advice
and support on aspects of the design activities, the
electronics and the manufacture.
Alarm project The alarm project is directed towards an electronic product
rationale that has been designed with the electronics integral to the
concept and design of the product. Pupils should be
encouraged to examine existing electronic products and
consider how the circuit boards, where present, are integral
to the structure and form of the product. Mobile phones and
other keypad devices provide good examples of the notion
that the PCB takes on the shape of the product and is part of
the products physical characteristics, in addition to its
technological function. It is hoped that pupils will also
endeavour to miniaturise their alarm by using small batteries
and small circuit boards where this is appropriate.
It has not been the intention to prescribe a particular age
phase or ability level to this project. In its simplest form the
workbook and project is ideally suited to KS 3 as a focused
practical task. Alternatively a miniaturised coded bag alarm
could be used as a GCSE or even AS level project. When
working at this higher level it is worth considering extending
the tasks relating to crime by accessing the Home Office
web site.
( www.homeoffice.gov.uk )
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Sheffield Hallam University
The starting point The importance of embedding this alarm project within the
context of crime reduction has been raised earlier and this
cannot be understated. This context should provide ample
opportunity to guide pupils into a design and make activity
based upon alarm systems.
Pages 4 and 5 are intended to guide pupils towards either a
‘hands off’ or a ‘surprise alarm’ project outcome. At this
point the teacher needs to make the decision about the
direction to take, or whether to allow both types of alarms to
be developed within the teaching group. In its simplest form
there is little difference in the electronics, the only difference
being an indication that the alarm is armed. There is,
however, a great deal of difference in the product concept.
Pupils’ attention should be directed towards this and to the
task on page 4. The note pad on page 5 provides
opportunity to develop a product design specification and
pupils should be encouraged to further develop this as their
design ideas begin to take shape.
Electronic As previously stated an important concept to develop
products, some through this project is the notion of designing electronic
things to consider products as complete entity. This means abandoning the
common practice of “now we have done the electronics let’s
get on with the packaging” and adopting “lets develop the
PCB concurrent with the product.”
Pages 10 to 13 are intended to assist pupils in structuring
their design thinking. On page 10 we begin by focusing
upon how the critical features of the product, the battery, the
PCB, the buzzer, etc. could be arranged in relation to each
other. The relationship of these features informs the
development of the design.
The design sketches developed on page 11 take forward the
possible arrangements of component parts in a logical
progression. This then moves towards a three dimensional
form through modelling (page12). It is here, by using scale
models, that sizes can be determined. Pupils should be
encouraged to develop forms in three dimensions through
modelling as early in the process as possible. Almost any
easily worked material is appropriate for this modelling
exercise. Pupils can record the development of their models
using a digital camera. Printouts of their pictures can be
added to their project workbook in the appropriate places.
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Sheffield Hallam University
The presentation drawing can be used to develop graphical
or ICT skills. ‘Pro/DESKTOP’ software is ideal for this
application. Pupils should be encouraged to annotate their
presentation drawing with relevant construction and
operational notes. This enables them to demonstrate their
understanding of how the product operates and how it will be
manufactured.
Pages 14 to 17 should be used for reference and to
encourage quality product outcomes. It is important when
developing shell type forming used to house an electronic
circuit that the method of joining the shells is considered.
With careful design the PCB can play its part in holding the
component parts together. Cable ties, split tubing, lacing
and pinning have proved to be particularly effective, yet
simple, means of joining the parts together. In the exemplar
bag alarm the PCB is a structural component that provides
the vacuum formed halves with rigidity and the PCB
mounted slide switches enable the clear acrylic rods to have
a positive switching action.
The electronics The principal component used in this circuit is a thyristor.
1. Thyristors Thyristors are semi-conducting devices formally known as
silicon controlled rectifiers (SCR), this is because they were
originally developed for the purpose of rectifying a.c. current
supplies. In this alarm circuit, however, the thryistor is used
as an electronic switch that once turned on stays on until
reset.
On page 7 both the thyristor symbol and the circuit layout are
shown. The arrowhead in the symbol indicates the direction
that the current will flow when the thyristor is turned on. The
current follows through the anode connection into the
thyristor and out through the cathode connection. Thyristors
are polarised components and must be connected the
correct way round. The gate of the thyristor acts a switch.
When a voltage is applied at the gate the thyristor turns on
and current flows freely turning on the output device, in this
case a buzzer. The useful feature of the thyristor is the
ability to keep conducting current even after the gate voltage
has been removed. In order to turn off or reset the thyristor,
the supply of electrical current to the anode has to be
interrupted in some way. The thyristor is then turned off until
there is once again a voltage at the gate.
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The electronics The batteries used in this project should be able to supply
2. Batteries sufficient current at 6 volts to drive the buzzer. A 6v supply
in combination with a 6v buzzer works well. The batteries
can be either four AA cells in a holder - these are relatively
cheap but rather large. Two 3v calculator batteries, like
those in the example on page 17 – these are more
expensive but smaller. Alternatively, four 1.5v small button
cells can be located in a single ‘N’ type battery holder. –
these are reasonably priced and small.
The electronics There are two switches in the alarm circuit illustrated on
3. Switches page 7. One provides the on/off switch for the circuit as well
as the function of interrupting the supply to the
buzzer/thryristor to reset the alarm. Without this facility the
alarm would keep sounding once triggered. The other switch
is the tilt switch (for a variation this could be a pressure pad).
The tilt switch, once activated, provides a voltage to the gate
of the thyristor and this turns it on. A brief action is sufficient
to do this. There is no specific symbol for a tilt switch (page
6) it is the same as for a push to make push button switch.
HEALTH & SAFETY Ensure that tilt switches used in school project work do not
WARNING contain mercury. Metal cased, robust tilt switches are
available from suppliers such as ‘Rapid Electronics’. It is
possible to buy cheap mercury filled glass tilt switches but
these are not suitable for use in schools.
The electronics Look closely at the circuit illustration on page 7 of the work
4. Resistors book. You can see that the gate of the thyristor is in effect
connected to 0v via two resistors, a combined total of 11k
(10k+1k). This value is not critical but it is good practice that
the gate is ‘held down’ at 0v i.e. connected to 0v, when it is
not switched on. When the tilt switch is on (tilted) then the
thyristor gate is connected to the +v supply via the 1k
resistor. This resistor limits the flow of current to the thyristor
so avoiding any damage.
The 1k resistor across the buzzer ensures that current
continues to flow to the anode of the thyristor even if the
buzzer is of a poor quality. Some buzzers, by the very
nature of buzzing, sometimes interrupt the supply for a split
second; this may be long enough for the thryristor to reset. If
this occurs this resistor value can be reduced to 560R.
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Sheffield Hallam University
The electronics In a surprise alarm this component can be left out. In the
5. The LED and its text there is reference to a flashing LED. For most flashing
series resistor LEDs a series resistor is not needed. If however an ordinary
LED is used, then a series resistor is required. It is worth
spending the time with pupils calculating the value of this
resistor using ohm ‘s law. This is a simple task and provides
a good opportunity to further develop numeracy skills.
Follow these steps:
1. Look up the specification of the LED being used, this will
typically have:

a forward voltage (Vf) of 2v

a typical forward current (If typ) of 15mA (0.015A).
2. What is the supply voltage (Vs)? in this example 6V
3. The maths to determine the value of the resistor is;

R = Vs – Vf
f typ
R = 6 – 2 = 266
0.015
Note: 220R is a preferred value that is close enough.
Teaching the Use the exercises on pages 6 and 7 in conjunction with
electronics circuit modelling. There is an ICT opportunity at this point to
model the circuit virtually using software such as ‘Crocodile
Technology’. For ‘real’ modelling ‘Locktronics’ kits are
good. Alternatively, prototyping board (bread board) can be
used. Working out a layout for a breadboard is a good
homework task.
It is important to ensure that pupils recognise the system
works in blocks of INPUT, PROCESS, OUTPUT. This can
stimulate discussion regarding other sensors and other
forms of alarm. If appropriate for your pupils, introduce the
term ‘transducer’ as a generic term for a device that
converts energy from one form to another. Transducers can
be either input or output devices. This term can be added to
the exercise at the foot of page 6.
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Extension activity, Coding the reset of the alarm is a very attractive extension to
coding the alarm the basic alarm circuit. This can be used to challenge more
able pupils at KS3 or incorporated within the design brief for
older pupils.
The concept is very simple and a coded alarm does not
require an on/off switch to effect the reset. Study the
extension to the circuit on page 8. With all of the three
switches closed current will flow through them rather than
through the thyristor because this is the route of least
resistance (effectively zero resistance). Thus the thyristor
turns off although the alarm will continue to sound until at
least one of the switches is opened to break the circuit.
Turn to page 16 where you can see, amongst the sketches,
the PCB for a coded alarm. If you follow the tracks through
where the three switches will fit, indicated by the word ‘pins’,
you can see that two of the switches on the board are in fact
connected at both ends, like this they would not switch off.
The reason for this is to facilitate coding. By cutting through
one line to each switch, different codes can be achieved
using the same PCB layout.
Circuit board The PCB masks provided at the end of these notes can be
masks used with or without the LED. If a flashing LED is used the
series resistor should be replaced by a wire link or the mask
can be modified.
PCB mask DAC1 is arranged to suit the layout on page 7 of
the workbook.
PCB mask DAC2 is a ‘stretched’ version with provision for
coded switch resetting.
Note that neither PCB mask has provision for on board
battery mounting. Batteries should be connected where
indicated at +V and 0V. or the mask modified as appropriate.
For PCB masks ‘DAC1’ and ‘DAC2’, please visit
www.designagainstcrime.org/schools
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Sheffield Hallam University