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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 ) DesignAgainstCrime – Alarm Systems Page 1 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. DesignAgainstCrime – Alarm Systems Page 2 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. DesignAgainstCrime – Alarm Systems Page 3 Sheffield Hallam University 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. DesignAgainstCrime – Alarm Systems Page 4 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. DesignAgainstCrime – Alarm Systems Page 5 Sheffield Hallam University 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 DesignAgainstCrime – Alarm Systems Page 6 Sheffield Hallam University