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Electronics Project - Object Lift Alarm

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Introduction

Object Lift Alarm Background My computer is one of the most valuable pieces of equipment in my home, and has been stolen previously. As such, I decided to make an alarm that would sound and give some kind of visual signal if my computer, or indeed and object, was lifted up off its base (attempted theft at the least). Specification * The alarm must sound as soon as the object has been lifted * The circuit must be able to be manually reset * The alarm must be able to be reset independently from the visual signal * The visual signal given must latch on until it is reset * The alarm must sound for a period of 5 minutes, then switch off * This period of time must be accurate to � 30 seconds * The circuit and components must be able to run off a low voltage supply, i.e. a battery pack My Electronic Solution (System Diagram) Alternative Electronic Solution My alternative solution would ideally involve a motion sensor, which when the object was moved (and a key switch turned - to 'arm' the circuit), would trigger a two (or more) tone siren, for maximum audible effect (would stay on for a time period of roughly 5 minutes, see specification) and a flashing high-wattage filament lamp which would latch on. Each would be able to be reset independently. Why I Chose My Solution I chose my solution rather than the one described above because it is simpler to build, and cheaper in price. ...read more.

Middle

The time delay of a monostable can be worked out using the formula: "Time (s) = 1.1 x resistance (M?) x capacitance (�F)". This can be used to work out the time delay produced by my monostable when triggered by a falling edge at pin 2: "1.1 x 0.56M? x 470�F = 290 seconds". 290 seconds is 10 seconds short of 5 minutes, therefore complies with the specification I provided earlier (time delay must be accurate to within �30 seconds of 5 minutes). Sub-System Testing The 555 monostable, when triggered by a brief falling edge from the Schmitt Inverter, produced an average (of 5 tests, which ranged between 327 and 329 seconds) timed output pulse which lasted 328 seconds (and was at 4.97V, according to the voltmeter used). The 328 seconds I timed may have been different from the 290 worked out using the formula for two main reasons; firstly the E12 series of preferred values for gold band resistors has a 5% tolerance range, allowing for a max value of 588K? and a min value of 532K?. Secondly human error may equate to a proportion of the time difference, depending on reaction time of the individual (in this case, me). The actual value of R4 I measured at 578 K? with an ohmmeter and the capacitance of C1 was measured at 459�F with a capacitance meter. Audible Alarm (Output Sub-System 2) This output sub-system consists of a transistor (labelled Q1), its protective resistor (R5), a buzzer (BZ1) ...read more.

Conclusion

Evaluation My circuit has been tested extensively, and has been found to match all of my specification points. The alarm does sound as soon as the object has been lifted, any time delay is a fraction of a second and negligible to the human mind. The circuit can be manually reset, by pressing switch 3 and/or switch 4 (see complete circuit diagram, page 6). The visual signal can be reset independently, depending on which switch is pressed (3 or 4). The alarm sounds for a period of time that is accurate to �30 seconds of 5 minutes, and then switches off. It met its accuracy target in all 5 tests, therefore I believe it satisfactorily meets the specification demands. The circuit can be run off a low voltage supply, as during testing I was using a 5V supply and the circuit worked perfectly and as it should do. I therefore believe it meets all demands provided by my specification. Costed Component List Acknowledgement of Sources of Information - List of "Inductive Output Transducers" from Mr. Woolley's class notes - Pin out for logic gates obtained from doctronics.co.uk - E12 series of preferred value resistors found on Wikipedia.org - Costed component list created in Circuit Wizard, prices from Circuit Wizard's Rapid Electronics database. - All circuit diagrams drawn in Circuit Wizard - Schmitt Inverter IC # found on electronicsinschools.org ?? ?? ?? ?? Jake Beazley GCSE Electronics Project Candidate No: 2833 Centre No: 13357 Page 1 of 8 ...read more.

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