An electronics firm wishes to introduce a range of low cost alarms. You have been asked to design and produce a prototype for further development by the firm.
Electronics
Folder
Problem
An electronics firm wishes to introduce a range of low cost alarms. You have been asked to design and produce a prototype for further development by the firm.
Possible Projects
Brief
I have decided to design and make a prototype low cost home burglar alarm.
Research into Alarms
Analysis of commercial alarms
From my research into commercial alarms I have found out the following:
* All alarms have more than 1 input.
* All alarms sound for a time after they have been triggered.
* Most alarms sound for a variable time of between 2 minutes and 20 minutes.
* All alarms have a sound output of at least 98dB.
* All alarms have either a delay between being switched on and being armed or they can be remotely switched on and off from outside the premises.
* The better alarms have a small delay between being triggered and the siren sounding.
Task Analysis 1
Task Analysis Detecting Burglars
Task Analysis Warning People and making the signal last
Task Analysis Getting out of the house and switching the alarm ON
Task Analysis Getting into the house without sounding the alarm and switching the alarm OFF
Specification
My alarm circuit must do the following things:
* It must sense burglars entering through the rear door.
* It must sense burglars passing down the hallway.
* It must sense burglars entering through the front door.
* It must sense burglars climbing the stairs.
* It must sound an alarm that can be heard 50m away (108dB plus).
* It must sound for 4 minutes.
* It must allow the householders 20 seconds to enter the house before the alarm sounds.
* It must allow householders to leave the houses without triggering the alarm.
* It must be possible to reset the alarm at any time to stop it sounding.
* It must give the householder a quiet audible signal that it has been triggered before the main siren starts to sound.
* It must be battery powered.
Research Sensing people entering the front or rear door
As the task analysis shows there are many ways of sensing people coming through a doorway. I built them all using SYSTEM ALPHA to ensure that:
* They work
* I know how they work.
I have decided to use the upside down switch unit with a proximity detector to sense if the rear door is opened.
I have decided to use a switch unit with a pressure pad to sense if the front door is opened. Since I know where the pressure pad is I can avoid standing on it after I arm the alarm. This allows me to leave the house without triggering the alarm.
The shadow sensor would not work unless there was a light shining beside the doors.
The sound sensor is very complex and may take too long to construct.
Research Sensing people climbing stairs
As the task analysis shows there are many ways of sensing people coming through a doorway. I built them all using SYSTEM ALPHA to ensure that:
* They work
* I know how they work.
I have decided to use the pressure pad a burglar would be likely to step on every step and a pressure pad could be constructed that was the size of the stair tread.
The shadow sensor would not work unless there was a light shining on the stairs.
The sound sensor is very complex and may take too long to construct.
Research Sensing people walking along the hall
As the task analysis shows there are many ways of sensing people coming through a doorway. I built them all using SYSTEM ALPHA to ensure that:
* They work
* I know how they work.
I have decided to use the shadow sensor, as there is a light shining in my hallway all the time. With a little adjustment the shadow senses a person's shadow 2 metres away.
The pressure pad would have to be quite large to ensure that a burglar would have to step on it.
The sound sensor is very complex and may take too long to construct.
Research
Sensing 2 inputs
I built the following circuit using System Alpha.
Research
Sensing more than 2 inputs
To see how to have more than 2 Inputs I decided to use crocodile clips to investigate the possibilities.
Research
Sensing 4 inputs
I can make a 4 Input OR gate using 3-2Input OR gates-please see crocodile clips diagram.
It will be easier to design using 3 pre-designed OR gate modules, but this would cost more, take up more space and take longer to make than necessary.
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Research
Sensing 2 inputs
I built the following circuit using System Alpha.
Research
Sensing more than 2 inputs
To see how to have more than 2 Inputs I decided to use crocodile clips to investigate the possibilities.
Research
Sensing 4 inputs
I can make a 4 Input OR gate using 3-2Input OR gates-please see crocodile clips diagram.
It will be easier to design using 3 pre-designed OR gate modules, but this would cost more, take up more space and take longer to make than necessary.
I can also make a 4 Input OR gate using a 4072 chip.
This contains 2-4 Input OR gates in the 1 chip.
It should be easier to use than any of the options above.
Development combining the 4 Inputs
To try and model the 4 inputs I will use crocodile clips.
This works perfectly and the output switches ON if any of the 4 Inputs are activated.
Reading from left to right:
* First Input pressure pad at the front door.
* Second Input reversed switch (proximity detector) at the rear door.
* Third Input pressure pad on the stairs.
* Fourth Input shadow sensor in the hall.
Of course crocodile clips does not allow us to use a 4 Input OR gate so I have constructed 1 from 3-2 Input OR gates.
Research timers
Although there are other timers:
* 4098 chip based.
* Counter based.
* 555 chip based
I have decided to use the 4098 based timer because:
* Although they are not very accurate they are easy to use can be positively and negatively triggered and can be reset.
I need a timer which can switch on for 20 seconds to allow the householder to enter the house without setting off the alarm, then after this time, if the system is not reset then a second timer switches on for 4 minutes switching on the main siren to alert people of the burglary. This requires me to find how to make timers switch on at positive going transitions and negative going transitions. I built the following system using system alpha and this does what I require.
I will need to investigate the data sheets for the 4098 chip to see if I can simplify the circuit.
Research into the 4098 chip
From the guide to system alpha modules I have discovered that we already have a circuit for a double timer which does exactly what I require. It has a positively triggered monostable timer linked directly to a negatively triggered monostable timer. Both time periods are adjustable from a few seconds up to 4 minutes. The double timer also has a transducer driver built into the first monostable. This enables me to make a quiet buzzer sound during this period.
The 4098 also has a reset facility. If reset is held low then the timer does not switch on, even if the input goes active.
Final System Diagram
Final Layout diagram
Parts list
INPUT
Power Connector BLACK DOT
0.1 ?F polyester Cap. 10-0960 0.04
1N4001 diode 1N4001 0.01
Total £0.05
INPUT
Dark Sensing Unit ORANGE DOT
Miniature LDR 58-0127 0.24
100k? preset 67-0145 0.04
Total £0.28
INPUT
Switch Unit DARK GREEN DOT
2 33k? resistor 62-0409 0.02
2 home made pressure pad 0.18
Total £0.20
INPUT
Switch Unit DARK GREEN DOT
33k? resistor 62-0409 0.01
2 push switch 78-0100 0.20
key switch 79-0295 1.60
Total £1.92
PROCESS
Double Delay Unit LARGE ORANGE
4098 i.c. 83-0426 0.25
16 pin chip socket 22-0160 0.06
1M? preset 67-0160 0.08
4M7? preset 0.20
33k? resistor 61-0409 0.01
47 ?F Cap. up to 24 s 11-0325 0.03
100 ?F Cap. up to 50s 11-0245 0.03
Total £0.66
PROCESS
4 INPUT OR GATE LARGE BLUE
4072BE i.c. 83-0316 0.20
14 pin chip socket 22-0155 0.05
Total £0.25
DRIVER
Transducer Driver LARGE YELLOW
MPSA 13 Darlington 81-0142 0.04
33k? resistor 62-0409 0.01
Total £0.05
OUTPUT
Sound
75 dB Buzzer 35-0035 £0.58
108 dB Miniature piezo siren 35-0272 £3.29
4 self adhesive p.c.b. pillars 33-2135 £0.09
Total £7.26
Plan 1
Plan 2
Diary 1
Task
Est.
time
Tools and equipment needed.
Time taken
Problems encountered
Use 2-D Design to produce layout of final circuit
20 min.
Computer 2-D Design and final system diagram.
60 min.
I didn't know how to use the double timer and had to ask my friends
Print 2-D Design layout.
5 min.
Computer 2 D Design printer
5 min.
None
Mill printed circuit board using CNC milling machine
5 min.
Computer CNC milling machine and copper board.
60 min.
I had to wait for the milling machine
Drill p.c.b.
0 min.
Mini drill.
20 min.
None
Clean p.c.b. with wet and dry paper
5 min.
Wet and dry paper
5 min.
None
Produce parts list
0 min.
Computer Microsoft Works and printer
40 min.
I couldn't find where the correct values for the double delay unit. The database didn't seem to have the values I wanted.
I couldn't find the correct components for the delayed arming part of the system
Solder power connector components in place.
40 min.
Soldering iron, stand, pliers, cutters. Heat mat and solder.
0 min.
None
Test the correct working of power connector.
50 min.
Alpha building and testing manual and multi-meter.
30 min.
I didn't know how to use the multimeter
Learn how to use the multimeter
20 min.
Alpha building and testing manual and multi-meter. TEACHER
0 min.
None
Insert and solder the last transducer driver and buzzer (I will use a standard buzzer not the 108dB siren).
I will fit the siren later
0 min.
Soldering iron, stand, pliers, cutters. Heat mat and solder.
0 min.
None
Test the correct working of the transducer driver and buzzer
0 min.
Alpha building and testing manual
30 min.
The buzzer didn't work
Diary 2
Repair the transducer driver and buzzer
?
Alpha building and testing manual and multi-meter.
20 min.
The buzzer was soldered in upside down.
Solder the reverse switch components in place
0 min.
Soldering iron, stand, pliers, cutters. Heat mat and solder.
5 min.
None
Test the correct working of the reverse switch
0 min.
Alpha building and testing manual and multi-meter.
0 min.
None
Solder the first switch components in place
0 min.
Soldering iron, stand, pliers, cutters. Heat mat and solder.
5 min.
None
Test the correct working of the first switch
0 min.
Alpha building and testing manual and multi-meter.
0 min.
None
Solder the second switch components in place
0 min.
Soldering iron, stand, pliers, cutters. Heat mat and solder.
5 min.
Test the correct working of the second switch unit
0 min.
Alpha building and testing manual and multi-meter.
0 min.
Solder the shadow sensor components in place.
0 min.
Soldering iron, stand, pliers, cutters. Heat mat and solder.
30 min.
The preset would not fit in the holes. I had to find a hand drill and drill 3*1.2 mm. holes for the preset.
Test the correct working of the shadow sensing unit
0 min.
Alpha building and testing manual and multi-meter.
60 min.
It was difficult adjusting the sensitivity of the shadow sensor
Insert the three wires connecting the inputs to the 4 Input OR gate
5 min.
Soldering iron, stand, pliers, cutters. Heat mat and solder.
5 min.
Solder the 4 input OR gate in place
20 min.
Soldering iron, stand, pliers, cutters. Heat mat and solder.
0 min.
None
Diary 3
Test the 4 Input OR gate
0 min.
multi-meter.
200 min.
The output was always on. I could not get it to switch off. Eventually I found out that pin 2 and pin 3 were always high. I had to short the switch wires for the reversed switches together. This made it look as though the 2 doors were closed. After this the OR gate worked. However it took a long time to test the 16 possible inputs of the OR gate.
Solder the double delay unit components in place
00 min.
Soldering iron, stand, pliers, cutters. heat mat and solder.
200 min.
No problems, it was just a bigger task than I thought.
Test the double delay unit
20 min.
Alpha building and testing manual and multi-meter.
20 min.
I tried but I could not understand the instructions.
Research into the 4098 chip
20 min.
Maplin CD rom
80 min.
I think I understand it now
Repairing the double delay unit
20 min.
Soldering iron, stand, pliers, cutters. heat mat and solder.
Alpha building and testing manual and multi-meter.
80 min.
I found many solder bridges and 1 of the delay unit capacitors was upside down. The second part of the delay unit still did not work until I replaced the 4098 chip. It works perfectly now.
Building a Case
Case
The case is made from 2mm thick black polystyrene. This material is not really strong enough for an
alarm case but gives a client a good impression of what it will look like.
Case
Case parts List
The case is made from 2mm black polystyrene
Part
No
Length in mm.
Width in mm.
Side
2
60
60
Bottom
60
53
Top and front (see plan)
222
49
Shelf
46
49
Platforms for velcro
2
49
0
Front stopper for battery
49
0
Central wall of battery container
52
28
Back wall of battery container
25
28
Back
58
49
Platforms for top
2
44
8
Case plan 1
Case plan2
Diary 1
Task
Est.
time
Tools and equipment needed.
Time taken
Problems encountered
Measure and mark parts on 2mm. black polystyrene
20 min.
Ruler, pencil, 2mm. Polystyrene.
0 min.
None
Score and snap parts out
00 min.
Safety ruler and scoring knife.
50 min.
It was more difficult to cut the polystyrene than I thought.
Clean edges with marking knife and 400 grade wet and dry paper
20 min.
Marking knife and 400 grade wet and dry paper.
20 min.
None
Mark and drill 6 holes on end of combined top and front
0 min.
Hand drill
5 min.
None
Mark combined top and front 63mm. from end (the end with the holes) and use strip bender to put 90 degree bend where marked above.
20 min.
Ruler, pencil, 2mm. Polystyrene, strip bender and 90 degree jig.
30 min.
I had to wait for the strip bender and then ask my teacher how to use it.
Glue, using polystyrene cement, top platform to each side 2mm. from the top 14mm. from the back.
0 min.
Polystyrene cement.
20 min.
None
Glue sides to bottom. The sides rest on the bottom.
0 min.
Polystyrene cement.
0 min.
None
Glue the back onto the bottom and both sides.
0 min.
Polystyrene cement.
5 min.
None
Fit the combined top and front to the case and see how much needs to be trimmed to make it fit properly.
0 min.
5min.
None
Trim top to size and finish sides with knife and wet and dry paper.
30 min.
Marking knife and 400 grade wet and dry paper.
40 min.
I had to continually recheck the fit.
Make battery container and ensure that battery fits
30 min.
Polystyrene cement.
60 min.
It was difficult to fit everything together before the glue set hard. I kept moving parts that I had already glued.
Glue circuit platform in place
0 min.
Polystyrene cement.
0 min.
None
Glue 2 velcro platforms in place and cut and fit velcro to platforms and top/ front.
20 min.
Polystyrene cement.
0 min.
None
Diary 2
Task
Est.
time
Tools and equipment needed.
Time taken
Problems encountered
Fit 6 3.5mm miniature jack sockets to front of case.
20 min.
Pliers
0 min.
None
Drill 4 - 4mm. Diameter holes, in circuit board, for stand-offs and fit stand-offs.
0 min.
Hand drill with 4mm. Drill bit.
5 min.
None
Unsolder LDR and siren from circuit board.
0 min.
Soldering iron, stand, pliers, cutters. heat mat, solder sucker and solder.
20 min.
Waiting for the soldering iron to heat up.
Solder wires from circuit to jack sockets.
5 min.
Soldering iron, stand, pliers, cutters. heat mat and solder.
5 min.
None
Fit battery connector.
2 min.
Soldering iron, stand, pliers, cutters. heat mat and solder.
2 min.
None
Fit switches and pressure pad to jack sockets.
20 min.
Soldering iron, stand, pliers, cutters. heat mat and solder.
30 min.
None
Stick board down in place.
2 min.
5 min.
None
Fit PP3 battery
min.
min.
None
Test circuit and adjust again to ensure that it meets the specification.
00 min.
200 min.
The darkness sensor part needed to be re-adjusted. This was very tricky.
Testing The specification for my alarm was as follows:
* It must sense burglars entering through the rear door.
* It must sense burglars passing down the hallway.
* It must sense burglars entering through the front door.
* It must sense burglars climbing the stairs.
* It must sound an alarm that can be heard 50m away (108dB plus).
* It must sound for 4 minutes.
* It must allow the householders 20 seconds to enter the house before the alarm sounds.
* It must allow householders to leave the houses without triggering the alarm.
* It must be possible to reset the alarm at any time to stop it sounding.
* It must give the householder a quiet audible signal that it has been triggered before the main siren starts to sound.
* It must be battery powered.
Testing reverse switch for back door
The proximity detectors worked every time that the door opened more than 10 cm.
Testing shadow sensor to detect people moving down a hallway.
After adjustment the shadow sensor could detect a persons shadow, with 100% accuracy, up to 1.2 metres away.
Testing burglars climbing the stairs.
The pressure pad circuit worked with 100% accuracy. However it was possible to ascend the stairs and step over the pressure pad. Burglars would probably climb stairs one at a time. Therefore this system should work every time.
Testing burglars entering the front door.
The pressure pad circuit worked with 100% accuracy. However it was possible to leave the house without triggering the alarm.
It must sound an alarm that can be heard 50m away.
I triggered the alarm and tested how far away that it could be heard. With the door open I could hear it 75metres away. With the door closed I could hear it 55metres away.
Testing the timer
The timer ran for between 3 minutes 10 seconds and 3 minutes 40 seconds. The average time was 3 minutes 35 seconds. Although this was below the specification it was not seriously below specification and the discrepancy was probably due to the low tolerance capacitors used.
Testing the 20 second entry timer
I adjusted the preset on the first part of the double delay unit and arrived at an average time of exactly 20 seconds.
Resetting the alarm
Pressing the reset button stopped the alarm 100% of the time.
Testing the quiet audible signal.
The standard buzzer could be heard 15 meters away.
My system therefore meets the specification in almost all ways and meets the specification in all important ways. To improve my system I would do more research into timers and find one that could time for longer. The 555 timer can time up to several hours and this would seem to be what I need but it is difficult to make it edge triggered.
Use of Industrial practices
I used a lot of CAD/ CAM when making my project. I used croc clips to produce a virtual circuit and used 2D Design to design the layout. I used a CNC milling machine to cut my circuit board and could use the file to produce many identical circuits. I could never have designed such a complex circuit without the CAD/CAM equipment.
Do not copy, use these ideas A*
L33T_CS Y11 project folder Page 1
