Cheap Burgler Alarm
I have designed here a simple cheap burgler alarm, that has the following features
Compact design -fits onto a 120x60mm circuit board
Cheap design, $30NZ -- $15US (exchange rate at time I brought components)
Balanced lines
8 Zones
1 Entry delay zone
1 24 hour zone
6 Armed zones
Has ability to change this by adjusting software
Runs from a 12volt supply
30 second entry exit delay, -by editing software easily changed
5 minute siren time
Has low voltage cut out protecting, if battery starts to flatten the alarm shuts down to save false alarms.
Pictures Of The Product
Schematic Diagram
For blown up image click on image
Basic Operation
First of all you have the power supply stage, this is D1, D2, C1, C7 and V1. This regulates the voltage to a steady, 5 volts. When I designed the circuit I designed to have a maximum of 13.6 volts feeding the input, however a higher or lower input voltage may be used as long as your siren and V1 can safely handle the higher voltages, or the output from the 5 volt regulator remains constant.
The balanced line network, the purpose of having a balanced line is that it offers protection if someone decides to put the side cutters through the wires to the sensor. What happens is the ideal design of sensor is a normally closed system. In this case if anyone decides to cut the wires open circuit and the siren goes off, but with that in mind the if someone shorts the wires together it won't go off. If a balanced line is used then if any wires are cut, or short circuited it will trigger an alarm.
Basic principle of the balanced line
On the sensor you have a 10k resistor as close to it as possible, when the sensor is closed the output of the circuit is 2.5 volts. If some short circuits the circuit, the output voltage suddenly raises to 5 volts. Or if some open circuits the circuit the output voltage drops to 0 volts.
R10 is the internal 10k resistor for the balanced line. In the circuit there are two op amps that are set up as comparators, these sense if the output voltage is where it shouldn't be. U2A is set to trigger if the voltage is over 3 volts (short circuit protection). U2B is set to trigger at 2 volts (open circuit protection). These input are then feed into the 2051 micro controller which works out weather to sound the alarm or not.
You have probably noticed that the zone inputs are feed into U1, U1 is an analogue multi-plexor, this is so I can cram 8 zones still using the same comparator network.
U2C this is another comparator, this is part of the micro controller power on reset circuit, I have complicated it slightly so that the system is given a reset if the supply voltage drops.
Parts List
| Designator | Size | Quantity |
| C1 | 220nF | 1 |
| C2,C3 | 33pF | 2 |
| C5 | 0.01uF | 1 |
| C7 | 1uF | 1 |
| D1,D2,D3 | 1N4007 | 3 |
| D3,D4 | LED | 2 |
| D6 | 1N914 | 1 |
| Q1 | MTP3055E | 1 |
| R11,R12,R14,R15 | 1K | 4 |
| R13 | 470R | 1 |
| R2,R8,R9,R10 | 10K | 4 |
| R3,R6 | 330K | 2 |
| R4,R5 | 220K | 2 |
| U1 | HCF4051BM | 1 |
| U2 | LM324 | 1 |
| U3 | 89C2051 | 1 |
| V1 | 78005CT | 1 |
| X1 | 10MHz | 1 |
| Connectors | 7 | 3x3 |
Circuit Board Layout
Here is the circuit board layout that I have designed for the components I brought.
Complete Layout
Click on image for clearer view
Bottom Layer
Click on diagram for clearer view
Silk Screen Overlay
Click on Diagram for clearer view
Usage (Manual on how to use it)
Multiple sampling for a zone to trip the zone must be held in for 1 second. The alarm will sample a zone four time within a second, if the zone is held open for all the four samples the zone is considered broken, this is to try and eliminate false alarms.
I have decided that for the armed switched to be activated when the armed input is high, reason for this is the 2051 has internal pull ups if someone cuts the wires it will pull up and arm itself. When held low it is in the disarmed position.
When arming the user will have 30 second to get out, after thirty seconds the alarm will go into armed mode and what ever state the zones are set in these are the states that the zones will have to stay in.
Zones 1 this is the entry delay zone, upon entry and as long as the zone is held broken the user will have 30 seconds to disarm before the system goes off.
Zone 2-7 these are ordinary zones that will go off if someone trips them whenever the alarm is armed, including when the alarm is on it's entry delay routine.
Zone 8 this zone is constantly armed, if broken alarm will sound immediately. To reset this alarm the alarm must armed then disarmed.
Siren sounding, on any conditions mentioned above the alarm will sound for 5 minutes, at the stage where the alarm will stop sounding after 5 minutes, the states that the zones are in will be reset, eg if zone 2 is broken, then zone 2 will be registered as broken and if someone closes it will go off again. To stop the alarm sounding disarm the system, zone 8 will require arming then disarming to stop it sounding.
What each LED means.
Zone LED, this lights up when ever any of the zones are broken.
Armed LED, once the alarm is in armed mode this LED will light up.
Software
The software has been written in C source and complied into Intel hex format, I have included both the source file and the hex file.
Information on this site the authour has no responsibilty on how correct information is with in.