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Making a simple but very robust alarm system with arduino

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DDT

New Member
The following simple diagram shows a concept which I'll try to execute in near future. Since I am new to Arduino, I'd appreciate any help. The alarm is mainly used for several window and door contacts (normally closed) in series. It will activate upon opening of any contact. The problem that I've had with previous circuits based on simple logic gates was the false alarm. I don't what was the source but my best guess is RFI noise due to long wire etc. By using Arduino I should be able to make more intelligent system which needs to meet some conditions before it can activate the alarm.

1. At the start all outputs are low. The power on delay starts and continue for 5 min. This is indicated by a LED. If any of the contacts are faulty i.e. open contact, the system won't proceed and the LED will be off.

2. After 5 min exit delay period, sensors I/O (analogue out/in) sends a current through wire and sensors. The sensor input pin also has low pass filter ,10K/0.01uF (not shown) to block RFI noises. The system goes into standby mode. I am going to use one of the sleep libraries to minimise the current consumption.

3. When any of contacts opens, the system won't react immediately, it waits 100ms and read again, repeat for three times. If the input pin reading is still lower than predetermined threshold voltage, the system then would react and sound the alarm. Any false alarm due to external noises shouldn't cause issue anymore.

Please let me know what do you thing and if you have any suggestions to further improve. Help with coding is also greatly appreciated.
 

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Wp100

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You want to have a look how commercial burglar alarms function and what features you can make use of.
https://www.security.honeywell.com/uk/documents/IE1-0040-Rev1-engineer.pdf

eg The use of Zones is helpful rather than having lots of sensors in series on one line, so having ground floor, first floor zones makes it easier to know where you have left a window open when you try to arm the system.

A 5min Exit delay seems ott, typically 30 to 60 seconds is all thats needed for Exit and Entry delays.

How are you activating the system, keypad, rf ?

No mention of a security loop on your sensors.
 

DDT

New Member
It's one zone with few windows and also a camp site. I am aware of commercially available models and functions. They don't meet my requirements, it must be portable, compact and have a very low power consumption as it's battery operated. This can be done by modification on Arduino mini pro and using sleep library, where power consumption can reach uA levels. The sensors are reed switch type.
 

Pommie

Well-Known Member
Most Helpful Member
If you don't have too many series contacts then you could put different value resistors across each switch, 1k, 2k 4k, 8k etc and when you have a fault, charge a capacitor through the loop to identify the open contact.

Mike.
 

large_ghostman

Well-Known Member
Most Helpful Member
How secure do you need? and why portable? I ask because the magnet contact ones are fairly easy to get around. In my workshop I use pulsed IR and a small reflective plate, the pulses are counted and 3 times wrong count sets the alarm off.

Also for windows I ended up using copper tape as well, just incase the window was broken. But difficult to suggest without know more. Coding wise there should be a fair amount of base code around the net for doing what you want.

No idea if its any use, but there are heap wireles IR devices now, these are battery powered and designed for sheds. The down side is a decent sizd moth set them off and the range is limited.
 

JonSea

Well-Known Member
Most Helpful Member
The place I used to work had an IR sensor that repeatedly gave false alarms (I lived across the street so I was the call-in guy when there was an alarm).

The alarm company finally figured out there was a bug in the sensor. Literally, a moth inside the case!
 
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