Instead of passive sensors reporting back to an active control box/panel with a siren, why not a distributed system with each sensor containing its own siren?
If one sensor is tripped then it sends out a signal and all of the other sensor locations squeal. There is no one point that can be brute forced to kill the system; you would need to crack the message sequence used to command the sensors to arm or disarm.
Maybe this has already been done...?
Cost of duplicated circuitry would be a factor, all that repeated power and control (how do you turn them off?) wiring too.
You could kill it by removing the power - unless each sensor/siren had a backup battery (more expense).
I can't see this being viable n a very competitive market.
After a couple false alarms he'd be running around ripping the batteries out of every device. This was the advantage of hard wired systems. They critical points in the system were monitored for tampering. Just removing certain screws or jumpering any of the wiring could set the alarm off.
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I'm talking about a wireless system, thus there is no single point that you can disconnect (or damage) to disable the system.
Each sensor box would have a battery, microcontroller, sensor and piezo speaker. Possibly a small solar panel also, since most of them will be mounted on windows. Most of the time would be spent in sleep mode with the uC occasionally waking up to broadcast its status, or flash a deterrent LED.
False alarms are an issue, so is ongoing development (I can't imagine that having to reflash 20 window sensors and a couple of PIRs stuck on the ceiling will be fun)
Probably not, but I'm intending to build it for use in my own home. :)
Have a look at the Zigbee wireless modules.
The chipcon CC2420 is a good start:
Also Microchip have a Zigbee development board but it's not cheap:
It'd probably be pretty spiffy if you got it all to work though.
Have a look at Zigbee RF comms. Its a cheap, low power, low data rate, networked system. It should be ideal for your use. There's a description of it in February and March 2005 issues of Circuit Cellar. They even have a contest going using the Zigbee devices.
Nordic have some cheap low power transceiver chips as well.
I had thought of this a while ago but using standard home alarm components, using a code hopping UHF car alarm in the sensors. This would send a secure signal back to the normal home alarm panel inputs. Depending on how many ZONES and devices you need, each ZONE would require a different receiver. Oatley Electronics have a receiver unit that can accept up to 15 transmitters.
This means each ZONE can have up to 15 sensors connected to it. You would then use the alarm output to trigger the sirens/lights etc, which could be hard-wired or remote(via another transmitter/receiver). Throw in the $50- Jaycar Nokia SMS kit and you would have one hell of a home alarm/notification/automation system...
Just a thought. Scott B.
Yes, that notification point can be a pain - having driven across the city recently to attend a false alarm. My current thinking is to duplicate the few sensors I have. ie position 2 at different locations in a room or hallway so that both would need to be active at the same time to cause an alarm. Has anyone tried this?
Cheers.
You could arrange the logic so that if the one sensor tripped twice within say one minute, or two different sensors tripped then activate the alarm. Cheaper than two sensors per room. Also, you can use dual detection sensors. These use IR and microwave sensors built into one unit, and both sensors need to trip to activate the output.
David
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