Wireless alarm project, a few questions ;-)

Hi all, I would like to build a burglar alarm using several small sensors wirelessly sending their data to an arduino to handle the response. So I need to be able to have one receiver (connected to the arduino) and several cheap transmitters (that I can discriminate). I am in France and the easiest RF band is 433 MHz, but I dont know for 2,4GHz. Ideally the emitter should be very simple and cheap, as I will need one for each sensor (almost), and the receiver must be able to know who is emmitting. Any idea? TIA, Emmanuel

I don't know if there are off-the-shelf solutions for this - there are some cheap modules based on the RFM02 and RFM12 that can do the transmission (RFM01 is receiver, RFM02 transmitter, RFM12 is transceiver), but they all need a uController to drive them. The 433MHz ISM band has restrictions on power and duty cycle, you need to check these out.

If your senders are to be battery powered that adds complication as you need to power down the transmitter most of the time.

These kinds of transmitters use very little power when not transmitting. http://www.sparkfun.com/products/10534 Build up a minimal arduino from a 168 or something smaller with power from 3.7V LiPo battery. Burglar alarm creates a low interrupt to wake the minimal arduino from power down sleep mode, it transmits,then goes back into sleep mode. Can add a 5V source battery charger chip and recharge the battery every couple of months. I do the same with a 3.3V promini as a remote control. Biggest drain is the power LED when its waiting around. Thinking about rebuilding it like above.

I used the 434 MHz version of this http://www.sparkfun.com/products/8945, will see what that datasheet has for power when not transmitting. I recall it has some time listed about waking up to first transmission.

I'd avoid LiPo - your insurance company would agree with me as reducing the risk of burglary only to increase the risk of fire seems, erm, non-ideal!

LiPo is not a problem with a proper charging circuit.

Thanks for the suggestions, I think that the simple RF link is probably the way to go. I would like to avoid to put an MCU to control it though, I am not sure if I can try something that would make a small transmit (on the sensor side) every 5 min or just sending a unique ID to let the control that it is alive, and then in case of a breach, the attached sensor (whatever this is) triggers a transmit to start an alert. I was wondering if I could do this with a few small chips (like 555 and others) to keep things simple. I dont even know if collisions are a problem with these transmitters: there will be several (like 10 to 15) transmitters. I can get them to transmit at different times, but the receiver must be able to handle the case of several alerts at the same time without being confused. Any experience to share ??? TIA Emmanuel

No, no, no, your receiver needs something it can recognize & discriminate http://www.mouser.com/ProductDetail/Atmel/ATMEGA8A-PU/?qs=sGAEpiMZZMv9Wvg%252bO4wHS3zXsAD%2fjbUD Plug in an inexpensive uC, 30 cent crystal, http://www.dipmicro.com/store/XC7-16000 two 9 cent capacitors http://www.dipmicro.com/store/C1K22-50 three 12 cent capactors http://www.dipmicro.com/store/C5K10-50 8 cent resister http://www.dipmicro.com/store/R4F10-4 some perfboard from futurlec.com that you can cut down to size. Load it up with virtualwire, will be so much easier to bring to completion.

Yes I guess that this will be much more flexible and easier than what I first thought! In that case I am even tempted by the nRF24 in the 2,4GHz band! I will check the prices and more importantly power consumption, but they have even more reliable comm and there is a good lib to handle them apparently. Thanks, Emmanuel

CrossRoads: LiPo is not a problem with a proper charging circuit.

Lithium polymer batteries can and do catch fire every day. Lithium-ion batteries are far safer. LiFePO4 are the safest of the Lithium ion batteries. NiMH never catch fire and are cheaper. No brainer!

[ if you don't believe me, read the lesssons learned section here: http://www.rcgroups.com/forums/showthread.php?t=160079

I've personally found a LiPo battery that swelled up all by itself after being stored in a charged state for 6 months - they can and do explode sometimes spontaneously from this. http://www.rcgroups.com/forums/showthread.php?t=205785 ]