Mains Wireless Interconnected Smoke Alarms.

Yes, a sonic screwdriver. ? Or anything built my Mike P-J. ?

Although digital systems don't necessarily clash or interfere with each other just because they're using the same frquency (unlike some of the old analogue stystems of yore) - provided eveything on the same frequency is 'singing from the same hymnsheet' as it were.

   - Andy.

some designs of 815MHz kit (a well known type of doorbell and some alarms come to mind, oh and at least one design of car tyre pressure monitor) are pretty poor and have very unselective receivers that are blocked by anything that transmits nearby over a wide range of frequencies, including GSM phones on the 900MHz band.

Some others really well engineered and have tight channel filtering and a high dynamic range and are only affected by things on the same or adjacent channel, or very strong signals indeed out of band, and as none of these low power devices transmit for more than a small percentage of the time, a simple wait and retry protocol will normally get through after a few goes, slotting in the gaps in the other systems transmissions. However, as things get more crowded, then the risk of collision and loss of connection rises sharply.  I'd be wary of a block of flats with a lot of these that were not part of one single synchronous system.


OF course a hostile attacker with a modest transmitter, would find it easy to deny service and cause a fault state.  More likely for a burglar alarm than a fire alarm, but in certain types of building, food for thought perhaps. (A variation on the attack where you reprogram the heating and bake out your enemy,)


Mike.

But presumably at least one smoke alarm would still sound even if it could not talk to its other family members. So all is not lost.


Z.

When I was investigating the subject of wireless smoke/heat alarms around a year or so ago I had a chat with the wholesaler about AICO items. Wholesaler basically said that they had experienced item returns due to various incompatibilities with other kit. Instances had been reported of certain brands of bathroom extractor fans, wireless heating controls and other devices prematurely setting off these particular alarms. Don't know the technical details behind this, but it was enough to deter many contractors around here using wireless smokes.

If you're going to being doing a lot of these you may want to consider a spectrum analyser but the value of one job will not be enough to warrant a purchase. 

Regards Ts

A spectrum analyser (or its poorer relative the scanning receiver with some sort of level indications )is a jolly useful tool for those of us who work in the comms world on a regular basis.

However, I'd caution that in unskilled hands it is possible to cause as much confusion as it solves. There is something of an art in estimating  how many microvolts  to expect at the terminals of different sorts of antennas when oriented in a particular way, and it is easy to fool yourself into thinking something is either very small or very significant when it isn't.

It would be worth looking at a known working system  to calibrate expectations before trying to debug a faulty one.

But there are now some very cheap if basic devices that bring simple spectrum analysis into the reach of the occasional  user, (as apposed to the £10k lab instruments that were once the only option) but even they need a lot of practice to get the best out of them (and some surprisingly cheap antenna analysers that are really a cut down network analyser too)

Mike.