Simon explained :
Hello -

I'm currently adding a bit of security to my house by buying some cheap
wireless PIR sensors and receivers from
http://www.screwfix.com/app/sfd/cat/cat.jsp?cId=101495 , interfacing a PC
to them, and switching on lights, sending email alerts, posting webcam
pictures to a website, etc. when activated. It's going well so far.

I'd be interested to know if anyone can see pitfalls in this approach, eg.
- PIR manufacturer going out of business and not being able to get
compatible receivers/transmitters

Buy enough plus spares.

- burglars who carry radio jammers (does this happen outside of crime
novels?)

Extremely unlikely, even if they already knew about the special system.

I would suggest the biggest pitfall is likely to be the reliability of
the PC itself - the likelyhood of it crashing. For really critical
stuff which needs to be able to restart the system should it crash,
consider a 'watch dog' system. PC sends a regular pulse signal out to a
discrete 'watch dog' circuit, if the circuit fails to see a regular
pulse from the PC, it reboots the PC. The pulse is generated by the PC
from running software, sent out via a bit on one of the ports.

- What am I losing by not using sensors intended for alarm systems
- Are inconspicuous PIR sensors less of a deterent than a big bell-box?
- I'm thinking of installing one of these at a property that already has an
"Anglian" branded wireless alarm system - are they likely to clash?

Nothing to stop you adding a big bell box and flashing lights.

Clash, unlikely if the PIR uses the PC wireless network frequencies
2.4Ghz and the Anglian 432Mhz licence free - no.

--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk