Re: Modifying/hacking PIR sensors....

[Patrick Lidstone <patrick-lidstone@xxxxxxx>]

At 20:49 15/09/98 +0100, you wrote:
>Hi,
>
>Does anyone on the list have any experience modifying or hacking
standard
240V
>PIR sensors - the sort of off-the-shelf 10 quid from B & Q type. 
I'd like to
>use one as a sensor, for example to activate video capture from a
cheapo
camera,
>but not to switch any load.

Maplin etc. sell just the PIR sensor modules - the bit that actually goes
inside the a conventional PIR, about 1/2" square. The outputs from
these
are at TTL (5V) logic levels, which makes them dead easy to interface to a
printer port etc. These modules are mounted on a PCB & cased up to make
home security PIRs - as used for alarms and security light switching - but
the TTL output should be easy to track down (usually the centre pin of the
sensor module). For your application, the sensor module element on its own
might be the best option, since you could then mount it in a hermetically
sealed environment for outdoor use.

>Presumably there's a minimum load these things like to switch, but how
little
>can I get away with?  I don't want to have to dissipate a lot of
power/heat,
>even although the on time can be kept fairly short.

The security light PIRs that I've come across use triacs for the switching
- so they'll need to see an AC load unless you modify them - and what looks
like a small switched mode supply for the PIR, which you'd have to bypass
if you were looking to use a DC only arrangement. If you need both a light
and a logic output, perhaps the easiest thing to do would be either to use
a separate light sensor to trigger the video capture board, or (horrible
hack) wire a relay in series with the switched load?

Patrick

______________________________________________________________________

Subscribe, unsubscribe, opt for a daily digest, or start a new e-group
at http://www.eGroups.com -- Free
Web-based e-mail groups.