On 28/04/2021 17:45, ARW wrote:
On 27/04/2021 17:39, Steve Walker wrote:
On 27/04/2021 16:49, williamwright wrote:
My neighbour has two infra-red beams across the two entrances to his
property. The alarm condition is 'open circuit'. The receivers are
close together so the two alarm outputs are connected in series to a
long cable back to the house. If there's an alarm a bell rings and a
big light comes on (because he's deaf). The light and bell are on a
five second timer so they go off 5 secs after they came on. There
then has to be a period of 'no alarm' (both receivers giving OC) and
then the alarm will sound next time one or both go CC.

It works just fine most of the time. It's just to let him know
someone's arrived. However, there's one snag. If either of the beams
are held in the alarm condition the bell and lamp don't operate if
someone comes in the other entrance. This can happen if the dustbin
is left in the wrong place or someone parks untidily. It's because if
one of the two circuits that are in series is OC it makes no
difference if the other goes OC.

So what's needed is a little something that will result in the
following:
If both circuits are not in the 'open circuit = alarm' condition and
then one or both is, the alarm will sound (like now).
If one circuit is held in the alarm condition for a period and then
during that period the other goes to the alarm condition, the alarm
will sound. There's no chance of a second cable back to the house.

So I suppose it needs something that will go OC on the feed back to
the house in the event of one of the receivers going OC, irrespective
of whether the other is OC or CC. There's a 12VDC supply at the point
where the cables from the two receivers meet. Ideas anyone?

Do the receivers only have one contact each or are they changeover? If
the latter, then change them to go CC on detection and feed each to an
interval timer relay, wiring the relays NC contacts in series.

Or look for timer relays capable of falling edge triggering and
interval timing.

What would happen if you just connected the two CC terminals on the
receivers together?


Sorry I needed to add to that. ie wired up as a 2 way light switch

--
Adam