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?

Bill