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