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Most PIRs have a N.C. output becoming o/c when triggered. The higher quality ones will have either a changeover contact or both N.O and N.C contacts. Either way you will need a relay (or electronic)interface if you are going to be using a 240V siren. Why not play safe & use a 12V siren? The other thing to consider is that the PIR only o/ps when it detects movement from one detection area moving to an adjacent detection area as defined by the type of lens. This will result in the siren sounding in very short bursts in time with the movement detected. You will need a latching cct for continuous operation.