I have acquired what appears to be a digital clock with twin ultrasonic transducers facing towards the front.
When I power it up, it is quiet, but as soon as I move in front of it, it initiates a loud clicking noise, almost as though by ticking like a clock it won't warn off intruders, but would warn the owner that there had been an intrusion. There is a plastic socket on the front with what appears to be a coupling coil buit in; perhaps there was some form of electronic master key to shut the thing off?
Any way, it has the identifying model no "BD100" on the front and seems to have been manufatured by Fotherby Willis Electronics Ltd of Leeds, UK, a division of Glynwed.
Any information on it would be gratefully received, failing which it'll be several hours of tracing the circuit.