albar Mk 11 - a b.f.s .

In November 1976 the circuit was described of an ultrasonic intruder alarm, the `Al bar`. By slightly mis-using the ultrasonic transducer it got away with using only one transducer as both transmitter and receiver. Unfortunately, this system has proved only 80%-90% reliable, i.e. 10%-20% of the units did not work as well as they should. As briefly noted last month, there are two possible solutions to the problem. Both circuits are discussed in greater detai! here.