I am looking to build a Sonar-system that allows me to measure depth (under water). I found some references to National Semiconductor's LM1812 but this chip is no longer available, is there a equivalent or replacement? The biggest problem is finding a transducer. Does anyone know what searchphrase to use?, company's that produce/supply these? or places to find transducers?. The biggest criterium for the transducer is that i have to be able to use it at depths of 150-200 meters. Basically i want to make a system that warns me if the distance to the bottom is getting to small, so maximum measuring-range can be relatively small.