I too have now googled “find sprinklers” and I am sure it is done by switching the operate current on and off fairly rapidly. One user even invented a modulating device by including a coarse saw blade and a bolt in the cct. One side of the loop connected to the saw, the other connected to the bolt, he would scrape the bolt across the teeth. I’m not sure whether it was a square wave generator or a saw tooth generator 😆