How do I attach a new Rain Sensor to my ancient Hydrotek sprinkler timer?
I pulled apart the blue-to-white from the transformer to the water on/off switch, and put sensor's black to transformer's blue, and sensor's red to switch's white.

Then I flipped the power on and turned the system on.  It ran fine.  I pressed the manual test sensor switch while it was running, and the system stopped.  Installation complete.