This instructable describes replacing the Cat Genie plastic sensor with an electronic sensor. The plastic Cat Genie sensor tends to get dirty very easily and causes the unit to make the annoying three beep error, as well as several other problems.
When the sensor gets dirty, not only do you get the three beep error, but many times the unit will not fill the bowl with water all three times causing extra stinkiness during the drying cycle (a great urine aroma). Also, since the unit thinks there is still water in the bowl it causes extra strain on the draining motor while it continues to try to drain. I have smelled the motor after a cycle and it doesn't smell like it's happy about all the extra work.
So, let's look at the sensor to see how it works. It is two plastic rod pieces connected by a small plastic junction at the bottom. The rods act like fiber optics, and the junction acts like a mirror. There are two 45 degree angles in the junction part, and these cause the light coming down one side to be reflected back up the other side. When water is in contact with this part of the sensor, the mirror effect of the plastic is negated and the light does not get reflected back up.
As long as we provide a changing IR light source to the Cat Genie's sensors (there is an IR emitter and sensor pair on the bottom looking down into this plastic sensor), then we can source it with data from an external water sensor.