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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.
Step 1: Design of the electronic water sensor
Ok, so we know how to interface with the Cat Genie. Provide an IR LED that can shine IR light into the CG's IR sensor and you are home free. Now we just need to design our own sensor that will be resistant to dirty (poopy) water.
There are many ways to sense the level of water in a tank, but I wanted something with no moving parts to get stuck, get dirty, etc. A simple solid-state electronic sensor would be best. After some google searches for electronic water sensor circuits, I found some ideas on how to best implement a water sensor circuit.
After experimentation on the breadboard, I found a good working solution using a Darlington Pair of transistors to provide a changing voltage to two other transistors, which then either light the IR and green LED, or it lights the red LED. This gives the user an indication of the state of the sensor, it will turn from green to red water hits the probes we install.
Below is the schematic of the sensor (credit to 2cats2dogs from litterbox-central.com for the schematic) and a picture of my prototyped board. Notice in the schematic that the IR LED is wired backwards, make sure to wire it correctly. I will fix this in a later version of the schematic. The resistor values are hard to read, so I will add notes for them. All transistors are 2N2222, CORRECTION: they are NOT 2n2222A, it probably doesn't matter though. The red/green wires go to the IR LED, the black/white wires go to the probes, and the two black wires go to a 12 volt wall wort transformer.