New ACTIVE water sensor mod

102 views

Skip to first unread message

Paul Gumerman

unread,

Aug 21, 2020, 5:38:44 PM8/21/20

to CatGenius

My previous mod to the water sensor worked fine for about a week or so, then seemed to become unstable; the CG would throw either a three-beep error or a one-beep error, I'd adjust the variable resistor to make it go away, it would run for a day or throw an error on the next cycle.

I got tired of that, and decided to deal with it.

First, the new probes: I took two pieces of 3/16" stainless TIG welding rod, cut them to length, and insulated each one with three layers of heat shrink tubing. Then placed the side-by-side, and used a couple of layers of heat shrink tubing to hold them together.

I have a "K-Weld" spot welder, meant for assembling lithium-ion battery packs and some nickel strip. I used that to weld a small piece of nickel to the top end of each probe, and then soldered my wires to the nickel strip. You can use *something* to make that connection, perhaps a small clamp, if you don't happen to have a spot welder available.

The important things about the probe assembly are 1) there is sufficient separation so that a water droplet will not "stick", and bridge the two probes, 2) the tips should be staggered by a small amount, perhaps 3/16", to discourage cat hair from bridging the probes, 3) at least 2" of the stainless should remain exposed at the business end of the probe to make sure that the water never reaches the place where the outer covering ends to avoid having a "bridge" of moisture there and 4) the water level will be set by the length of the *shorter* of the two probes.

The circuit is dead simple. There's a single chip, a CD4093B quad-NAND gate with schmitt trigger inputs, and two resistors. The resistors form a voltage divider, cutting down the +5 to about 3.3 volts. That's well above the 4093's B input threshold. When the probes hit water, the voltage goes down to the 0.1 to 0.3 volt level, which is well below the input low threshold. The schmitt trigger inputs make for clean and certain state transitions. The extra power consumption of this circuit is in the microwatt range.

See photos below:

Yes, that's a 14 pin chip in a 16 socket; I already had it soldered in place when I realized the mistake.