A newbie switching an analog signal (I think)

Thread Starter


Joined Dec 23, 2019
Hello everyone and thanks in advance for help with this not fully formed question!

I have a PLC-like device that lets me programmatically control various physical power outlets for my aquarium. That device allows for input devices like a thermometer, pH reader, etc. One of those input devices is a water overflow detector, which, from the software interface, shows up as on or off when it detects (or doesn't detect) the physical presence of water on the ground. This device is very simple. After splicing its two wires, I know that the software shows an 'on' signal when the wires are connected and an 'off' signal when they aren't. When I take a multimeter to those two wires, I see quickly varying voltage reading between ~0.3V and ~4.6V, so I assume it's an analog signal of some kind?

I want to retrofit this input switch through a simple circuit whose effect is like me connecting those two wires together. That connection should be triggered by this LED phototransistor which senses the presence of water in a plastic tube. This device is just made up of this IR diode and this NPN transistor. When it senses water, I want to effectively connect those two wires going to the controller. But I guess I can't do this with normal transistor because I'd mess with those varied voltage readings I observed.

Here is a simple diagram of where I am so far.

In short, assuming that what I'm seeing is an analog signal, how would a newbie cheaply/easily switch that signal using a LED phototransistor? What characteristics do I need to know about the signal and how do they affect what I'll need? Thank you in advance and happy new year!