I got one of these off ebay and it has 2 outputs marked DO and AO. I assume this means digital output and analog output. I seem to get a digital output on both pins. Has anyone any experience with these things that can shed some light on their outputs?
Maybe D is a straight connection to the LM393 and A is the same output but via a voltage divider?
Seeing a 393 is a voltage comparator, my guess there will not be an analog output.
The schematic shows a pot for adjusting the comparator threshold voltage. However, the photos do not show the pot. The three solder pads on the board underside look like they were intended for a pot. The top side shows a pair of 10K resistors where the pot would be. The parts count suggests the two resistors replaced the pot. The A0 input may then be an external sensor adjuster and the board uses a fixed divider. Since you saw a digital signal on A0 then the trimming resistor may be from the comparator minus pin to ground or Vcc, depending on which way you want to change it. You will likely have to reverse engineer the circuit to be sure.
From some other threads, A0 is an analog output. I would think they just used one of the comparators as a non-inverting amplifier.
See figure 28 here.