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That would only be true if rain and the traces were perfect conductors, but they aren't. This kind of sensor will show a difference between a few drops and several. It doesn't have to get completely covered to reach minimum resistance, but it can tell the difference between two drops, five drops, and twenty.That said, this isn't a rain gauge, and it's main job is to show whether or not there is surface moisture. It can detect trace amounts that a rain gauge won't. It can also detect dew and frost, and, more importantly, it can show how quickly surface moisture evaporates. This is important to know when, for example, one is scheduling automatic watering, since wet leaves aren't good for most plants.
Or you could just buy a quad ADC and use it for four inputs, no multiplexing needed. After researching this today I picked up a quad ADC from Aliexpress for $2.25. Just search for ADS1115 for 16 bit and ADS1105 for 12 bit. You can read the channels with I2C.
There's a voltage drop across diodes because diodes have resistance (and capacitance, but that's another story). The drop depends on the current passing through the diode. Diodes aren't really one-way valves that are totally closed in one direction and totally open in the other, although in most applications we can use them that way, as long as we stay within their specifications.
Yes. The output would be influenced by both sensors.Since diodes have a drop across them that varies by current, for more accurate readings you'd be better off using a multiplexer chip. You can get 8x1 and dual 4x1 multiplexers for a dollar or less.
Without the diodes you'd have both sensors in the circuit at the same time, and the one powered down could be shorting the output of the other to ground.
Better quality is mostly a function of the glass.You could do this with a point-and-shoot if it had a compatible shutter release interface. At ten feet you'd still need decent focal length and, for the blurred background, a large aperture.