Electronic – arduino – Does the Arduino Uno “Clean” the power input from the adapter

Here is a circuit that should work:

Your basic idea of using a high side PNP switch was fine. The problem with it is that this exposes the high voltage to the micro pin.

In this circuit, Q2 is the high side switch that turns the 12V output on or off. Q1 switches the high side switch, thereby isolating the micro from the 12V on the base of Q2.

When the base of Q1 is held at 5V, the emitter will be 4.3V, so there will be 1mA thru R1. Most of this also flows thru the collector of Q1, which thereby acts like a switchable 1mA current sink. Most of this can only come from the base of Q2. Figuring the two transistors each have a gain of at least 50, then the output is good for at least 45mA for Q2 to stay in saturation. The purpose of R3 is only to make sure Q2 is really off when Q1 is off.

Here's an example of a possible solution using MCP23017 and 4066 switch. In that case you read on 14 and 15. In the example there's two sensor. One attached to U3:A:Y/U3:B:Y and the second one attached to U3:C:Y/U3:D:Y.

You control the MCP23017 with I2C, he will turn on or off the switches and so, select which inputs are connected to pin 14 and 15. One output on the MCP32017 control two switches, one wire pair, so one sensor.

I've only place the "useful" connections and tried to make it as simple as possible. So it's just an idea, not a fully working as-is solution. For example I suggest you to place flip flop with latch to control once the MCP23017 are updated when to reflect the updates on the 4066 and keep them as-is until the next latch or find some 4066 switches with latch.

NOTE : I haven't found touch component in the lib excepted this one... But you get the idea with your two wires for your sensor.

For 64 sensors, you can do it with 32 4066 chips and 4 MCP23017. The pins used on the arduino will be 2 for the sensors and 2 analog to control the 4 MCP23017 through I2C so 4 pins in total, even a UNO or ATTiny can do it.

EDIT

Instead of the 4066 you can use CBT3245A, you'll need 16 of them instead of 32 and they are quite cheap too. Also, another alternative would be to use MAX4618, you'll need 16 of them too, but they are quite expensive. 1.8$ each in DIP package on digikey a 4066 is 0.4$ and a CBT3245A in SO package is 0.11$, so 12.8$ in total for 32*4066, 28.8$ for 16*MAX4618 and 1.76$ for the 16*CBT3245A (but SO package, can't find DIP)