I've found one simple schematics for capacitive sensor on electronics-project-design.com, and implemented it as follows:
The B? is 9v battery. Sorry for the odd-looking schematics: gschem usability is near zero. And the device is NE555, not LM. The plate is ~10x10 cm aluminium foil. Q1 hFE is over 300, if that matters.
At first, I checked with a 'scope at the Q1 collector and 8k resistor instead of led and smaller resistor. And, wow! the plate worked most of the time as a proximity sensor.
Then I decide to try with a led (without the 'scope), and the proximity sensing was there no more. (Needless to say, the circuit is not connected to anything, works on battery, build on a breadboard) But then the thing worked even better as touch sensor... Except for I needed to touch the ground T0 to activate. To be honest, when the plate is connected to the ground, touching R1's open end has the same effect, which is reasonable as the body makes a capacitor with the plate.
Thus, the questions:
1) Does this design have any problems if the logic output will be connected to some other circuit? Should I use optocouplers for instance to prevent noise? update: one problem I found after writing this is that when I switch off the table top lamp, the sensor works no more... Is some kind of oscillator needed after all?
2) Is it possible to turn this into proximity sensor (1 cm distance is enough) or does it make sense to go with ICs like QT113 and the like? Will proximity work outside, where there is no low-freq electric lines noise?
I have not yet decided whether the sensor output will just plainly switch something ON/OFF or whether MCU will be required for more advanced logic. Probably the latter. But for sure there will be two sensors used.
There are so many capacitive sensors design out there, so I chose what seemed simple.