Trying to use AT42QT1011 for a touch (and if I am lucky - proximity sensor) as follows (from datasheet):
The Cs is 10nF, Rs about 330 kOhm (also tried without). Vdd is 3V from batteries. SYNC/MODE is permanently connected to Vdd.
Getting no signal with or without pads ("sense electrodes") on Vss / SNSK in any combinations.
What is interesting, I am getting pulses on SNS, and also when I pull-up OUT, I am getting heartbeat pulses, as per datasheet. So I am assuming the IC is working. Checked pins of IC do have proper connections and no short-cuts between neighbouring pins.
However, I never got even a glimpse of "1" on OUT, and I suspect that something is wrong with my setup. I would understand false positives / too much sensitivity, but having no response at all puzzles. No more ideas how to troubleshoot this further.
(this is a follow up from Simple capacitive proximity/touch sensor problem after which I decided to try specialized IC)
Update: I suspect I should use 2.2 nF, not 10pF capacitor for Cs. (fixed, now 10nF)
Update 2: here goes actual schematics. I still do not understand why it work backwards: when not touched, OUT issues pulses ~80ms (in addition to ~1 ms heartbeat ones) - LED visibly blinks. But when I touch the end of Rs, only heartbeat pulses are left (seen on the scope).
However, when I attach large electrode, LED no longer blinks, but when my hand nears the electrode, there is a burst of pulses, which is visible as a weak single blink...
From datasheet:
Output The output of the QT1011 is active-high upon detection. The output will remain active-high for the duration of the detection.
There is a third kind of behaviour, when the electrode is very large: LED is on all the way, sometimes off.
So something must be wrong.
This is on proto PCB (IC and capacitors) and breadboard (transistor and LED), and I am planning to use a shielded cable to connect electrode with a PCB. I guess I must not connect the shielding to anything.
simulate this circuit – Schematic created using CircuitLab
Also tried to add 1kOhm resistor between OUT and transistor, the result was the same.
Update 3: When I connect scope channels to OUT and SNS (both DC coupled, 1 MOhm input resistance), GND to Vss, then I observe perfectly working sensor. Now I need to understand how to achieve at least the same with debug probes disconnected.
Update 4: Experimented with Cs capacitance. And it occured to me, that I needed to make Cs different, so I used 22 nF capacitor, and now the sensor works as expected! Maybe, those 80ms pulses were ICs way to tell the Cs is too large/small (undocumented feature?).