[SOLVED] Multiple TTP223 problem

[dzem]

Hello all!

I have run out of ideas on how to resolve the issue I encountered when working on my project. I haven't found any similar problem on the net, so I'm still assuming I'm doing something wrong, but I can't figure out what it is.
I would like to use twelve touch sensors TTP223B - a cheap and very popular module on a well-known Chinese shop. The fabric setup outputs high signal when the sensor is touched. There is also a red LED placed, to notify the status of the output. I've soldered my board with sockets for touch sensor modules. The problem is that I'm unable to make sensors work when more than one sensor is plugged into the board. It seems that the more sensors is plugged in, the bigger the chances of failure.
For example, in this photo there are three sensors plugged and all of them are working correctly:



When my board is powered I can plug the next sensors and still everyone works. In particular, I can add all twelve sensors and things look good:
1.1 the whole circuit takes some minimal amount of current - measured in dozens on microampers.
1.2 each output shows about 2.8V when triggered by finger and 0V when not (board is powered with 3.3V - due to the TTP223 datasheet the output voltage drop is expected)
1.3 the sensor activation takes about 7-8mA (most likely for powering the build in led)
1.4 activation of multiple sensors take the predictive amount of current - the number multiplied by 7-8mA
So far, so good.

However, when I turn the power off and turn it again, ALL sensors are not working correctly:
2.1 the whole circuit still takes some minimal amount of current
2.2 the activation of sensors give too low output voltage - it can be from about 1.1V to 1.6V. Far too low to be recognized as high signal. It is also not enough to make the led shine or makes it barely shine.
2.3 when the sensor is activated the current rises extremely to 25mA.
2.4 activating more than one sensor raises current up, but not linearly - 42mA for two, 63mA for three
2.5 activating four sensors makes sometimes the whole circuit start working correctly... That is weird.

The number of sensors plugged, when the circuit stops working is not obvious. Sometimes two is enough to start the issue, sometimes eight, but the common thing is that the problem is happening after powering my board off and on again. If I start with the powered board you can plug all sensors, one by one, and have everything work. Also, after some time circuit stops working which suggests that it may be tied to the low power mode described in the datasheet.


What I've done so far:
- I've excluded the board soldering imperfections - there are no shorts, output signals are not connected (if it does any matter, all of them are soldered to 16-pin analog multiplexer)
- I've got more than twelve sensors so I can say that it is not a problem with one particular item - I was replacing "suspicious" ones with new ones without any luck
- the datasheet says that the power supply needs to be stable, so I've added a 10uF capacitor to each power input sensor. No difference.
- I've sensor module paths and found that configuration pins (TOG, AHLB) are not grounded. Probably they are grounded internally in the chip, but just in case I've added the ground - no change.

At this moment I run out of concepts on how to trace the problem down. Any ideas what it could be and how to deal with that?
Thank you

[lakis70]

Do you have some of the blue color modules? They different as they have the sensor at the same side of the pcb with the electronics parts and the keyboard as i see is like the blue modules.

[dzem]

I mentioned that the outputs from sensors are connected to the analog multiplexer:

The multiplexer is connected to the power, but the rest of configuration pins (S0-S3) are unconnected. I assume this is causing some random reverse power (?) that gets back to the sensors through the output pin and makes them unstable.
I have connected multiplexer pins to ESP, run the code to control them, and read the output signal. After that sensors start working stable and reliably.