I have 10 circuits each consisting of a Touch IC (Atmel at42qt1010) and electrode, and 2 x 350mA Constant current regulators (NSI50350AST3G) and LED's. All connected to a Arduino Mega, by ~1 meter cables.
The header pin is as follows:
- +5v (for Touch IC)
- LED2 (HIGH switches circuit on)
- TOUCH OUT (When touch is detect this is HIGH)
- LED1 (HIGH switches circuit on)
- +7.5v (for Constant current regulators)
So on all circuits 1,5,6 are commonly connected. 2,3,4 are isolated and connected to the MCU.
The problem occurs when switching pins 2 & 4, doing so seemingly causes random circuits to output 3 as HIGH. Even if I switch the LED'S on say circuit 1, circuit 5 may falsely start detecting a touch.
The only way I have found to remedy this is two always maintain the same loading, for example on circuit 1 if LED1 is ON & LED2 is OFF, if I toggle there states at the same time, everything is OK. However if there is a delay say 20ms, it will cause the problem. Or if I want them both off, the major problem I have now...
This behavior can also be seen (not to the same extent) when +7.5v, is not even connected.
Attached is my schematic. I have 10 of these, connected to an Arduino Mega. +5v is provided by the Arduino and USB +7.5 is provided by an external supply. GND's are connected.
I am not using PWM, just HIGH and LOW to control the LED's
Please help! I thought I had this sorted but a couple of days before the project goes out I find this bug.
After following the advice given here, and reading these further application notes, I have revised my schematic and also incorporated a three channel LED, rather than the two previous LED's. I will also plan to run these with a PWM signal now.
The Application note 'Secrets of a Successful QTouch Design - Atmel Corporation'
The Application note 'Power Supply Considerations for Atmel Capacitive -touch IC's' Shows a LM78L05 regulator
PCB Layout 3.0 - Routing and trace widths still need to be optimised.
1.Should I use the LDO regulator mentioned in the above application note instead?
2.Will I be OK using this 7.5V - 10V across 10 of these circuits, therefore having 30 PWM high current circuits impacting on each other? It will be provided by a switched mode psu such as http://uk.farnell.com/xp-power/jpm80ps07/psu-80w-7-5v-10-7a/dp/1109830
3.Have I gone overboard with the 3x 100uF electrolytic caps?
4.Anything else I should do? I will then re-design the PCB.