Electrical Issue: PIC input false triggering, Signal Noise suspected.

[abecker]

I am posting this reply to my first inquiry for those in the future who are merely browsing may want to know the results without going down through the many threads in detail. Having said that I would encourage any person who needs to solve a similar problem to dig down through (the threads of) this exchange of great ideas and suggestions.

I am happy to report that my false triggering problem has been solved no doubt to a combination of systems suggested here. I will summarize what I did for others to consider.

- I added bypass caps to all power supply traces across the +5 & Gnd.
- I installed a common ground bus connected to the house power earth ground.
- Since I am using a wall-type 5vDC power supply without a ground and a plastic enclosure, I grounded my main control board to the bus.
- I used a shielded cable between the reed switches and the control enclosure and connected the shield to the ground bus(see attached schematic.)
- I added bypass caps to each of the 3 reed switch signal lines.
- I decreased my pull-up resistor values on the signal lines from 4700ohms to 47ohms
- I have not yet received my inductors but have etched a small carrier board (no room left on the main board) to connect them as shown.



I have not needed to put in place the software-based solutions but they are very interesting and may find their way into future work.

Once again I thank all who contributed to this discussion. There are many new details for me to apply to the next project.

[AvionicsMaster1]

It would be interesting if you could do one of the above solutions at a time and narrow it down to which one or combination of the several solved it.

The 100uH coil going to ground isn't doing anything and could be deleted. I'd suggest only having one ground for the reed switch and shield preferably as close to the PIC as possible.

Did you choose the cap and the coil values for any particular reason? I'm assuming you're using a PWM controller and wonder if you could tune them closer to your PWM frequency to reduce spurious noise further. Usually I have many caps laying around and only a few coils so reducing your cap size would be the easiest.

Love the train.

[Dave]

Wow, Twisted/shielded cable, Inductors, 47 Ohm resistors, and capacitors.... Why not just use some opto isolators at the PIC side and run some current, say 10 mills or so thru the reed switches with 20 gage unshielded wire and call it a day. I can't beleive this topic is still doing.