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

[abecker]

No offense taken.

Nobody has ever accused ME of being a genius but my tenacity has lead me to many learning opportunities.

I will try several of the strategies offered by the contributors and hopefully if the problem gets resolved soon, I will report back.

[richard]

I discovered this curious method of debounce/noise filtering on the arduino forum this week . the idea translated to pbp easily and makes my worst cheap and nasty tactile switches perform as smooth as silk.
here is a little demo (might add there is no cap used at all on the switch )

'****************************************************************
'*  Name    : debounce.BAS                                      *
'*  Author  : richard                   *
'*  Date    : 2/17/2016                                         *
'*  Version : 1.0                                               *
'*  Notes   :  pic16f1825                                       *
'*          :   switch debounce                                 *
'****************************************************************
#CONFIG
             __config        _CONFIG1,    _FOSC_INTOSC & _CP_OFF & _WDTE_ON  &  _PWRTE_ON  &  _MCLRE_OFF  & _CLKOUTEN_OFF
              __config      _CONFIG2, _PLLEN_ON & _LVP_OFF            
#ENDCONFIG
 
OSCCON=$70 
DEFINE OSC 32
ANSELA=0
ANSELC=0
TRISA = %011010
TRISC = 0 
 
 
led1 var lata.5    ;  lit when  sw operated
led2 var lata.2    ; toggle when sw   operated
sw var porta.3     ;  the switch (active low ie has pull up resistor)
old_sw_state var bit
sw_state var bit
sw_buff  var byte
clear

main :
    gosub ck_sw
    if  sw_state != old_sw_state then   ; the sw has changed either on or off
        led2 = !led2
        if !sw_state then led1 = !led1      ; if state is changed to on
        old_sw_state=sw_state            ;remember state
    endif
goto main

ck_sw:
    sw_buff= (sw_buff<<1) | sw
    if (sw_buff=$7f ) and sw then
       sw_state=1
    elseif  (sw_buff=$80 )and !sw  then
            sw_state=0
    endif
return

[abecker]

Hi Richard,

This a very interesting strategy and I have added it to my collection of code snippets.

On this project my approach is to try the hardware methods first since they (like ground shielding the signal line, etc.) are just good practice anyway. If that doesn't do the trick I will try some of the software solutions. I hope this method will be a way to somewhat isolate the cause of the false triggering or at least give an idea what might be going on.

This project has even forced me to get out my really old and clunky oscilloscope and get some new probes for it. Now I just have to figure out how to set it up to check for noise on the signal line. I'm a bit rusty on using the scope for diagnostics. It's been quite a while since my last class in EE.

Thanks for your thoughts,
Allan

[mark_s]

There are a lot of good suggestions here. One possible cause could be a magnetic field being set up by current passing through the rails.
This could be causing the reed switches to randomly close. Or magnetic coupling into the long leads. Google "Right Hand Rule"

It would be easy to test. Leave everything connected as is, but move the reed switches a few inches away from the tracks.

Hope you figure it out

[Heckler]

@mark_s
I would add that moving the reed switches a few inches away but leaving all the wiring as is would likely cause the reed switches to stop functioning but that is OK as it would allow you to determine if mark_s's theory holds true.

If you still have false trigering then it was because of either the magnetic coupling or possibly vibration.

At least that is what it seems you were getting at with your suggestion (or am I missing your point Mark?)

@richard

Here is how I devised a software debounce that would not block the rest of my main code loop

Main:
if swflag = 1 and sw1=open then swflg = 0  '(sw debounce) clear swflg only when switch released
   
if swflag = 0 and sw1=closed then   'sw1 button detected and valid
        swflag = 1      'set flag that switch has been closed
        (code goes here that you wanted the switch to do)
endif

(more main code loop goes here)
GOTO Main

it requires the swflag = 0 before it will acknowledge the sw1 is pressed. Other wise the switch will be ignored.
the only way for swflag to return to 0 is for you to release sw1

[mark_s]

@mark_s
I would add that moving the reed switches a few inches away but leaving all the wiring as is would likely cause the reed switches to stop functioning but that is OK as it would allow you to determine if mark_s's theory holds true.

If you still have false trigering then it was because of either the magnetic coupling or possibly vibration.

At least that is what it seems you were getting at with your suggestion (or am I missing your point Mark?)

Dwight,

Yes, just move the reeds temporary to see if they are triggering. If that is causing the problem, maybe just by rotating the reeds 90 degrees would solve the problem? Abecker made two statements that are key - "It worked superbly on the bench" and installed with the reeds disconnected, it quit triggering. So it's probably interference entering the cable or the reeds closing?

Abecker should take note when it is triggering. On acceleration the motor will be drawing the most current. On deceleration it could be generating inductive spikes back into the power supply. Does it false when the train is not moving?

Brings back memories, my dad had a large H0 gauge train table when I was a kid. We couldn't wait for him to get home from work
so we could run it.

[abecker]

Mark,

I'll address the second remark in this reply. The first time I discovered the problem (and it did occur consistently) I would have to say there is no acceleration or deceleration possible because the speed control is set and remains at a preset DC voltage (I have no means of attenuating the speed control, electrically or mechanically.) Knowing this I suppose it is safe to conclude that there is a considerable inrush of current when the control relay closes.

As I recall from the first real run the PIC firmware closed the "Run" relay contacts and the trolley moved a few inches in the correct direction. Then operation jumped to the next logical state which was sensing the trolley position and stopping at the station. The only problem being, it was not near enough to the reed switch to activate it using the magnet below the car.

You ask a very good diagnostic question which I have not tried yet. I should test for false triggering without the trolley (dc motor) running but voltage applied to the track and overhead wire. This should rule out any influence of those objects. Lately I have been asking myself the uncomfortable question as to whether I have been wrongly suspecting DC motor EMI of causing the problem. This test would help eliminate the transmission sources from the cause.

I hope my work later today will shed more light. Thanks for your insights.

[abecker]

Hello Mark,

I have invested some time in running the long signal leads in shielded cable and grounding it and everything I can think of to the earth ground of my 110vAC power cord. The main control board being in a plastic enclosure also now has a ground strap to the same ground. If I don't get positive results from this and other remedies offered here my last recourse is to fabricate a steel enclosure and ground it as well.

There is one construction hitch to moving the reed switches (although I agree it would be a sensible test) in that the two lead wires run down through holes in the table. I would have to fasten some type of disconnect there to make them removable. I soldered the leads to the grounded cable with the shortest possible length to reduce the chance for EMI pickup. I try to avoid many connectors for I feel each one is a potential weak point.

Later today I will have everything wired and ready to test except the adding of inductors since they are still on their way from Mouser.