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

[LinkMTech]

I would really hate to redesign and rebuild my present main board yet again to integrate the inductors.

Lastly since I am new to working with inductors could you spec or describe what I would be looking for number-wise from say the Digi-Key online catalog?

Hi abecker,
The inductors should be in your controller circuit because just as your intuition suggests, you will end up with a short antenna at the input of your box.
If you have room at the reed switch connector, these inductors can be installed at the molex connector you have there by soldering each one inline with the wire going to the switch and the other side crimped to the connector pin. Be sure to shrink tube each one to keep from shorting to its neighbor.
And it looks like you can put your filter caps at each (R1 pullup resistor and PIC input) to the nearest GND.

[richard]

another way is to filter the input with a software routine

reeds var byte
reeds_flg var byte
reed_1_active var reeds_flg.0            
reed_2_active var reeds_flg.1
reed_3_active var reeds_flg.2
reed_cnt var byte[3]
reed var byte
define reed_threshold 64

chk_reeds:
reeds=portc & $70  ;c4 c5 c6  active low
if reeds < 112 then
reeds=reeds >> 4
for reed = 0 to 2
if ! reeds.0 then
        reed_cnt[reed]=reed_cnt[reed]+1 
        if reed_cnt[reed]> reed_threshold then reeds_flg.0[reed]=1
else
        reed_cnt[reed]=0
endif

reeds=reeds>>1
next
endif
return

if you call the chk_reeds subroutine regularly (must be regular) say in this case 100 times per second, then if a reed input is consistently low for 64 (counts in a row the reed_threshold) then the reed_x_active flag will be set.

[abecker]

Thanks Richard.

This has great potential in my code because as you commented, once I set close the realys to the trolley power all I have to do is loop through the routine that checks for a low sensor. To make it easier, the trolley dbeing on a track can only go in on direction at a time so in each stage of travel I know exactly which sensor to poll. Not like I would need an interrupt to see which of the three sensors went low. The time the magnet is over the reed switch is eon's compared to the rate at which the pic can poll the sensor line.

As I mentioned to one other reply, the pic advances to the next logical set of events just when the trolley gets moving and is several feet away from any sensor. This is what leads me to think some DC motor noise or even arcing of the pickup on the overhead wire is being picked up at the sensor pins and causing a false signal.

I will study your code and possibly with it and many of the other outstanding suggestions here, find a solution which I will gladly report on.

[abecker]

LinkM,

I appreciate your idea to inline the inductors and naturally shrink-tube saves the day for so many inline solutions like this.

I did strike upon the idea of soldering the caps to the traces under the board because the board is on standoffs anyway.

By the way in my searching the net for use of inductors and caps for noise filtering, I discovered some LC filter arrays in a single IC type package with 4 per chip, by TDK (series MEA) but it looked like they were narrowly designed and spec'd. for telecom use. Not understanding enough about LC filters I didn't know if the product series could have helped my situation. It was a very slick idea though having 4 filters on the chip. If that worked, it would be nice to put that on the main board in any new design I might come up with in the future.

Thanks for all your help.