PICs and harsh environments

[Nicmus]

Hi everyone,

I’m planning on building few gismos for automotive application based on PIC microprocessors and I will need to interface with some custom sensors and other modules. I will be dealing with ADC input, and digital inputs and outputs. Because this will be a long term step by step project I would have used and unused PIC I/Os for expansion.

The question I have is does anyone recommend a particular configuration of the I/Os so they are safe from the extremely harsh environment on both aspects: electrical and mechanical / chemical (vibrations, water and (or) other fluids).

I have some experience and I got good results potting the PCBs with different epoxy type products which will take care of the mechanical aspect. I will also make sure I will filter my 12 V in as well as possible along with the 5 V line and the mandatory decoupling capacitors next to VDD – VSS pins.

What I’m looking for is some kind of protection so my I/Os don’t get randomly zapped or fooled by EMI. I’m not dealing with extremely fast signals so I guess just a T like configuration, limiting resistor series + capacitor to ground + limiting resistor series might do. Should I go to the extreme of using voltage suppressors?
Also, how do I deal with an ADC input without messing up the input voltages (I expect 0 – 5 V full scale) and without loosing resolution?

Any input will be appreciated.

Nick

[erice1984]

I recently made a circuit thing, it worked great in my "lab" environment but once I put it in the car, it made this high freq ringing, extremely annoying and took it out. In the mean time I am trying to figure out what I 'left' out of the build.

I would say yes to the capacitors... this I noticed from looking at other schematics and re-reading what I should have already known about it.

I am curious to see what advice is given from the seasoned vets, /subscribed

[aratti]

The question I have is does anyone recommend a particular configuration of the I/Os so they are safe from the extremely harsh environment on both aspects: electrical and mechanical / chemical (vibrations, water and (or) other fluids).

Potting the PCB with epoxy products will solve the mechanical and chemical problems, as far as the electrical isolation is concerned, use optocouplers TLP504 for digital I/O and HCNR200 for all your analog staff.

Al.

[Nicmus]

Hi Al,

Thank you for your input.
I've been using optocouplers with good results before but had never used HCNR200 for analog I/Os. I'll give it a try.

Regards,

Nick

[Ioannis]

And a regulator for your logic/analog/digital circuits. Do not connect anything directly on +12V. Instead use lower Vcc for analog or digital about 8-9 Volts and 5 or 3.3 for the controller. So a regulator with large enough capacitors will almost eliminate your problems along with the other suggestions by Al and erice1984.

I once (and more inexperienced in electronics) had a major problem with C-MOS used as an alarm in my dad's car. Every time the alarm was triggered, the horn was generating so much electrcal noise that reset all circuits. A 7808 saved the alarm from the trash.

Ioannis

[erice1984]

I figured out the decoupling capacitors after I had made this circuit and it made this high pitch ringing noise... very annoying. At least I think that was why it was making noise.

[erice1984]

use this diagram, I forgot to add a few things..

[mister_e]

R3 and R4 will give you some problem, this makes a great voltage divider but will give you false logic level.

Is this a Typo or those 220nf are 220uF?

[mister_e]

something is weird in the way you hook Q3 and Q4

[Ioannis]

Yeah, GND must be Vdd...

Ioannis