PICs and harsh environments

[erice1984]

*sigh* That is irritating....

Fixed T1 and T2

Yet again, another change.... am I the only one this happens to?

[aratti]

erice1984, few comments on your last schematic.

a) NPN transistors are wrongly used (emitter should be grounded).

b) PNP should have a base pullup resistor (4,7K will work).

c) Diode D2 should be placed across emitter and collector of T4 to be of any use.

d) Connector SL1 has two wires missing, since you will need ground and Vdd


Al.

[erice1984]

I was thinking of replacing T1 and T2 with Optocouplers.

[mvs_sarma]

I was thinking of replacing T1 and T2 with Optocouplers.

How about cleaning the power supply? it can also bring in Nonsense. better use a choke of say 1mH and definitely have a 0.1 ML C capacitor across the Vdd and Vss pins and very close to the chip.
Ensure that the device is not close to high current carrying wires, neither the wiring parallel to such wires. If needed use a screened cable

[erice1984]

Interesting Idea, As far as high power wires, this Circuit will be located under the dash, slightly to the left of the steering wheel - about 4" from the fuse box

[erice1984]

schematic .

[mister_e]

Why T4 is PNP, but T3 is NPN?

Opto-Coupler will not work as-is. You should tie collector to VCC, emitter to your PIC I/O + Pull-down. Or variant of it, emitter to GND, collector to your I/O with a pull-up. Internal ones, if available on these pins, could work. Assuming the signal are 12V (or something over 5v), a simple current limiting resistor, or voltage divider could also work.