Brent, David and Isaac:

Thank you for your replies.

Brent, the PCB would be inside a plastic (ABS) case and fitted under the da=
sh, inside the cabin.  It would not be near heat sources or engine room com=
ponents, but no doubt it would receive RF noise from the driver's mobile ph=
one and from that mobile 10W HAM radio David spoke of.  The power to the PI=
C is filtered as shown in the following schematic:

https://cl.ly/jsiM/Image%202017-04-06%20at%209.15.20%20AM.png

(+V1 is the 12v that feeds my H-BRIDGE, which in turn connects to a 6.5-ohm=
 speaker/siren)

Isaac, having a capacitor connected from the PIC input pin to Ground would =
keep the voltage HI on that pin longer when a noise glitch is trying to pul=
l it to ground, but as you point out there is charge time to consider.  Eve=
n a rather small 0.1uF cap combined with a 1M-ohm pull-up resistor connecte=
d to 3V would take 100ms to charge.  That's rather a long time even for tim=
e insensitive applications.

James W.



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