On 6 Apr 2017 at 9:31, James Wages wrote:

> Brent, David and Isaac:
>=20
> Thank you for your replies.

You're welcome.

> Brent, the PCB would be inside a pastic (ABS) case and fitted under
> the dash, inside the cabin.  It would not be near heat sources or
> engine room components, but no doubt it would receive RF noise from
> the driver's mobile phone and from that mobile 10W HAM radio David
> spoke of.

Ok. Not the worst environment. Not the best environment.

> The power to the PIC is filtered as shown in the
> following schematic:=20
>=20
> https://cl.ly/jsiM/Image%202017-04-06%20at%209.15.20%20AM.png

Circuit looks good.
=20
> (+V1 is the 12v that feeds my H-BRIDGE, which in turn connects to a
> 6.5-ohm speaker/siren)=20

Also good. Your micro and control electronics remain well isolated from the=
 higher=20
current paths expected there.
=20
> 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 pull it to ground, but as you point out there is charge
> time to consider.  Even a rather small 0.1uF cap combined with a
> 1M-ohm pull-up resistor connected to 3V would take 100ms to charge.=20
> That's rather a long time even for time insensitive applications.=20

Does it seem reasonable that a person would change DIP switch settings and=
=20
expect to see results in less than 0.1s? That does not seem like a long tim=
e to me.=20
The filtering time constant is what is giving you noise immunity, the longe=
r the=20
better. You decide the value you can live with, can be smaller than 0.1uF.

Design involves compromise & balance. Increasing pull-up resistance reduces=
=20
current, at the expense of noise susceptibility. Increasing capacitance red=
uces=20
susceptibility, at the expense of speed.


--=20
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