> Opto couplers are cheap and simple. It's highly unlikely that the LEDs
> will light except when driven by the switch closures.

Oh yeah?  I have heard of just this happening in an Automotive
environment (big long truck actually). The trick is to put zeners in
series with the optocouplers so they need a signifcant current AND
a significant voltage before they light up. Have seen this in other
commercial equipment too where a certain amount of induced EMF
is practically unavoidable. Hope this helps.

Brent Brown
Electronic Design Solutions
16 English Street
Hamilton, New Zealand
Ph/fax: +64 7 849 0069
Mobile: 025 334 069
eMail:  brent.brown@clear.net.nz