Hi,

yes Opto isolation is a cool idea but he may be having some budget constraints.

Shahzad, you may use ULN2003 transistor array already arrange in a Inverting configuration.

But the thing is as long as the loop is at 5 volts then transistor will remain ON. the moment the loop floata to 0volt then the transistor may not be for sure OFF, cuz you may be using some pullup at the base with the +ive rail.

So you will never get a ground at the put when the input floats.

In such a case Mahmood's idea of opto couplers is the best working idea.


Babar