Harrison;
Easiest way that I know is to drop the 4-20mA across a 250 ohm resistor, then
 run
the 1-5VDC signal to the base of an NPN transistor THROUGH a 3.3V zener diode
 and
resistor in series.  The series reistor limits current in the on position and
 allows
you to limit leak current to an off level in the non-zener-on-position.  The
 emitter
goes to ground.  The collector, via pullup, goes to the input pin on the pic.
 If
you skip the zener, and try high values for the base resistor, you may get it to
work today, and the next batch of NPN's that you get with a beta that is off by
 a
factor of three don't work for shit in your circuit.

Harrison Cooper wrote:

> Isn't it nice when a client says...and use THIS sensor (none other), to turn
 on
>  and off
> your stuff (i.e., an inhibit TTL signal).  Well, as it happens to be, the
 output
>  of
> this particular level sensor is 4-20mA.  When it reaches a certain level in a
>  tank, the
> output is in that form (low level is 4mA, high level is 20mA).  So, at 20mA,
 we
>  are supposed
> to inhibit our stuff from running. Simple..done it before, but either with a
 PLC
>  or a hard
> contact.  Sorry to bore you all with this....
>
> SO...rather than reinvent the world, has anyone built a circuit, doesn't have
 to
>  be PIC based
> exactly, to monitor a 4-20mA loop and just switch a TTL signal when it reaches
>  20mA? Yea, I
> could build a window detector, or use a ADC and PIC, but time is a problem so
>  just need a simple
> and inexpensive solution.  Otherwise, I can use a couple LM311's and such...
>
> Private mail is fine to save on bandwidth.  hcooper@es.com