Andy, my first thought is to also to use a V/F or VCO. While there are
several V/F and VCO chips you might want to take a look at classic old 555
timer circuits. One circuit that comes to mind is to use a voltage
controlled current source to control the charge of the 555's timing
capacitor. TI has an application note using a 555 and their TL011 current
mirror which is incredibly simple. It only requires 2 caps, 2 resistors, and
a diode. The reference design uses a 10-50V input and a 15V supply but you
can easily change it. The advantages are low cost and a single opto-isolator.

   My next choice would be a serial A/D. Another option here is the PIC12C67x
though I'm not sure of current pricing vs low-cost A/Ds. It would allow
pre-processing if needed. In both cases, you can use a dual opto-isolator.

   Someone else mentioned isolation amps from AD and Burr-Brown but they are
fairly expensive. However, they are very accurate and rugged.

   - Tom

At 09:09 AM 5/27/98 -0400, Andy Kunz wrote:
>At 11:59 AM 5/27/98 +0930, you wrote:
>>Hello
>>
>>Maybe set up a Voltage to Frequency converter powered by the 3-30 Volt
>i/p, use a standard opto isolator and count the pulses?
>
>That's a pretty neat idea.  I'll have to investigate that.
>
>Thanks!
>
>Andy
>
>
>==================================================================
>Andy Kunz - Statistical Research, Inc. - Westfield, New Jersey USA
>==================================================================
>
>