> I'm looking for a single supply voltage to frequency
> converter.
...
> 10KHz is fine.
> US$ 5 for all the parts including an optoisolator.
> Size isn't a problem but fewer parts is always better.
> Square wave is what I need.
> 3% accuracy 0 to 50 C.

Perhaps PIC10F, a capacitor and a few resistors could do the job.

- let the voltage input to charge the capacitor;

- when the PIC's comparator detects the certain voltage is reached on
the capacitor - the PIC's pin would be changed from "input" to
"output" and the capacitor would be discharged;

- the pin would get back to the "input" state waiting to detect the
voltage on the capacitor;

The higher the input voltage - the higher frequency of the process.
With the proper parts it should work for "3% accuracy 0 to 50 C", I
believe.
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