This reader with TTL logic compatible output uses an LM139 comparator. The circuit was found in the 1994 National Semiconductor Linear Applications Handbook.
Using the LM3909 this circuit was found in the January 1992 issue of Radio Electronics magazine....
This circuit can work with almost any JFET. The circuit is from an Electronics Now from April...
The presented differential amplifier, indicated for instrumentation, was obtained in the 1994...