This reader with TTL logic compatible output uses an LM139 comparator. The circuit was found in the 1994 National Semiconductor Linear Applications Handbook.
This circuito from the 1977 GE Semiconductor Manual circuit provides timings from 0 to 90 seconds...
Suggested by General Electric in a 1965 document, this circuit can accept a greater number of...
This circuit with an unusual configuration was found in the January 1974 issue of Radio...