This reader with TTL logic compatible output uses an LM139 comparator. The circuit was found in the 1994 National Semiconductor Linear Applications Handbook.
I found this circuit in a Popular Electronics from November 1999. The magazine is no longer...
This circuit of a 1991 PE Hobbyst Handbook allows the brightness of the incandescent lamp to be...
This simple operational amplifier tester was published in the November 1987 issue of Radio...