For a current of 5A and a supply of 6 V this circuit was found in documentation from the 70s. Transistors admit modern equivalents. Q1 is a PUT uncommon today.
This reader with TTL logic compatible output uses an LM139 comparator. The circuit was found in...
This circuit can be used with common transistors, from general purpose to medium power. The...
Although it is a common configuration today, these circuits were taken from an old publication. The...