Found in Linear Technology documentation, this circuit drains only 70 uA of the monitored circuit. The components used are from that time.
In the figure we have the way to activate a red LED that needs 1.7 V or even others with higher...
This circuit was obtained in a 1979 publication, but it can be assembled with the BD135 instead of the...
Found in Radio Electronics from the 90's, this circuit employs a 3140 operational amplifier, still...