I discovered this circuit in Radio Electronics magazine in 1962. The circuit drives a special control motor. Power comes from the power grid.
This simple indicator circuit, found in a 1997 documentation, indicates when a soldering iron up...
In the figure we have the way to linearize the response of a TTL port with the use of a feedback...
The operational amplifiers used in this circuit are old. The circuit is from documentation of the 70's, but...