Using the 2N2646 this circuit was found in a Radio Electronics magazine from July 1968. The transistors are from the time and the lamp can be replaced by a high-power LED in a current version.
We find this circuit in an American 80's documentation. It makes use of IC sensor which is mounted...
This is a configuration obtained from an old manual, using an operational amplifier of the time....
In the figure we have the way to add hysteresis to an operational amplifier. The relationship...