This circuit without sensors in series triggering an SCR was found in volume 5 of the 1995 Encyclopedia of Electronic Circuits. The circuit can have the number of sensors changed.
An advanced motor control software plug-in for Microchip’s MPLAB® X Integrated Development...
This relay without lock uses a general-purpose transistor. The relay is according to the supply....
This signal follower is from a 1970's documentation. So it uses the traditional configuration at the time...