This circuit was suggested by an Electronic Design from the 90s. Its purpose is to detect resistance relationships in circuits with resistive sensors for control purposes, by firing a relay.
This simple circuit for a common 110 V power grid was found in a 1980s American documentation. The circuit uses...
Short pulses can be found in this 90's EDN circuit. The load is 50 ohms and the transistors are from that...
This configuration, quite common and found in other sites, is from European documentation from the...