This circuit from a Popular Electronics from the 90s uses a common optocoupler and a general-purpose transistor to control a sensitive relay that can be from 6 to 12 V.
With this configuration we have a voltage follower, amplifier with unit gain, in which the output...
This circuit is from the Linear Circuits Application of Texas Instruments, using an operational amplifier...
This amplifier, suggested by National Semiconductor (Now Texas Instruments), has high input...