We found this circuit in an 80's manual. The same configuration can be adapted to make use of modern operational amplifiers. The power supply is symmetrical.
With this luminous remote control, which we found in a 1990 publication, using a flashlight you...
We found this circuit in a 1967 Electronics World. The circuit can be designed to control other...
This circuit can be used in a signal level indicator with LEDs. The circuit is from an Electronics Now...