In this circuit the LED lights up when the voltage drops below the value set in the trimpot. The transistors can be the BC548. The circuit is from a 1999 American publication.
November 2016 - The new OLED modulator for displays with 2 x 16 format can be obtained from Mouser...
This is a configuration that can be used in analog calculations by adding and subtracting...
This circuit is from old documentation, but it can be implemented with more modern equivalent...