This circuit was intended to drive a vactrol* in electronic music applications. Is basically a 4 bit R2R network feeding a V to I converter that drives the LED in the vactrol.
* A vactrol is a type of analogue optocoupler composed of an LED and LDR in a light sealed enclosure. Increasing LED current = lower resistance of the LDR. The relationship is very non linear. Can be used to make Voltage Controlled Filters & Voltage Controlled Amplifiers in situations where linearity is not paramount.
