This is an 8-bit DAC-controlled pulse-width modulation circuit. The 4 pushbuttons control the pulse width via 2 counters.
The outputs of the counters is fed to a set of resistors. The most significant bit has a value of 10k, with each lesser but getting a resistance twice the previous.
The voltages are combined in DAC Stage 1 (the bus connecting the resistors), peaking at about 120 microvolts. This is then fed to DAC Stage 2 (lower middle), a set of op-amps which multiply the voltage to a range between 0 and 12 volts.
A voltage divider, which includes two potentiometers, manipulates the output range of the DAC stage to have a range which coincides with the upper and lower peaks of the 555-generated sawtooth. Specifically, the upper potentiometer sets the lower voltage limit, and the lower potentiometer controls the upper voltage limit. A drawback of using a 555 to generate the necessary waveform is slight inconsistencies, resulting in pulse-width variations.
Both signals are fed to a final op-amp (middle right) which generates the actual PWM output. The output drives a transistor switch, which itself controls a single LED.
A similar circuit could, theoretically, be used as an LED driver. A set of the could drive an RGB LED using standard hex color codes.
Scope Legend:
Purple - Output current through LED.
Green - 555 Sawtooth Signal
Orange - DAC Output Voltage
Red - DAC Stage 2 Voltage
