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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 sawtooth. Specifically, the upper potentiometer sets the lower voltage limit, and the lower potentiometer controls the upper voltage limit. The use of a sawtooth generator rather than a 555 allows the waveform to be more consistent. As a side benefit, eliminating the 555 timer and its related components fed up enough space to include displays showing the hexadecimal value of the current PWM setting.
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 - Sawtooth Signal
Orange - DAC Output Voltage
Red - DAC Stage 2 Voltage
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