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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 DACs. The output of the upper DAC, representing the lower nibble of boots, is passed through a voltage divider to reduce it's voltage range to 1/16th. This is then added to the DAC representing the upper nibble.
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
Red - Raw DAC Voltage
Orange - Adjusted DAC Output Voltage
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