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This circuit could be used to control the speed of a DC motor with a pulse with modulated signal output. It can also be used to control the brightness of an LED. The input being a simple AC voltage signal the first op-amp turns it into a square wave, this is called a Schmidt trigger. Basically the positive feed back the output of the first op amp makes it hit the voltage rails on the top and bottom creating the square wave. The square wave feeds into the second op-amp which is an integrating amplifier with the cap placed on the negative feedback branch. The output is a triangle waveform. This triangle wave feeds into the third op-amp, which is a comparator op-amp, it compares the voltage input of the triangle wave to the voltage input from the potentiometer that you set. The pot voltage is the red straight line, whenever the blue triangle wave is above this line, the third output hits positive rail, or 6V, once the triangle wave goes back below the set red voltage line from the pot, the output(orange) goes to zero. Therefore, just by adjusting the potentiometer, you adjust the duty cycle of the signal and power to the DC motor. When half the time voltage output is +6v and the other half 0v then half power is going to the motor. This switching on or off happens so quickly that to the motor it seems more like a constant signal. The long the +6v output that's on compared to when it's off, the more power to the motor. This can also be used to control brightness of an led. The light is either on or off, but changing the duty cycle changes the time of when it's on compared to when it's off, though it turns on and off so fast that to you it will always look like it's on. If it's on half the time and off the other half than the led will like half has bright. If it's on one fourth time and off three fourths time, it will only look one fourth as bright and so fourth.
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