Fading an LED on and off circuit using 2 Op Amps

Astable Op Amp uses the changing voltage of a capacitor to keep changing the output state (and vice versa). If you’d like to apply the capacitor’s voltage across an LED as well, you can use another Op Amp wired as a voltage follower.

LM358 Op Amp fades an LED on and off while charging and discharging a capacitor circuit schematic
LM358 Op Amp fades an LED on and off while charging and discharging a capacitor circuit schematic

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Anode LED is connected to the positive supply (+5V) in my example circuit.

  • Lower Op Amp follower voltage will put a higher voltage difference across the LED and it’s protective resistor. The LED gets brighter.
  • Higher Op Amp follower voltage puts less voltage difference across the LED and it’s protective resistor. The LED gets dimmer.

Op Amp inputs don’t pass current (just a very small leakage). They just look at the voltage applied to them as far as must circuit are concerned.

  • Op Amp follower non inverting (+) input looks at the capacitor’s voltage in this circuit. The Op Amp follower’s output voltage is connected directly to the inverting (-) input. The Op Amp output voltage instantly goes up or down as needed to make the voltage at both inputs the same.


The astable multivibrator Op Amp  has 2 changing voltages at it’s inputs as well. The capacitor is at the – input, while a divided supply voltage that is shifted based on output state is a reference voltage at the + input.

  • As soon as the capacitor charges above the reference voltage, the output goes low, which drops the reference voltage, and starts discharging the capacitor.
  • As soon as the capacitor discharges below the reference voltage, the output goes high, which raises the reference voltage and starts charging the capacitor.

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