Fading LEDs schematic

Posted on May 8, 2012

This circuit is used to slowly illuminate and fade a pair of red LEDs (light emitting diodes). The fading LEDs could be installed as 'eyes' in a small pumpkin or skull as a Halloween attraction, or mounted in a Christmas tree ornament. Or, they might be used as a fancy power indicator for your computer, microwave oven, stereo system, TV, or other appliance. In operation, a linear 3 volt (peak to peak) ramping waveform is generated at pin 1 of the LM1458 IC and buffered with an emitter follower transistor stage. The 22uF capacitor and 47K resistor connected to pin 2 establish the frequency which is about 0.5 Hz.

Fading LEDs schematic
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Fading LEDs schematic - image 1
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You can make the rate adjustable by using a 100K potentiometer in place of the 47K resistor at pin 2. The circuit consists of two operational amplifiers (opamps), one producing a slow rising and falling voltage from about 3 volts to 6 volts, and the other (on the right) is used as a voltage comparator, the output of which supplies a alternating voltage switching between 2 and 7 volts to charge and discharge the capacitor with a constant current. Each of the op-amps has one of the inputs (pins 3 and 6) tied to a fixed voltage established by two 47K resistors so that the reference is half the supply voltage or 4.5 volts. The left opamp is connected as an inverting amplifier with a capacitor placed between the output (pin 1) and the inverting input (pin 2). The right opamp is connected as a voltage comparator so that the output on pin 7 will be low when the input is below the reference and high when the input is higher than the reference. A 100K resistor is connected between the comparator output and input to provide positive feedback and pulls the input above or below the switching point when the threshold is reached. When the comparator output changes at pin 7, the direction of the current changes through the capacitor which in turn causes the inverting opamp to move in the opposite direction. This yields a linear ramping waveform or triangle waveform at pin 1 of the inverting opamp. It is always...

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