In this circuit the intensity of LED will vary in a ramping fashion. The circuit consists of three ICs: Two 555 timer ICs and one LM393 op-amp. IC1 and IC2 are wired as oscillators to produce 10 KHz and 1 Hz frequencies respectively.These two frequencies are given to the inputs of the op-amp LM393.LM393 is wired as a comparator and its output will be a PWM signal. This PWM signals controls the FET Q1 to drive the LED. The LED will rise from OFF state to full brightness slowly and then slowly fades to OFF state and this operation repeats. The resistor R4 controls current through the LED.

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Notes.

    Assemble the circuit on a general purpose PCB.
    The ICs must be mounted on holders.
    The power supply for this circuit can be anything between 3 to 12V.
    I recommend 6V for this circuit.
    The effect can be varied by changing the values of C1, C2, R1 or R2.

   

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