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The circuit below was designed to drive a LED with an intensity ramping mode. Two 555 timers generate two different triangle waveforms.
The upper device generates a 10KHz signal while the lower unit produces a 1Hz signal. The two signals are fed to a voltage comparator. The result is a pulse width modulation (PWM) signal, which with the aid of the FET, drives the LED in such a way that its average light output slowly ramps from about zero light to maximum and then slowly dims back down.
The circuit should operate over a supply voltage ranging from 3v to 12v. You can easily vary the ramping time by changing the value of the 1M resistor. For an interesting effect, you can place a 1N4148 diode in parallel with the 1M resistor, with the cathode (banded end) side connected to pin 3.
Content By http://www.discovercircuits.com/DJ-Circuits/rampingLED-driver.htm
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