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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.
Click on Drawing Below to view PDF version of Schematic
