I have posted a fade in fade out (fifo) pummer that operates as you
described. The circuit consists LM358 dual opamp, one of which is
used for a new triangular waveform generator and the other a current
limited buffer to drive the LED. The LED is off for 4 or 5 seconds
and then increases slowly in brightness as the current ramps up to
20mA. The LED stays on at full brighness for 4 or 5 seconds and then
fades slowly back to off. The LM358 standby current is 300uA.
The LM358 can be replaced with LM324 quad opamp to make a dual fifo
Just duplicate the circuit to vary the brighness of two leds at two
different frequencies. The LM324 standby current is 600uA.
Note how the waveform generator uses the voltage drop across R2
caused by the input current to set the lower threshold. The upper
threshold is 100mV less than the saturated high output voltage
The result is a highly exponential triangular waveform which lingers
at the min and max peaks to generate the desired on and off delays.
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