In this instructable i show you a simple circuit to fade a LED using a sinusoidal waveform.
Step 1: Circuit
In the circuit, a twin-T sine oscillator, build around Q1 is used to slowly fade a LED in and out.
The twin-T filter is formed by R3,R4,C1 (first T) and C2,C3,R6 (second T).
The T-filters give a 180 degrees phase shift at the resonance frequency, at which the impedance of the filter is at it's minimum.
The divider created by R1 and R2 is used to offset the voltage that is converted into a current by Q2 and R7. R1 and R2 are dimensioned to get a sinusoidal voltage over R7 that is goes to 0V, so the LED goes completely off.
R7 is chosen so the current through the LED is approximately 10mA.
R6 is added to limit the current so the LED can not be damaged while experimenting.The oscillator has a period of about 3 seconds.
Use a high efficiency LED for LED1 for the best effect, because their brightness is pretty high at 10mA.
The oscilloscope picture shows the voltage over R7 (=100E), that represents the current through the LED.
Step 2: Build the Circuit
I build this circuit on a breadboard, but i have no PCB-layout for it.