(R)C - (PNP) Darlington - Diode: LED Fader

This circuit provides a "soft-OFF" function to an LED string (or DC lamp) using a capacitor, darlington transistor, and diode. When the switch is opened the LEDs/lamp do(es) not extinguish instantly but slowly fade(s) off. This happens because the voltage at the emitter of the darlington does not immediately drop to zero. This in turn is due to the base terminal being connected to a capacitor which slowly charges through the darlington. When the switch is closed, the lamp fully turns on and the capacitor is discharged through the series-connected diode and resistor. The rate of fading can be modified by adjusting the value of the capacitor. The base-emitter shunt resistance allows the negative electrode of the capacitor to discharge closer to the power supply voltage. Bleed current also flows through the base-emitter resistor when the switch is open and the capacitor is charging and supplying current to the LEDs. The resistor in series with the diode determines the charging rate of the capacitor. Lower values results to faster charging but increases the peak charging current.