The following article explains a simple circuit which can be used for alternately switching LEDs with gradual brightening and fading effects.
The circuit can be effectively used for generating spooky effects in idols, for example it can be used for illuminating the eyes of a Jack-o'-lantern during haloween celebrations.
The circuit is overly simple and requires just a couple of opamps and a few other passive components for implementing the proposed brightening and fading actions in the LEDs.
The opamps can be a couple of separate IC 741 or a single IC with dual opamp such as IC 1458, 4558 or a TL072.
The opamp A1 is used for generating a gradual rising and sinking voltage, which ranges from 3 to 6 volts.
The opamp A2 is simply configured as a comparator for supplying an alternately varying voltage between 2 and 7 volts in order to charge and discharge C1 and C2 through a constant current input.
Thus the above operations become responsible for generating a linear peak to peak ramping signal at pin#1 of A1.
This signal is amplified with a couple of transistors wired as emitter followers to pin#2 of A1. Here the LEDs become the emitter loads of the transistors.
R4, together with C1 and C2 determines the rise and fall frequency of the connected LEDs.
R4 can be replaced with a 100K pot for making the fading rate manually adjustable.
The circuit should be operated from a 12V DC power supply for supporting at two LEDs on each channel.
For accommodating more LeDs, the collector of T1 and the upper end of R7 should be connected to a separate high voltage supply may be to a 30V supply which would then allow the connection of 6 LEDs on each channel.
Parts List for alternate brightening and fading LED circuit
R1, R2, R3 = 56K,
R4, R5 = 120K,
R6, R7 = 150 OHMS
C1, C2 = 33uF/25V
T1 = BC547,
T2 = BC557
LEDS = 5mm, 20mA,