I’m looking to create a simple circuit that will have 8 randomized “pulsing/fading” LED’s


I am looking to make a simple (cheap, small spaced) circuit that will light 8 LEDs in a random order AND have a pulsing/fading in and out effect. I was originally going to program a randomized effect, but I found a schematic using a 4026 counter and display driver and 555 counter that will do the deed instead (saving time and hair pulling). The problem I am having is figuring out how to get the fading effect on each LED and how I would implement that with the randomized effect.

The circuit I found is here: http://rookieelectronics.com/sequential-random-led-flasher/.

I know that changing the capacitor value will slow/speed up the blinking speed, but won't add the fading effect I'm looking for. My best guess is either attach another capacitor between the LED and driver, or to implement a BJT into the circuit. I'm pretty stuck so any advice would be great!

I should also mention I plan on making this DC with a 6V battery. It can be higher voltage if needed, that was just the original plan.

Thanks and have a great day!

Best Answer

The CD4026 has a Display Enable pin that you can apply PWM to. The question is, how to make the PWM vary in time with the LEDs? One possibility is to use the triangle waveform that the NE555 is already producing as it clocks the CD4026.

In the circuit below, OA1 and OA2 form a classic Pulse Width Modulator. OA1 generates a high frequency triangle wave on C2. OA2 then compares this to the much slower ramp produced by the NE555, and outputs a PWM waveform with varying duty cycle. This should cause the brightness of each lit LED to go from 0 to 100% and back again during its time segment.


simulate this circuit – Schematic created using CircuitLab

If you want the LEDs to fade in and out at a different speed to the randomizer then use another 555 (or another opamp) to generate the brightness ramp. If you want the brightness of every LED to vary independently and/or randomly then you will need an even more complex circuit, in which case it might be easier to program an MCU to do the job.