Apart from using LEDs as power indicators or Joule thieves, I haven’t really used them much, so maybe it’s time to create something really fancy! 😊
I searched for some LED flashing circuits that can control at least 4 LEDs with minimal components, and finally found two circuits that I liked, both using Schmitt trigger oscillators, one that makes LEDs fade in and out, and another that feeds BCD to a 7-segment driver IC, making 7 LEDs flash randomly.
Fader [HERE]
Random flasher [HERE]
Push-Pull Circuit
The push-pull circuit uses one Schmitt trigger for each LED, and since Schmitt trigger ICs usually have 6 triggers, it’s no problem to make 6 LEDs fade in and out at different rates. The downside of this circuit is that it will never fully turn on the transistor/LED, with a maximum current of only 4mA through the LED.
Since I want the LEDs to fade in and out at different rates, I couldn’t use the same resistor/capacitor values for all oscillators. Here is the table of resistor values used for each oscillator, with all capacitors having the same value (2.2uF / 22uF).
| Lead | R1 | R2 | R3 |
|---|---|---|---|
| 1 | 100k | 4.7k | 47k |
| 2 | 15k+27k | 2.2k | 15k |
| 3 | 27k+47k | 4.7k | 47k |
| 4 | 47k+100k | 4.7k | 47k |
| 5 | 47k | 4.7k | 27k |
| 6 | 1M+1M | 10k | 47k |
Random LED Flasher
The random flasher circuit description mentions that the slight differences between Schmitt trigger oscillators are enough to make it random, but after building and placing it for about an hour, it still flashes the same LEDs, so I replaced two 47k resistors with one 68k and one 33k.
I also added 2N3904 transistors to drive high-brightness LEDs with 50mA current on the 7 outputs, since the 7-segment decoder IC (74HC4543) I used has a rated current of only 20mA.
