Simple Pseudo Random Glitter

A question recently asked on the Elektor website forum was how to make several white LEDs ‘sparkle’. The helpful author has not only provided a useful suggestion (use a random effect), but also devel-oped a suitable circuit and even designed a PCB layout. You can download the Eagle files for this from the Elektor website page for this article (www.elektor.com, archive # 080329-1.zip).

But first let ’s consider the basic question: artificial sparkling or glittering can best be simulated by having the different light sources switch on randomly at a par ticu-lar frequency. Surprisingly enough, it is not all that easy to generate truly random se quencesele ctronically. However, the electronic ran-domness does not necessarily have to be perfect for glitter applications. Patterns that appear to be random are suf-ficient for the desired visual impression.

Based on this principle, the author uses two 556 timer ICs to generate signals whose frequencies (850 Hz for IC1a and 180 Hz for IC1b) can be divided by each other with-out yielding an integer divisor. A decimal counter oper-ated in an unconventional manner uses these two signals to produce a constantly pseudo-random pattern on its ten outputs, which repeats itself only very infrequently. This behaviour is obtained by applying the higher frequency signal to the CLK input of counter IC2, with the CLK Inhibit input on pin 13 being driven by the lower-frequency signal. The result is ‘genuine pseudo-random’ blinking.

LEDs can be connected directly to the ten outputs, since a CMOS output can anyhow only supply a few milli ampères. However, it is recommended to use series resistors (2.2 kΩ to 4.7 kΩ) to reduce the load on the IC out-puts if the supply voltage is higher than 10 V. If you want to have more than ten LEDs glitter, you can naturally build several copies of this circuit. Link