Pop music and lights effects are inseparable. The circuits that make the effects possible vary from simple to complex. The circuit presented here is a simple one and is a sort of running light whose rate of change depends on the frequency and intensity of the sound.
The signal is applied to the clock input of counter IC1 via a single-transistor amplifier, T1. Its (amplified) level must be high enough to overcome the switching threshold of the counter, while its frequency determines how often the counter is clocked.
The input may be fed with the signal of a preamplifier, but it is also possible, as shown in the diagram, to connect an electret microphone across it when it functions entirely contact-less. After *amplification, the signal is applied to IC via 131, which controls the sensitivity of the circuit.
Since audio frequencies are too high for making a good visual effect, the signal frequency is scaled down by 1C1 when S1 connects to pin 11 with pin 15. When the switch is in the other position, pin 2 is connected to pin 15: the counter then divides by 1 and the effect assumes a completely different character that no longer resembles a running light.
The actual running light is provided by IC2, a counter with the integral 1-from-10 decoder. which is clocked by the QO output of IC1, of the ten outputs of IC2, each of which is connected to an LED, there is always one high. The ten LEDs have a common bias resistor, R5, an arrangement that is perfectly feasible, since only one LED lights at a time although it often seems as if more do so.
The circuit may be expanded by adding an LED at pin 12 carry out of IC1. This LED must have its own resistor 560 Ω.
The power supply must be able to provide a current of up to 100 mA: at low frequencies, the current is appreciably lower.
The colors of the LEDs can be chosen to individual taste.