Audio Running Lights Circuit

Here is a project for audio running lights circuit which uses filament bulbs instead of LEDs. The circuit employs seven channel darlington array IC ULN2004 and seven reversed biased zener diodes having different breakdown voltages.

circuit diagram of audio running light

At low audio level, zener diodes having low breakdown voltages will conduct and corresponding lamps will light up. When output level increases, the other diodes will also conduct.

As a result, you can see a very fine display of ‘moving’ light bulbs from top to bottom or from left to right, as per their arrangement.

AF input signal fed via capacitor C1 is rectified by diodes D8 and D9. This rectified audio signal is given to the cathodes of all zener diodes.

Adjust VR1, for satisfactory operation of the IC. At minimum input the first lamp will glow and at maximum input all the bulbs will light up.

PARTS LIST

Resistors (all ¼-watt, ± 5% Carbon unless stated otherwise)

VR1 = 10 KΩ

Capacitors

C1 = 220 µF/25V

C1 = 470 µF/25V

Semiconductors

IC1 = ULN 2004

D1, D2 = 1N4148

ZD1 = 1.2 V zener diode

ZD2 = 2.7 V zener diode

ZD3 = 3.6 V zener diode

ZD4 = 4.5 V zener diode

ZD5 = 6.8 V zener diode

ZD6 = 9.1 V zener diode

ZD7 = 12V zener diode

Miscellaneous

Bulb1 – Bulb7 = 12V bulbs

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