Car-Stereo LED Power (VU) Meter

This circuit senses AC audio voltage supplied to the car-radio loudspeakers and displays it as power using a LED bar graph, achieving at the same time an attractive visual effect. It is designed to cover common car-radio output power ranges, but can easily be modified to suit different needs. It is supplied from the 12 V car electrical system and is suitable for classical CC (Capacitor-coupled) as well as BTL (Bridge Tied Load) types of amplifier with no changes to the circuitry or connections at all. In fact, only the meaning of the LEDs changes — with BTL, the LED increments equal four times the CC value on the same load.

Resistors:
R1,R5 = 22kΩ
R2,R6 = 10kΩ
R3,R7 = 820Ω
R4,R8 = 470Ω
Capacitors:
C1-C4 = 470nF, lead pitch 5mm
C5,C6,C7 = 10µF 63V radial
Semiconductors:
D1 = 1N4001
D2-D21 = LED, 3mm dia. or rectangular-face
IC1,IC2 = LM3915N-1 (National Semiconductor)
IC3 = 7805
Miscellaneous:
Heatsink for IC3 (10 K/W)