I built this circuit to use it in my car stereo, to display two bar-graphs, from the audio signal taken directly from the speaker terminals. Each bar-graph is made from 10 LEDs.
Since the stereo unit contains two identical audio channels, I'll describe only the Left one:
The input signal of the Left audio channel is directly connected to C8, which is used as a DC-blocking capacitor. Through C8, the input signal is applied on R11 potentiometer, which is used to control sensitivity. From R11's wiper, the signal passes to the non-inverting pin (pin 3) of IC3A, via R10 resistor. IC3A is an LM358 operational amplifier, and with D4 and D2 diodes form an ideal half-wave rectifier. R7 and C7 form a low-pass filter which is used as a peak detector. C7 is charged through R7, and discharged through R9 at a rate determined by the RC constant. This way, peaks and troughs of short duration are averaged, and at C7's terminals there is a voltage which is directly proportional to the amplitude of the input signal. This variable voltage is applied on the input pin (pin 5) of a precise VU meter IC (LM3915). LM3915 senses analog voltage levels and drives ten LEDs on a bar-graph, providing a logarithmic 3 dB/step analog display.
Stereo VU-Meter
Besides providing a bar-graph, LM3915 can be also used at “dot” mode. At this specific mode the display changes from a bar graph to a moving dot. The mode of operation depends on S2. When S2 is closed (on), LM3915 provides a bar-graph display. In contrary, the graph changes to a moving dot, when S2 is off.
LED VU-Meter schematic
How to use and calibrate the VU meter
Connect VU-meter's ground to your amplifier's ground. Then, connect C8 and C2 capacitors (VU-meter's inputs) at your Left and Right speaker terminals, respectively. Use only one wire from each speaker. Which wire is the right-one, can be found experimentally.
The calibration process is quite easy: During playback or listening to radio, raise the volume off the amplifier at its maximum level, and adjust R4 and R11 until you get a full scale graph (all LEDs on) at each channel. Alternatively, you may calibrate the VU-meter for full-scale at a different volume level (e.g., half maximum volume).
Have fun..
John D.
Components list
R1, R2 = 1K, 1/4 watt
R3 = 560 ohm, 1/4 watt
R4, R11 = 10K, trimmer
R6, R9 = 47K, 1/4 watt
R7, R8 = 1.5K, 1/4 watt
R5, R10 = 10k, 1/4 watt
C1 = 100 uF, electrolytic, 25 volt
C2 ,C8= 22 uF, electrolytic, 25 volt
C3, C7 = 1 uF , electrolytic, 25 volt
C4, C5, C6 = 100nF
D1, D2, D3, D4 = 1N4148
D4-D24 = LED
IC1, IC2 = LM3915
IC3 = LM358
S1 - S2 = switches
