A pair of analogue VU meters pulled out of an old tape deck have been hanging around for a while, so I put them to use decorating the front of my PC.
Schematic:
The circuit consists of one ordinary half-wave precision rectifier per channel. VU meters are supposed to have full-wave rectification, but since music is symmetrical, it won't make any worthwhile difference as long as it is averaged over a time (300ms in this case) much longer than the period of the lowest frequency (50ms for 20Hz).
The circuit is powered from the +12V which is easily available inside a PC. Note that the output is biased to a point a bit above ground to keep the output of the op-amps within their usable range. The meters themselves are referenced to the same voltage because it's not possible to use a DC blocking capacitor in series with the output, since the output is DC.
The values of the resistors in the potential divider (R6/7 and R11/12) contribute to the rise/fall time of the meter, along with a 1.2K resistor and 100uF capacitor in parallel with each meter (not shown in the schematic). The values were determined empirically to get the required 300ms rise time to 99% full-scale deflection.
Schottky diodes have been used, for their lower forward voltage drop. Level adjustment is via the potentiometers at the input.
PCB layout:
Double-sided board was necessary for this. The power connector is one I rescued from an old CD-ROM drive. Otherwise, there is nothing in particular of note about it.
Photographs:
Here is picture of the whole thing assembled. It is mounted to the inside of one of the 5¼" blanking plates that came with my case. The meters are sandwiched in place by a strip of aluminium bent into a u shape. The circuitry is attached by little brackets to the same strip of aluminium. The backlight LEDs are taped to the bottom surface of the meters.
And finally a photo of it installed. The backlight LEDs are in fetching shades of blue and orange, to match the rest of the case lighting.
