VU meter

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.