Adding a VU Meter to your PC
by Philip Chien
Note - You will need mechanical and electronic skills to build the project in this article. Use this information at your own risk; do not complain if it doesn't work for you.
Computers are functional tools. But sometimes a little glitz is nice too. Many folks modify their computer's cases to include a transparent window, internal lights, and even glowing cables inside the computer.
I wanted to add a stereo LED VU meter to my computer. I found a nice kit which measures 1.5 by 4 inches, perfect for my intended use. Stereo VU circuits are extremely simple and if you want to duplicate this project it's easy to purchase a kit or build your own VU circuit from scratch. The computer's audio output drives the VU meter - any sounds sent to the computer's line out jacks are "displayed" on the VU meter.
My motherboard has an AC97 audio connector. That connector is a 10 pin header on your motherboard which transmits audio to your computer's front panel audio jacks. This project should work with the newer HD audio connectors too but I have not checked that out.
My case didn't have built-in front panel jacks so I added them with a 5.25" all-in-one adapter. The Ultra Media Dashboard features various flash memory card readers, USB, SATA, and IEEE-1394 ports, audio in/out jacks, and a digital audio SPDIF jack. Several cables connect the Media Dashboard to your computer's motherboard, including one for the AC97 connector. The key disadvantage to this unit is the USB jacks are not powered; it's unfortunate the company didn't decided to make it a fully powered USB hub.
If your case already has front panel audio jacks you can tap into the wires which go from the motherboard to those jacks.
You need to tap into three lines - ground, right audio, and left audio. That's pins 2, 5, and 9 respectively on the AC97 motherboard connector. Use a voltmeter to verify that you're connecting to the correct wires. I used squeeze together telephone tap connectors because they're less invasive than cutting the cables and resoldering them.
My VU meter mounts into the 5.25" bay directly above the Media Dashboard, minimizing excess cables inside the computer.
The VU board has two connectors - a three pin header for audio and a two pin connector for power.
I repurposed an old internal CD audio cable to plug into the three pin audio connector. The cable had a four pin connector and with a little bit of work I was able to reconfigure the connector into the three pins I needed.
It was a tight fit, but I decided to tap into the lines inside the Media Dashboard. It has an open frame which made it possible but it took a lot of patience to slip the taps around the correct wires, insert the wires from the audio cable with the modified connector, and squeeze the taps together.
I cut apart an old Molex Y power splitter cable to get a male Molex 4 pin connector to get the 12 volts to power the VU board. My computer's got plenty of extra power connectors so I'm not concerned about running out of power connectors in the future.
I had a couple of spare 5.25" faceplates, from when I installed the DVD drive and Media Dashboard as well as the faceplate from bay which I decided to use for this project. Faceplates are rarely interchangeable among different computer cases though. They're pretty ordinary plastic and easy to drill or cut for DIY projects.
Drilling the holes for my VU meter's LEDs was the biggest challenge in this project. I started by drawing a template in Inkscape. I measured the distance from the first LED to the last and then divide by the number of LEDs, with the (correct) assumption that the LEDs were evenly spaced. I went through a couple of iterations before I made a precise cutout. I've got access to a laser cutter which made the job a bit easier. All of my experiments and measurements were done on index cards before I cut the plastic faceplate. My faceplate had an internal marking that it was ABS plastic which is compatible with the laser cutter. Do not try to use a laser cutter to cut PVC, the chlorine gas created by burning PVC is poisonous!
If you don't have access to a laser cutter you can drill the holes by hand. You can print the template on a self-adhesive label, stick the label on the 5.25" faceplace and use it as a drilling guide for a drill press. Remember the old adage - measure twice, cut once. It's not easy to erase an incorrect hole!
I originally planned to build an entire front panel audio setup with line out and microphone jacks, so I drilled holes for 3.5 mm. jacks on my faceplate. This was before I decided to use the Ultra Media Dashboard which already has line in and line out jacks. I filled in the extra holes on my custom faceplate before painting it. I may drill another faceplate without the extra holes and add a logo at some point.
My faceplate is copper colored for a simple reason - I ran out of black paint and painted the panel with what I had available. It turned out very nice so I'm glad that I ran out of the black paint.
I found a nylon mounting bracket which I was able to adapt by cutting it in two. The plastic pieces are permanently glued to the back of the faceplate. The VU board is attached to the plastic brackets with small nylon screws and nuts.
Once I put everything together I was pleasantly surprised that everything worked on the first try. But there was one important test to make sure that it was correctly hooked up and I didn't reverse the audio channels by accident.
There are a couple of YouTube videos which alternately play a sound clip through the left and right speakers with a corresponding text on the video. It's a quick way to verify that your right and left speakers are correctly hooked up. With my particular VU kit's audio connector it isn't a big deal if you've hooked it up backwards, all you have to do is flip the audio cable upside down.
There was a nice bonus with the Ultra Media Dashboard which I hadn't expected when I started this project. Its SPDIF connector is just an RCA extension cable - a female RCA jack mounted on the unit's front connected to a cable with a male RCA connector. Presumably the user is expected to thread the cable through an opening in the back of the computer and plug it into a SPDIF jack on the back of the computer's case.
My MSI motherboard has a barely documented two pin SPDIF connector. The manual labels the two pins as 2+ and 3-. No information on why there's no pin 1 or what + and - mean. The motherboard has a two pin connector with an empty hole where pin 1 would be located. I used a voltmeter to verify that the - pin was a ground but + was not 5 volts. Most SPDIF backplane brackets require three pins and none of the ones I saw corresponded to my motherboard. It turns out that the third pin (5 volts) is only needed for the fiber optic TOSLink connector. If you only need the digital audio through the RCA SPDIF jack all you need is a two pin connector with a straight cable without any extra components. Once I figured this out I chopped off the male RCA connector from the Media Dashboard's cable and soldered on a two-pin connector (tip of the RCA connector to pin 2 and ring to pin 3) which plugs into my motherboard.
I'm very pleased with the results. I'm considered adding a small LCD display, infrared remote receiver, indicator LEDs, and an auxiliary power jack if I do decide to drill a new faceplate. If I was to design the VU circuit from scratch I would add brightness and sensitivity controls.
The VU meter is a very nice addition to the Home Theater PC (HTPC) I'm putting together.
(My apologies for the quality of this demonstration video. Because of where the HTPC is located I had to remove it and put it on the workbench with a small monitor to record this demonstration. It's much more impressive when the computer's in its normal on its shelf next to the large screen TV.)
photos copyright 2012 Philip Chien. All Rights Reserved.
Demonstration video from the 1978 movie "Superman", music by John Williams.
A quick video to check whether or not your speakers are reversed.
Pinouts for the AC97 and HD Audio motherboard connectors.
A LED VU Meter Kit similar, but not identical to the one I'm using.
Order the Ultra Media Dashboard from Amazon.com.
Order 3MT-R-100 22-18 Gauge T-Tap connectors from Amazon.com.
A simple circuit for a LED VU meter.
About the author
Philip Chien enjoys working with computers and electronics.
copyright 2012 neatinformation.com. All Rights Reserved.