This is the wiki page for a project TP is thinking about.
- 1 Overview
- 1.1 Background
- 1.2 Potential Panel Types
- 1.3 Control
- 2 Current Status
- 3 Log
- 4 Current Issues
- 5 Next Steps
I would like to build a giant VU meter (like, wall sized, maybe 10 feet tall?) using electroluminescent tape (EL) or similiar (maybe some other kind of lighted panel would be cheaper?). For those that attended the grand opening party, there was a guy walking around with a very cool light-up t-shirt that was exactly what I'm talking about, only *slightly* smaller.
http://www.instructables.com/id/LM3915LM3916-VU-Meter/ shows a project to build a simple VU meter that measures overall volume from an audio source (a single bar that bounces to the loudness of, say, a song). This is basically an analog circuit that outputs directly to an array of LEDs, although with the right output stage we could make it run just about anything (EL tape, desk lamps, etc).
Expanded Basic Meter
The meter I really want to build is probably better called a panel. I want to place 8 bars into a grid shape, and tune each bar to just a portion of the frequency spectrum (bandpass filters). We've all seen this somewhere before - the coolness comes from making it 10 feet tall, right? This expanded version of the above meter is really just eight of those meters each being feed from a seperate bandpass filter.
The next level up idea would be to multiplex the output of the bandpass filters into a PIC or similiar microcontroller's ADC for sampling. The controller could then light the appropriate output lights, probably by setting an externally scanned memory location or toggling inputs to a massive latch array, so that instead of a VU meter/panel what we really have is a 10' tall 8x8 monochrome screen that just happens to be running a VU program. The cool part of going this extra step is we can add things like a startup splash screen. We could also select between different output routines for variety (horizontal vs. vertical vs. radial vs. ?). The display pixel resolution may need to be increased past 8x8 - at least to the point of being able to draw a Hive13 logo on it or something. The price of the glow tape is based on area I think, so it /may/ not add *too much* to the price of the project to get a resolution of 64x64 or even 128x128. The design rolling around in my head right now has a max res of 256x256, but the approach may be too slow and I haven't so much as found a sheet of paper yet. If we end up with some other kind of lighted panel square adding massive resolution could end up (relatively) increasing the price much more, although overall it still may be cheaper than the EL tape.
Potential Panel Types
Cost is definitely a factor here. It looks like the big trade-offs are between initial construction cost, operating cost, and coolness factor.
Appropriate widths of EL tape cut into squares.
- Extreme cool factor
- Relatively low power to run
- EL tape should last a ridiculously long time
- Expensive to build
- Capactive load - needs at least a dirty AC signal (Class C style spikeyness would be sufficient I think, but without a sample to play with it's hard to tell).
Incadencent Light Bulbs
Square 'shadow box' type wooden things with regular light bulbs hidden behind an opaque-ish sheet of plastic or similiar.
- Make panels ourselves - much cheaper
- Lots of watts of power needed to run a large array
- Severely less cool, but still possibly cool-ish
- Inductive load - will need a relatively clean AC signal to nicely (of course straight-stick wall power and light bulbs where literally made for each other).
- Light bulbs will need to be changed occasionally
Compact Floruencent Light (CFL) Bulbs
Same as above but with lower power bulbs. May not be able to switch on and off quickly enough, but this can be tested.
- Less power than regular light bulbs
- One-for-one interchangable with regular lightbulbs
- Still more power than EL tape
- CFL bulbs are more expensive than regular ones
Super LED Arrays?
Same as above two but with say, five (5) or so super LEDs behind each panel.
- LEDs last much longer than light bulbs
- (maybe) - Resistive load; DC power
- Power levels approaching EL tape, certainly much better than the larger bulbs.
- Still not as cool as EL tape
- Replacing a burnt LED will probably require a soldering iron
(Your panel idea here?)
(Think 'big square thingy that lights up quickly')
These can be run from the LM3915/3916 IC chips mentioned in the background article, with a driver array for the panels. For the expanded version, a bandpass filter is needed for each bar to seperate the incoming signal into bands.
Needs a programmable controller with an ADC channel, a way to 'funnel' multiple bandpass outputs into that ADC channel, some sort of output buffer stage, and the same driver array as the basic versions.
I had previously been kicking around the thought of somehow using a phase-locked loop (PLL) circuit to create a single bandpass filter that could be software controlled vs. multiplexing several hardwired filters. After looking into some of my reference texts and around the 'net a little, I now realize that this wasn't really the brightest idea.
The bright idea is to use a Univeral Switched Active Filter - this can be used to create a bandpass where the resistive values of the filter are controlled by switched capacitors. This particular chip (MF10) is good for 0.1 Hz to 30 kHz at a 50:1 ratio. Basically, if you feed it a clock signal at 50kHz, it will act as a bandpass centered around 1kHz. Very cool (Thank you National Semiconductor).
Once the incoming signal has been filtered around the target frequency of interest, a voltage follower, rectifier, and filter cap will give us a DC voltage relative to the strength of the incoming audio signal at that frequency. Once sampled, we adjust the clock for the next band, wait a few microseconds for the filter output to settle, and take the next sample. My favorite part of all of this is of course the ability to determine how many and which frequencies to sample from software without any hardware changes.
Looking around for ideas I came across the 74HC595 shift register. Should do nicely. These shift registers have two clock inputs - one for shifting and one for latching. By placing 32 of them in series we can drive a 16 x 16 display with three (3) lines (Data, Shift, Latch).
While shifting serial data may not seem the fastest, consider:
- 16 x 16 is 256 bits, which is only 16 words of data
- These registers can take a max shift clock of 10 MHz (!)
- Shifting occurs behind the scenes - the display does not update visably until the latch clock is pulsed (think 'off screen buffer')
Typically a refresh rate of 30 frames per sec or so is enough to provide smooth animation effects. If we assume a shift clock of 100kHz (pretty slow), a 16x16 display driven this way would have a theoretical refresh rate well over 300 frames per sec (!!).
256 LEDs is an awful lot of soldering. Wiring 256 light panels sounds like massive work. These are, however, doable. Any bigger resolution and I think we cross the plausibility threshold. At 16 x 16, we can do some graphics, tho:
If no one has noticed yet, I'm a big fan of the PICs. They're cheap, fast, flexible, and simple to program if you don't mind assembly. Once all final design details have been hammered out a particular model can be chosen.
I/O Requirements so far:
- Signal Input
- (I) ADC Channel
- (O) Frequency control for bandpass
- (O) Serial Data Bit
- (O) Shift Pulse
- (O) Latch Pulse
The LEDs can be driven directly by the shift registers using pull-up resistors. This will invert the logic (0 = On, 1 = Off), but that's a minor detail to be handled in software.
Full Sized Installation
The full sized version will require some components between the shift registers and the panels.
(Needs research - dependent on type of panel we chose)
I think this project has approached the late planning stage.
- These tape sellers are hard to get prices and samples from. Half the websites don't list prices, or don't offer small quanities, or require you to buy some expensive power supply with your tape (I intend to make my own supply - the tape requires an AC current).
- The sites that do list prices show that this stuff ain't cheap.
- Find a tape sample (good luck)
- Play with tape sample
- Build small (but complete) LED version of advanced meter (reasonably cheap - I think I can fund this)
- Decide on type of lighted panels to go with
- Get enough Hive members excited about project so that we can get the funds to build Giant version onto a wall in the hackerspace