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Giant VU Meter Project

  1. Jan 2, 2007 #1
    Hello all,

    I'm a freshman electrical engineering student at the University of Kentucky. I'm also a DJ at our all-volunteer student radio station.

    I was browsing around the awe-inspiring project blinkenlights site and recalled the quasi-famous Green Building VU meter hack at MIT.

    Now on our campus we have a convienently tall building atop which sits our transmission tower. I'm toying with the idea of doing a large installation for some special event for which that building is turned into a very large VU meter (showing the volume of our broadcast). Now, this building is habitated so we don't have the luxury of running wires from floor to floor (even if we wanted to). My idea was to get 10 or so radios (one on each floor), tune them to our station, and send their outputs to a circuit that turns a light on ad off. Make the set up more sensitive on each successive floor as you go down (either by adjusting the comparator reference or the radio volume) and you get nicely synchronized lights (we are in control of the transmission, so we could broadcast some calibration signals to aid the process).

    I figure the circuit will consist of nothing more than a relay and comparator. The radio (hopefully found cheap and in bulk on ebay) output is connected to the input of a comparator, and the comparator output is connected to a SPST-NO (single pole, single throw, normally open) relay (like this one). My present problem is that I can't seem to find a comparator IC that can supply enough current for the relay coil (200mA in the example relay given above). Should I use a transistor? Am I just really bad a searching for comparators?

    I'd prefer the comparator supply voltage to be something for which wall-warts are widely available and radios are likely to run on (to avoid multiple power supplies). 5 to 9 volts or so.

    So is my approach at all sensible? I'm eager to order parts for a prototype, but I'd hate to do it without a basic sanity check.
     
  2. jcsd
  3. Jan 2, 2007 #2
    Sounds real neat to me. But consider the wrath of the electricians who will have to be changing a lot more fluorescent bulbs than they are used to. Fluorescent bulbs rapidly lose their life span the more they are turned off and on.
     
  4. Jan 2, 2007 #3

    berkeman

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    Yeah, what kind of response time can you get turning the room flourescent lights on and off? Seems like you'd trash the ballasts pretty quickly. I wonder what kind of response the MIT building had -- cool idea, though.

    If you had access to an automated building, you could do it just over the lighting control network. You'd still have response time issues probably, though.
     
  5. Jan 2, 2007 #4
    The idea is to put our own lamps in the windows, and only light up one column of windows. The relays need to switch less than an amp (we're talking one or two 100 watt bulbs).
     
  6. Jan 3, 2007 #5
    I would say using a transistor would be the easiest way of doing it. Something like a 2n222a would be able to run a 200mA relay. That would be very cool to see!
     
  7. Jan 3, 2007 #6
    Now I have a better idea of what you have in mind. Lighting up whole floors would really be a phenomenal sight and as long as it was only for a short period of time should not be a problem for the ballasts and lamps, like for one concert. This would be very expensive if there was not a light contactor for each floor which I doubt there is. I thought about an alternative away from the incandescent bulbs. For one thing the elements are relatively slow brightening and dimming, this may or may not be a desireable effect. Since it is after Christmas there should be a sale for Christmas neon strings of lights. The neons would turn on and off fast and you could make geometric designs in the windows. Power consumption would also be very low.

    I think from the bottom up and am still pondering getting the song signal strength coupled to the relay. If it was a digital receiver I think it would be easier than an analog.
     
  8. Jan 3, 2007 #7
    I should have read your post more carefully. If you use incandlescent bulbs at 100W then you would need a relay with contacts rated at least 1A for one bulb and for two bulbs then contacts rated at 2A. Due to the fast switching I would double the contact rating.
     
  9. Jan 3, 2007 #8
    If you are using your own lamps it should be pretty easy, but lots of electronics to build, all low cost though.

    Step 1: The noisy end.
    Take a spare audio feed and stick into a standard VU meter circuit, this gives your rectified audio level.
    Stick that level into an A/D
    Pass the A/D output into a demultiplexer to get a set of digital outputs.
    Put this output into an AM or FM modulator (See Notes)
    And transmit the modulated signal at your building

    Step 2: The Bright end (Build one for each floor)
    Build an AM/FM reciever to match your transmitter
    Demodulate the recieved signal (See Notes)
    Use the demodulated signal to switch a solid state switch connected to your lamp.

    Notes:
    Check what power you are allowed to transmit in a particular band without licence before picking your transmitter frequency. Also check what frequencies are being used in the area for securities comms, the bands kit etc.

    The system sample speed and propogation delays need to be faster than the brain can react to image changes else the sound and light will be out of step, anything less than 100mS should be OK off the top of my head.

    You can either use a single transmitter that sends a coded pulse "111000" put together in a shift register in which case each receiver switches the lamp on if It's selected bit is lit up. You will need to frame the coded pulse so that the reciever knows when each block starts.
    Or you can use a block of narrow band transmitters through a wide band antenna. Each bit will transmit at a different frequency and each reciever will be identical but tuned to a particular band. No need to build a coded pulse or frames for this method.

    You could also do this optically using the same basic architecture but I think the recievers would be harder to set up.
     
  10. Jan 3, 2007 #9
    I think building transmitters and receivers and using A/D convertors is WAY more complicated than necessary. I've actually done a project like this using about 7 incandescent lights. They guy I did it for knew that he'd often be replacing bulbs but it wasn't an issue for him. All you need to do is get an audio source at each window. As stated earlier, this could simply be a radio receiver tuned to the pertinent station. There are opto-couplers available that are packaged up with a triac to drive things at 120 VAC line voltage. This is what I used. Drive the optocoupler with the output from a comparator that would be driven from a rectified/filtered version of the audio. You will want some adjustment built in to the comparator as well. Depending on how good of control your radio station has of its audio, you may want some sort of AGC at each site.
     
    Last edited: Jan 3, 2007
  11. Jan 3, 2007 #10
    Ah, this is the insight I was hoping for! Where can I find such triacs/opto-couplers?
     
  12. Jan 3, 2007 #11
    Not sure where to find it. I was a 3rd party in the project. I picked the parts out and recommended the scheme that I descibed above. I was not responsible for keeping documentation. I did see it work though.
     
  13. Jan 3, 2007 #12

    dlgoff

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    For your next project, maybe the building could be a giant graphic equalizer display. lol
     
  14. Jan 5, 2007 #13
    I may be mistaken but I think that the term is opto-isolators. We use them all the time in various electronic equipment.
     
  15. Jan 5, 2007 #14
    I am reaching back but I was told that a opto-isolator was connected just like a Darlington Pair. I think that a Darlington Pair is the same as a general purpose NPN transistor. If indeed that is what you are looking for.
     
  16. Jan 5, 2007 #15

    berkeman

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    Staff: Mentor

    Not exactly. An opto-isolator (or optocoupler) is used to galvanically isolate a controlling signal from an output transistor:

    http://en.wikipedia.org/wiki/Optocoupler
     
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