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Vintage stereo receiver re-lamp with LEDs

Discussion in 'LEDs and Optoelectronics' started by OJ Bartley, Nov 22, 2012.

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  1. OJ Bartley

    OJ Bartley

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    Nov 22, 2012
    I have just started a project revolving around cleaning up and updating a 1970's Pioneer stereo receiver. I won't go as far as calling it a "restoration" because all I'm capable of at this point is monkeying around and hopefully not destroying it, but I do want the end result to look and sound good. Here she is all cleaned up and working:

    [​IMG]

    What I would like to do on the "look good" front is replace some of the old incandescent fuse-type bulbs with cooler-running and sharper-looking LEDs. My first efforts will be towards:

    1) the STEREO indicator lamp (6V 0.03A 4mm sub mini lamp); and
    2) the five dial scale lamps (8V 0.03A fuse lamp) and signal meter (same type)

    Here's a shot of the existing lamps in working order:

    [​IMG]

    I'm pretty much a novice with regard to electronics although I have done some soldering, and read a few circuit diagrams way back in high school, but I'm pretty handy and don't think this is out of my scope (with a little help). In my research, I found lots of people who have done things like this before and I've seen everything from using automotive LED fuse lamps that seem to work, to custom built LED fuses, to serial strings of LEDs run from the unit's main power.

    I have read about lots of other projects, and see what's available for sale, and I decided to go in my own direction. What I would like to do is build a small circuit board for the fuse lamps, using 2 or 3 LEDs, the appropriate resistor, and (from what I can understand) a blocking diode. This is where I need some help from more experienced people...

    I ordered a batch of LEDs online, rated at 3.4V (typical forward voltage) for 20mA. I also ordered some resistors, a batch of 1n4004 diodes, and some blank circuit boards to put everything together. I figure unless I can find some fuse end caps, I can make rigid loop connectors to fit into the fuse holders.

    I have done the math, and come up with the following:

    1) For the easier STEREO indicator (this is a single bulb on a lead poked through a hole in the face):
    6V - 3.4V = 2.6V
    R= 2.6V / 0.015A (using half the provided power)
    R= 173 ohms (round up to 180, 190, 200, whatever is easy to get)

    0.015² x 173 = .039W = 39mW (1/4W resistor is fine)

    I'm 90% sure I connect the resistor to the +ve lead? What about the blocking diode? How does it fit in to the layout?

    2) For the dial lamps (fuse bulbs in holders as seen above):
    This is where it gets tricky... My understanding is that if I use 2 LEDs in opposite orientation it will reduce the potential flicker. I'd like to do that, and also use 2 for the added brightness and ability to diffuse the light a little.

    So do I subtract 8V - 3.4V - 3.4V = 1.2V?
    R = 1.2V / 0.015A
    R= 80 ohms

    But, assuming that works, now I'm at a loss as to how to connect everything. I could probably follow a diagram, but can't actually design it myself. If anyone could help me out here, I would really appreciate it. Am I way off base, or is this something that has a chance of success? Sorry, this got pretty long. But if you've read this far - thank you, and any comments will be helpful. I'll post lots of pics of the process.
     
  2. KJ6EAD

    KJ6EAD

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    Aug 13, 2011
    Measure the actual voltage being supplied to the lamps and determine whether it's AC or DC first and whether it's stable or varying. The voltage for dial illumination is probably fixed but the voltage for the stereo indicator is another matter. Doing the math based on the lamp rated voltage is a trap. If it's AC, you'll need the diodes, otherwise not.

    LEDs are relatively monochromatic and unidirectional light sources compared to incandescent lamps. This means you should choose an LED color that matches the blue filtered light color in the stereo as closely as possible (430nm?) and try to achieve as much diffusion as possible either in the selection of the LEDs or the construction of the luminaire.
     
    Last edited: Nov 22, 2012
  3. OJ Bartley

    OJ Bartley

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    Nov 22, 2012
    Thanks for the reply. I believe you are right, the dial is AC, and the stereo indicator lamp looks like it is DC. So in that case I should be OK with just the resistor on the positive lead?

    I can try to measure the voltage, but I don't really know what I'm doing. I'm sure I have a multi-meter at home, probably still in its original package. I'll give it a try... so I'm looking for the actual voltage at the bulb socket?

    As for the LEDs, I chose some flat top bright white ones for better light distribution, and to go for a nice clear bright blue. The incandescent bulbs give the dial a yellowy turquoise look, but when I held a white CFL bulb behind the dial it was a great pure blue:

    [​IMG]

    My plan was to mount 2 white LEDs at opposite ends of the circuit board so that one was above and one below the dial. I'll have to experiment with placement and maybe I will need to diffuse, but I've seen single LED conversions without much hotspotting.
     
  4. KJ6EAD

    KJ6EAD

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    Aug 13, 2011
    How do you know the dial is AC? It doesn't matter which lead you put a limiting resistor on.

    Yes.

    The flat tops will have a wide dispersion so that's good. Since you're getting enough transmission of the color you want through the filter, you don't have to use blue LEDs in this case but it's usually better to color match.
     
  5. OJ Bartley

    OJ Bartley

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    Nov 22, 2012
    I found it referenced by other users on the AudioKarma forum, where I started this project. I can check to make sure though.

    I actually ordered blue of the same type as well so I could test the look. I was wondering if I could achieve a gradient by using blue LEDs at the bottom of the dial and white at the top. At least this way I have the option to try a few different things. I did plan to use blue behind the signal meter which has yellowed with heat and age.
     
  6. BobK

    BobK

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    Jan 5, 2010
    If the bulb was driven by AC you can wire your two LEDs back to back with a single resistor.

    Bob
     
  7. OJ Bartley

    OJ Bartley

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    Nov 22, 2012
    Thanks Bob. So I can wire the 2 LEDs in series with the resistor. Now, can I reverse the polarity of one LED to compensate for the 60Hz flicker?

    I have read that even with one LED in reverse, it doesn't really do a proper job of blocking the reverse current present, which can be harmful to the LEDs? Something about not being a true diode, which is why I should include the 1n4004 in the circuit, is that right? I know it may still work without one, but I think the longevity is the question. And I have already ordered them, so I may as well be on the safe side and include one.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The two LEDs need to be in parallel, with each LED pointing the opposite way. (anti-parallel)

    Each one conduct for opposite half cycles.
     
  9. OJ Bartley

    OJ Bartley

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    Nov 22, 2012
    Thanks Steve. So am I looking at something like this? (remember, complete novice circuit designer here...)

    [​IMG]

    Where would the blocking diode fit in?
     
  10. BobK

    BobK

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    Jan 5, 2010
    Yep, that circuit will work. No blocking diode necessary. As steve said, each LED conducts on half the cycle and it protects the other LED from seeing too high a reverse voltage.

    Bob
     
  11. OJ Bartley

    OJ Bartley

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    Nov 22, 2012
    Thanks again for the reply, Bob. I have read elsewhere that just reversing one LED may not provide adequate protection over the long run. On the other hand, I have also read that LEDs have come a long way recently and reverse voltage isn't as much of a threat as it used to be.

    I'll certainly be happy to give it a try both ways, but since I ordered a batch of 1n4004 diodes anyway, I'd like to include it, if for nothing more than peace of mind and learning more about how these things work and fit together.

    Is there any negative to adding one (or more) to the circuit?
     
  12. BobK

    BobK

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    Jan 5, 2010
    No, you can add another diode in series. Just subtact another 0.6V when calculating the resistor. A typical LED has a reverse voltage limit of 5V so, with the forward voltage less than that, it is fully protected by another LED anti-parallel, but you can add extra safety if you want.

    Bob
     
  13. OJ Bartley

    OJ Bartley

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    Nov 22, 2012
    Bob, you're fast! Do I add the diode in series with the resistor, or do I have to add one in series with each LED? Are they directional? I'm just not sure where on the diagram it would go, I'm still kindergarten-level concerning circuit diagram layout and reading. :eek:

    Thanks for all the help.
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Diodes and LEDs are all directional.

    It's a string of things in series., Resistor, diode, LED; or LED, diode, resistor; or LED, resistor, diode; or diode, LED resistor; it's all the same (order doesn't matter).
     
  15. OJ Bartley

    OJ Bartley

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    Nov 22, 2012
    Should it look something like this with 2 LEDs?

    [​IMG]
     
  16. KJ6EAD

    KJ6EAD

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    You don't need the blocking diode when you have LEDs in reverse parallel since they protect each other from excessive reverse voltage. If you leave it in the circuit, only one LED will light, the one on the left.

    I've scribbled the corrections on your schematic.
    [​IMG]

    You would only need the blocking diode when operating a single LED or several in series. All of this applies only to LEDs operated from AC. A blocking diode with 2 LEDs would look like this (the resistor value will be different).
    [​IMG]
     

    Attached Files:

    Last edited: Nov 24, 2012
    KilgoreCemetery likes this.
  17. OJ Bartley

    OJ Bartley

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    Nov 22, 2012
    Awesome! Thank you for those layouts, it really helps me grasp what's going on, especially along with Steve and Bob's advice. It looks like I have 2 decent options to try.

    1) 2 LEDs in serial with the blocking diode. If the 60Hz flicker is noticable, I will likely be better off with...

    2) 2 LEDs in anti-parallel configuration without the extra diode.

    Now that I have the basics covered for one- and two- LED circuits, what about if I wanted to add a third (maybe 2x white and 1x blue, or vice-versa)? Do I completely throw things off if I have 8V at the source, and the 3 LEDs I'm using want 3.4 + 3.4 + 3.2V, or does that just mean I don't need the resistor anymore?
     
  18. KJ6EAD

    KJ6EAD

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    Aug 13, 2011
    It means you have to wire them in parallel because you don't have enough voltage to run them all in series at full current.
     
  19. OJ Bartley

    OJ Bartley

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    Nov 22, 2012
    Could I wire the 2 white LED in parallel and the blue in series? Something like this?

    [​IMG]

    Or is that just combining the 2 whites in parallel?
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,448
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