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LED color organ

Discussion in 'LEDs and Optoelectronics' started by whiterabbit, Dec 5, 2013.

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  1. whiterabbit

    whiterabbit

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    Sep 14, 2013
    I've followed along with this Instructable: http://www.instructables.com/id/Color-Organ-Triple-Deluxe-II/ but I seem to have gotten myself stuck, and I'm not exactly sure how to troubleshoot it...

    I've followed the schematic and gotten everything soldered together, but it doesn't respond as it should. Power seems to be getting through to each LED chain, but there is no responsiveness to input music. The LEDs are on all the time, even with no audio input. The intensity DOES respond to the rotary pot.

    The images may be hard to decode, I had to take pictures with my phone; I can go back tonight and edit them so they have a parts key if that will be helpful.

    I plan on having the LEDs spaced out further from the board which is why they aren't attached currently. To test, I just set up three chains of each color (R G B, three LEDs in series for each) on a breadboard and connected them with temporary jumpers. I also only used a 9v battery to test this because it was more convenient at the time, but I do have a 12v DC source I can use as well.
     

    Attached Files:

  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    That circuit is designed for a 12V power supply. If you use a 9V power supply, the biasing conditions for the transistors won't be quite right. That doesn't explain why the LEDs are constantly ON, though.

    The fact that the LED brightness responds to the sensitivity pot implies that the input amplifier might be oscillating. The oscillation would pass through all three filters and cause the LEDs to stay lit, and varying the position of the sensitivity pot would vary the amplitude and/or frequency of the oscillation.

    In that case, you should check your wiring of the input stage. Your construction looks OK generally, and I can't see any definite errors, but I can't follow it fully. I'm not sure about the 47 ohm and 470 ohm resistors to the 0V rail. Also, the 1.2M feedback/bias resistor looks like it might be the wrong value.

    Double-check your input stage. Also, what transistors did you use?
     
  3. whiterabbit

    whiterabbit

    90
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    Sep 14, 2013
    I'll break out the 12V power supply tonight after work first and foremost. Note to self- 'close only counts in horseshoes and hand grenades' :p

    All the transistors are the same as per the Instructable- http://megads.com/files/datasheets/mps2222a-datasheet-3m.png

    I will check the whole circuit over, specifically the input stage, against the schematic tonight. Could you expand a little bit on the oscillation of the input amp? As in, what happens that results in the oscillation as opposed to a steady, non-oscillating amp?

    Also, I noticed that the LEDs would also get much brighter if I put my hand near the circuit or even touched the leads. This seems similar to the ambient noise I was getting while working on rewiring my record player, which I think you said was related to improper grounding- is that right/close?
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,270
    Nov 28, 2011
    If the input amplifier is oscillating, it's most probably because of an error in the connection of the components that results in the overall feedback around the amplifier being positive, with an overall gain of more than 1, instead of negative, at a particular frequency.

    A common emitter amplifier (Q1) is an inverting amplifier. In other words, the signal coming out (at the collector) has the opposite polarity to the signal going in (at the base). This inversion is equivalent to a phase shift of 180 degrees, i.e. a delay of half a cycle at any frequency. If there is a path from the collector back to the base that adds a further phase shift of 180 degrees at any frequency, and the signal loss from collector to base at that frequency is less than the transistor's gain from base to collector, the circuit will oscillate at that frequency.

    The brightness change when you move your hand around the circuit is probably due to capacitance between your hand and the circuit affecting the frequency of the oscillation.

    Capacitance is a natural phenomenon that occurs when two conductors are separated by an insulator. In this case, the amount of capacitance is very small, because of the distance between your hand and the circuit, but a circuit operating at a very high frequency can be affected by small amounts of stray capacitance; this effect is used in a theremin.

    Instability can also be caused by improper arrangement of connections on the 0V or +V rails but this is unlikely at the low currents you are dealing with here.

    Start by double-checking the connections of all the components in the input amplifier stage.
     
  5. whiterabbit

    whiterabbit

    90
    0
    Sep 14, 2013
    Success!

    Let me first start by saying I just got *EXTREMELY* lucky. I just learned the hard way that capacitors can explode.

    Normally I sit on the floor so I can get more close-up with what I'm working on. For some reason when this happened I had decided to sit back up on the couch like a normal person. Plugged everything in- and nothing..... which was disappointing because before today it was at least showing signs that power was flowing. So as I leaned forward to investigate.. POP. Definitely going to start wearing goggles when testing circuits from now on.

    Anyways- after a calming sandwich break I returned to investigate and try again. Prior to the cap exploding I had gone over each piece and all their connections while comparing to the schematic. The only thing I found wrong was that I hadn't connected ground on the audio jack. So that would probably explain why it wasn't picking up the audio signal.

    I also used a 12v source this time instead of 9v, and connected six LEDs of each color instead of just three. This time it worked as expected.

    I'm chalking up the initial failure to both laziness and n00bishness of reading electrical schematics.

    The only thing I can think to explain why the cap popped today was that I MUST have somehow clipped the power leads on backwards... which sounds so incredibly stupid I don't know how I possibly made that mistake... but I must have, right??

    I think I may as well leave that capacitor out of the circuit because I already have a separate filter on the permanent 12v supply that will end up powering this LED organ [see https://www.electronicspoint.com/rewiring-7-band-eq-t263472p6.html]. If I'm misunderstanding the purpose of the exploded capacitor, please let me know...
     
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