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Word Clock circuit help

Discussion in 'General Electronics Discussion' started by Craig Dudley, Dec 8, 2014.

  1. Craig Dudley

    Craig Dudley

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    0
    Dec 8, 2014
    Hi all,

    I am currently attempting to build a word clock for my girlfriend for Christmas, following these tutorials:

    http://www.highonsolder.com/blog/2011/1/8/arduino-word-clock.html
    http://www.instructables.com/id/Sleek-word-clock/

    I am building it using an Arduino instead of the micro controller that they have used in these tutorials just for ease really.

    I've followed the following circuit schematic:
    http://www.highonsolder.com/display....JPG?__SQUARESPACE_CACHEVERSION=1294531692634

    But have replaced the ATMega for my arduino.

    I am now at the stage of prototyping my circuit with a breadboard and it is at this pint that I have run into troubles. I think I have followed the schematic correctly, however when I power it using my arduino there doesn't seem to be enough power to light up any of the LED's. At this stage I am only connecting one group of LED's at a time so there should be no reason that they wouldn't power with 5V going through the circuit (I think).

    All the components seem to be OK and the arduino powers fine when powered out of the circuit. Maybe its worth mentioning that when I first connect the arduino to the circuit the power lights are bright but after 30 seconds or so they are only dully lit.

    Sorry if this is a really basic thing that I have missed out here, my electronics knowledge is just about nil and this is my first experience in it.

    Any help would be greatly appreciated.

    Here is an image of the circuit that I currently have, if it helps?
    WordClockCircuit.JPG
    Thanks
    Craig
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    Hi Craig and welcome to Electronics Point :)

    Have you used resistors in series with the LEDs to limit the current? If not, you need to. Try something like 220Ω in series with each LED to start with.

    What kind of LEDs are you using? Can you show the full wiring for one of them as an example?

    What does your firmware do with the "PWM" signal that goes to the output enable pins of the 4094s?

    Does the Arduino continue to operate and update the LED control signals even when the LEDs are dim?
     
  3. Craig Dudley

    Craig Dudley

    6
    0
    Dec 8, 2014
    Hi Kris,

    Thanks for the quick reply.

    Each LED has a resistor of 10Ω I think Here is an image of them:
    WordClockLEDs.JPG
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    What type of LEDs are they? Do you have a part number for them? If not, can you measure the voltage across one of them when it's illuminated?

    Can you confirm the resistor value is 10Ω? It is important!

    What is the maximum number of LED+resistor combinations that are connected in parallel and driven from a single output of a ULN2003?
     
  5. Craig Dudley

    Craig Dudley

    6
    0
    Dec 8, 2014
    The LED's spec are the following:
    Size : 5mm
    Lens : Round Top, Water Clear
    Forward Voltage : 3.0-3.4v
    Forward Current (typical) : 20mA
    Luminosity (at 20mA) : 14,000mcdLead Length, Cathode (-) : 15mm (minimum)Lead Length, Anode (+) : 17mm (minimum)

    The resistors are 1K ohm

    Maximum LEED + Resistor combinations is 7
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    OK, I think you're going to need a higher voltage supply for the LEDs.

    The ULN2003 ICs that are driving the LEDs have a VCE(sat) voltage of about 1V. In other words, their outputs pull down to about 1V above the 0V rail. The LEDs typically need let's say 3.2V. That adds up to about 4.2V. Using a 5V supply, you only have 0.8V remaining across each 1kΩ resistor.

    That corresponds to a current of 0.8 mA which will not illuminate an LED very brightly. Typically they are run at 10~20 mA for indication applications like this.

    Although the ULN2003 is specified for a maximum output current of 500 mA, that only applies if only one output is active. With more outputs active, the allowable current per output drops. In that design, a reasonable worst case would be four or five outputs enabled at once; according to the data sheet, with five outputs activated, the maximum allowable output current is about 150 mA per output. Divide that by 7, the maximum number of LED+resistor circuits on a single output, and you get 21 mA. So let's aim for 20 mA per LED.

    Since your series resistors are 1 kΩ, if you want 20 mA per LED, you need 20V across each resistor (from Ohm's Law). With the 4.2V dropped in the LED and the driver IC, you need a supply rail of about +24V for the LEDs.

    Alternatively you could use a lower voltage such as 12V. With the existing 1 kΩ resistors, that would give a current of about (12 - 4.2) / 1000 = 7.8 mA. That's a lot better than the 0.8 mA you have now! But you can increase it by reducing the resistance of the series resistors. For 20 mA LED current the resistance needs to be (12 - 4.2) / 0.02 = 390Ω.
     
  7. Craig Dudley

    Craig Dudley

    6
    0
    Dec 8, 2014
    Thanks Kris.

    I've learnt a lot more about all this just from reading your last post!

    I have found that the problem I am experiencing with the arduino powering down actually happens before I have connected up any of the LED groups. Does that make any difference?
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    It sounds like it might be a separate problem. Does the Arduino run properly with nothing at all connected to it?

    BTW, to clarify: You can (and should) power your CD4094s from the 5V supply from the Arduino, but the LEDs need a separate supply. The negative sides of the two supplies must be connected together.
     
  9. Craig Dudley

    Craig Dudley

    6
    0
    Dec 8, 2014
    Yeah the Arduino runs fine when it is not connected to the circuit and lights up the LED group with no trouble. I hadn't actually thought of lighting the LED's with a separate supply I will give that a go.

    Could it be that there is a short circuit somewhere?

    If so how would I identify one?
     
  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    There could be many reasons for the problem. I would start by connecting your meter across the 5V rail on the Arduino and seeing what happens to that. Also, double-check your connections to the off-board circuitry, and perhaps take a few close-up photos so we can check them too.

    You don't have many connections - just 5V to the 4094s and a few control signals, right? That should be easy enough to check.

    You might also benefit from drawing a complete schematic of what you have built, working from the actual construction. This may help clarify what you have built, and show up any mistakes.
     
  11. Craig Dudley

    Craig Dudley

    6
    0
    Dec 8, 2014
    OK I'll give that a go.
    I'm away for the next 2 days so my plan is to jump back on this on Friday.
    I'll keep you updated!
     
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