On-Off Momentary LED circuit for keyboard status lights?

Discussion in 'General Electronics Chat' started by HaloArchive, Nov 21, 2014.

  1. HaloArchive

    Thread Starter New Member

    Nov 21, 2014
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    Hello All,

    I have soldering skills but not so much on the design side. I am looking to make a circuit that has a feed of 3.3v-5v on my keyboard that when I hit the caps-lock or num-lock or scr-lock that it actuates the respective LED. The switches are mechanical so, they are momentary switches but I want to hit the switch, the led stays on until I hit the switch again.

    Any cool circuits you guys have for this? -- Preferably from readily available parts on amazon or somewhere with fast (2 day) shipping.

    Thanks!

    EDIT: These are the LEDs that will be used: http://www.amazon.com/microtivity-I...601802&sr=8-1-spell&keywords=microivity+il188
     
    Last edited: Nov 21, 2014
  2. HaloArchive

    Thread Starter New Member

    Nov 21, 2014
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    After looking a little, I realized I dont have a number pad so, only capslk and scrlk. I want to use a Red and a Green LED respectively.

    Here is some data for the LEDs:
    Forward Voltages are Red, Orange and Yellow 1.9-2.0V | Green: 2.9-3.1V | Blue, White, Pink and Violet: 3.0-3.2V.
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Doesn't any normal keyboard made in the last 30years do this automatically? There are dozens of used keyboards at my local thrift store that could be bought for $1
     
  4. HaloArchive

    Thread Starter New Member

    Nov 21, 2014
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    I am building my own and using a beagle-bone as a controller. Pointless ... yes, cool ... yes!
     
  5. RichardO

    Well-Known Member

    May 4, 2013
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    If you are building your own keyboard then you will be scanning the switches with I/O lines from the microcontroller. The microcontroller will do the toggle function and LED driving using software.
     
  6. HaloArchive

    Thread Starter New Member

    Nov 21, 2014
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    Unfortunately I wish to not hog clock cycles for something that can be controlled with hardware especially since I will not be using a traditional keyboard matrix. I will be doing some other things with this board as well. Regardless, I would like to build the circuit I am looking for in my original post for another project to. Could be a great learning experience :)
     
  7. HaloArchive

    Thread Starter New Member

    Nov 21, 2014
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    Anyone?
    I attached a pic of my project :)
     
  8. HaloArchive

    Thread Starter New Member

    Nov 21, 2014
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    It looks like some people use a JK-FF. Could I use a TI SN74HC109N (http://www.ti.com/lit/ds/symlink/sn74hc109.pdf) and connect the following:

    Vcc to the beaglebone 3.3v
    Gnd to beaglebone Gnd
    1J and 1K to 3.3v
    1CLR to Gnd

    Button between 1CLK and Gnd
    10K resistor between 1CLK and 3.3v

    LED between 1Q and Gnd using whatever resistor is appropriate.

    Sound right?

    Thanks!
     
  9. absf

    Senior Member

    Dec 29, 2010
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    Even if it works with HC109, how are you going to connect the keyboard matrix to the JKFF? Does grounding one side of the keypress allowed by your keyboard circuit?

    Allen
     
  10. HaloArchive

    Thread Starter New Member

    Nov 21, 2014
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    Grounding one side shouldnt be a problem. When the key is pressed, the diode on the matrix will allow the signal through and will also allow vcc through the piggyback circuit for the light. Unless im not thinking of this correctly?
     
  11. absf

    Senior Member

    Dec 29, 2010
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    Well, if you think so then just follow the wiring of HC109 as attached....

    LED for KB status.PNG

    But first try it out on a breadboard with 3.3V on Vcc. If it works as you planned, then connect the input to your keyboard matrix.

    cheers...........

    Allen

    PS...May be you need a power-up reset on the HC109, if the LED isn't on OFF state every time you switch on your keyboard.
    Try it out first and let me know.
     
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  12. HaloArchive

    Thread Starter New Member

    Nov 21, 2014
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    Thank you very much Allan,

    Quick couple questions ... I see this chip is a dual JKFF, would it be possible to use this one chip and connect both of the FF's to control both of the lights with a different resistor for the green LED? Or, is the power rating on the spec sheet only appropriate for the total combined power output thus only allowing me to drive one led per chip?

    ----------
    And for the second question, is there a way to use a couple of these chips to make a rotating led setup like a shift register also using a momentary button? Example:

    initial state: LED 1: On, LED 2: Off, LED 3: Off
    press button once: LED 1: Off, LED 2: On, LED 3: Off
    press button again: LED 1: Off, LED 2: Off, LED 3: On

    Thanks!
     
  13. absf

    Senior Member

    Dec 29, 2010
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    Sure, you can do that.


    That should be easier with a 4017 chip. I tried with your JKFF and I came out with the pattern:

    initial state: LED 1: On, LED 2: Off, LED 3: Off
    press button once: LED 1: on, LED 2: On, LED 3: Off
    press button again: LED 1: on, LED 2: on, LED 3: On

    Di you try google "making a shift register using JK flip flop"

    That should give you some clue...........

    Allen
     
  14. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Or use a shift register… 74HC595, although supporting circuitry necessary may make it undesirable.

    I do like the 4017 chip.

    I'd say, don't get hung up on the JKFF. Remember the old saying, if all you have is a hammer then everything starts to look like a nail.
     
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  15. HaloArchive

    Thread Starter New Member

    Nov 21, 2014
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    Well, Id prefer to use a bunch of 74HC109's because that is what I have on hand and dont really want to wait several days for another chip :)

    Allen,

    That would work as well ... Lets say I used the LEDs with the 3.0v forward voltage, do you have a schematic for it?

    After the third press, does it go back to the initial state or an all-off state?

    And yes, I did initially convert the JKff's into Dff's from my college days and tried to make a shift register but I think I had things wired incorrectly.


    Im at the point everything is a nail :) But yes, in the future when I run low on JKff's I may decide to widen my spectrum of available logic chips.

    ... as of right now I have 20 dual jkff's 74HC109
     
  16. absf

    Senior Member

    Dec 29, 2010
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    OK, here's the led chaser using your JKFF HC109. It needs a 555 timer for the clock and an extra 74HC04 NOT gates. If you dont have HC04 you may use NPN transistors and resistors wired as inverters.

    See attached HC109 led chaser.PNG

    The LED chaser would have the pattern mentioned above and all LED would be Off when it reaches the n-1 LED. The nth output is used to reset the JKFF and cannot be used to light up a LED. You can cascade as many JKFF as you want provided your DC supply is sufficient to provide the current needed. For example using 6 JKFF would give you 6n-1 ie (6x2)-1 or 11 LEDs.

    Hope you have a 555 in your spare box.

    Allen

    [update] R5 may not be needed. you can remove it.
     
  17. HaloArchive

    Thread Starter New Member

    Nov 21, 2014
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    Excellent, I will build inverters with a 2N3904 and two 1k resistors. Seems it should work. I will breadboard this tomorrow and report back.

    Thanks so much!

    EDIT:

    Just realised the 555 was for driving the chaser. However, it will also be operated off of a momentary pushbutton. I will just place the pushbutton similar to the first schematic.
     
  18. HaloArchive

    Thread Starter New Member

    Nov 21, 2014
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    Alright, update post ... It looks like I need to debounce the switch ... only works sometimes. What would I add onto the schematics?
     
  19. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
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    Here is a good reference to debouncing switches. You can use a simple RC circuit to debounce your input (few parts). I was looking for the specs on your input voltage, but didn't see them. If they're in the range of CMOS logic gates, you can use an SR latch as a debouncer.

    The RC circuit is as follows. Note in this example, the switch is connected to ground. Later, is an example where the switch is connected to Vcc.
    [​IMG]

    And here is an example of how it is used connected to a JKFF. Note in this example, the switch is connected to Vcc.
    [​IMG]
     
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  20. absf

    Senior Member

    Dec 29, 2010
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    Since you dont have 4027 JKFF, you may use HC109 as the debouncer as in the circuit above.

    I think it is time to get some more chips like 555, 4017, inverters, NAND and NOR gates if you want to progress with your experiments.

    BTW, is the circuit in post #11 working? Did you try it out on your keyboard?

    Allen
     
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