Opening switch with capacitor + ic ?,

Discussion in 'The Projects Forum' started by namara, Oct 2, 2010.

  1. namara

    Thread Starter New Member

    Oct 2, 2010
    9
    0
    HI,
    im building a circuit for an LED clock display
    i would like the display to turn off after a period of time automatically,
    apart from using a time delay switch,
    it seems like there is another way to do this using a capacitor and IC,
    i.e the IC outputs a high when the capacitor reaches max.
    what type of IC can sense the capacitor max.

    THANKS FOR ANY HELP,:)
     
  2. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    You can use a comparator, like the LM339 (which has four-in-one.) Remember, the LM339 is a current sink, so it needs a pull-up resistor to generate a logic signal.
     
    namara likes this.
  3. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    I agree, just use an RC tank and compare the voltage on the cap to some arbitrary reference (such as a resistor voltage divided). Depending on what you're doing, you may want to charge the cap through a diode and let it decay thru the resistor.
     
    namara likes this.
  4. namara

    Thread Starter New Member

    Oct 2, 2010
    9
    0

    hi tom,thanks for advice,
    im fairly new to electronics,
    i have drawn a circuit on multisim,
    i was wondering if you wouldnt mind having a look at it ,
    to see what needs to be changed---its a small circuit,
    so to recap--the objective is to have a mini solar panel
    charge the battery,the circuit would be triggered to emit the led(clock)
    (roughly-560mA), turning off after roughly 2 seconds,

    thnks graham,
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,179
    1,800
    DO NOT forget to observe the common mode range restriction on this part.
    DO NOT make the mistake of trying to sense a voltage that is close to Vcc
     
    namara likes this.
  6. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    Post your circuit, I'll have a look at it. (Please do not PM me.)

    I don't understand your LED figure. 560mA is too much for a typical LED - is this a power LED of some sort?
     
  7. namara

    Thread Starter New Member

    Oct 2, 2010
    9
    0
    i didnt explain that i meant,
    a 7 seg.display(x4) ,
    140m.A per 7.seg.

    unfortunately its not possible to upload multisim files here,
    and theres no other format to save the file in,in the program,
    iyl send you it via email if thats ok,
    mcnamcnamara@hotmail.com

    thanks,
     
  8. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    I don't have Multisim, so can't open it.

    Use the Print Screen button (Prnt Scrn, Print Scr, Print Scrn, whatever) on your keyboard and paste the result into the image editor of your choice. On Windows, Paint works well; just press Ctrl-V in Paint to copy the screenshot to the editor. Then save the file as a PNG (important otherwise it will be massive and take long to upload) and post it here.

    140mA is a bit much for a 7 seg display. 20mA per segment. I haven't seen many driven above 10mA per segment, or 70mA per display, or 280mA for 4 displays.

    Tom
     
  9. namara

    Thread Starter New Member

    Oct 2, 2010
    9
    0
    another not very well description by me ,i meant, 140mA per display,
    with 4 displays,
    i would like displays twice the usual size,so thinking
    the current will be more like 1A+

    the png file should be with this,
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    I agree that an RC tank and a comparator will work, or a 555 circuit. See my post here:
    http://forum.allaboutcircuits.com/showthread.php?p=277593#post277593

    for comments and a diagram of how I did exactly what you want. Note that I already had a spare comparator on the quad LM339. Starting with nothing, it might be better to look at the 555. Simpler, I think. Less parts, more control.
     
    namara likes this.
  11. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    A few points before I go to the land of Zzz's:

    You are operating the LM339 at 1.2V. It will not work at this voltage, it requires at least 2V, I'd recommend 2xAA to give you 3V.

    The LED should be in parallel, going to ground, and remember to put a resistor in series to prevent your circuit from cooking it. For 3V, 150 ohms works well. I assume you want this to indicate the switch is on. Also, I'm not sure but I think the cap should be in parallel as well.

    There should be a 1k resistor to the battery positive connecting to the output. The output will then go from almost 3V to almost 0V, instead of just sinking current, which isn't particularly useful except for driving LEDs.
     
  12. Papabravo

    Expert

    Feb 24, 2006
    10,179
    1,800
    And of course you have completely ignored the common mode voltage range restriction.
     
Loading...