Is this a correct 7555 circuit?

Discussion in 'The Projects Forum' started by Fenris, Jun 21, 2009.

  1. Fenris

    Thread Starter Active Member

    Oct 21, 2007
    288
    2
    Hi All

    I am drawing up a vero board LED matrix oscilloscope from a schematic found on the web. But the 7555 based clock circuit doesn't look right. I have added where I think the +VE supply should go if it is a working iteration of such a clock. But it just doesn't look right to me but I may be wrong........in all likely hood :D Could some one just say yay or nay please.

    regards

    Fenris
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Yay. ;)

    I'd increase R10 to around 2k though.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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  4. Fenris

    Thread Starter Active Member

    Oct 21, 2007
    288
    2
    Thank you both :) I have never seen this variation and couldn't find anything that looked exactly the same. I managed to work out the +12V line though :) Although I guess it's kind of obvious you don't want a variable resistor between the supply and pins 4/8........ isn't it?

    thanks again

    regards

    Fenris
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Fenris,
    I made a slight modification to your circuit:
    [​IMG]

    R10 actually didn't need to be increased; discharge current will be a maximum of 8mA. However, R11 needed to be added; otherwise if you set the 1M pot to 0 Ohms, the discharge pin of the 555 would be trying to short out the power supply, which would result in smoke. :eek:

    7555 timers wired as astable multivibrators (like your schematic is) charge the timing capacitor (C3, C4 or C5) to 2/3 Vcc, and then discharge to 1/3 Vcc. Since 2/3 Vcc is in your case 12 x 2/3 = 8v, the maximum current through the discharge pin will be 8v/1000 Ohms = 8mA.

    Now if R9 (the 1M pot) were set to 0 Ohms, there would still be R11 in the current path. Since R11 is 4K3 Ohms, maximum current would be 12v/4300 Ohms = 2.79mA, for a maximum total discharge pin current of 10.78mA. This is well within the capabilities of the 7555 timer.
     
  6. Fenris

    Thread Starter Active Member

    Oct 21, 2007
    288
    2
    Thank you Sgt much appreciated! This circuit is the clock which drives the 4017 pair which in turn drive a trio of 4049's which in turn drive the column lines of the matrix. Similarly the row lines are driven by a pair of 3914's which then drive a trio of 4049's. From the pictures on the guys site this circuit seems to have a very good presentation of audio signals :)

    Heres the site addy. I did find that the guy had messed the pin ID of pin 4 and pin 8 of the LF353. Pin 8 was marked as pin 11 and the +ve and -ve was the wrong way round as well.

    http://www.geocities.com/rlaude2000/ledscope.htm

    The LF needs a dual rail supply the rest of the circuit is single rail.

    regards

    Fenris
     
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