Pls help me find problem with 555 circuit

Discussion in 'General Electronics Chat' started by JimG, Jan 31, 2010.

  1. JimG

    Thread Starter Member

    Dec 7, 2009
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    Below are schematic and board images for my circuit. Its job is to generate a linearly increasing signal at the output of around ~229 mV per min rate of rise. (The output signal is used to calibrate another instrument).

    LED1 sets the control voltage at around 2V, so the voltage across C1 should be a sawtooth that cycles between 1V and 2V, with a period of 254s.

    R8, C3, and Q2 make up a timing circuit to charge C1 quickly upon startup.

    R1, Q1, and D1 provide a constant charging current of 12.6uA into C1.

    The problem is this: a couple of seconds after startup, C1 passes the threshold voltage of 2V. At this point, the NE555 discharges C1. The NE555 output drops low. So far, so good.

    But....the NE555 reset pin is also dropping low at this point. So the NE555 output stays low, the discharge continues to sink current, and C1 does not begin to charge. Expected behavior is for the NE555 output to go high when C1 has dropped to 1V, and begin the slow charging cycle.

    I've modeled this extensively and have not yet found the flaw in the design. I have also built and used a similar circuit successfully (the differences being that this new circuit's sawtooth is 10X steeper, plus the addition of the R8/C3/Q2 quick charge circuit at startup).

    What really has me scratching my head is that the 555's reset pin is going low despite it being tied to V+.

    Thanks for any suggestions.

    Jim
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Why do you have a resistor connect to pin 4? It can be tied directly to the Vcc line. Generally the only reason for a resistor at this point is to allow something like a transistor to turn the device off. Try removing R6, it is not needed, and is way to high a value for this chip.

    I'm having serious problems following your schematic, so I'll redraw it a little to see if it can't be made clearer.
     
    Last edited: Jan 31, 2010
  3. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    Any chance there is a short between the output on pin 3 and the reset on pin 4 of the 555?
     
    Last edited: Jan 31, 2010
  4. Wendy

    Moderator

    Mar 24, 2008
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    Wish I had said that. :D
     
  5. JimG

    Thread Starter Member

    Dec 7, 2009
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    Great call!

    Before mounting the chips in the sockets, I checked all the signals with the continuity beeper on my DMM, and 3/4 tested OK. But a short here did seem to be the most likely explanation. So I ran an exacto blade in the gap between the solder traces for these signals.

    Must've been a splatter or something in that gap that wasn't showing on the DMM, but that was shorting out the reset pin at 12V.

    Fixed now, and working perfectly. Many thanks!

    Jim
     
  6. JimG

    Thread Starter Member

    Dec 7, 2009
    53
    0
    Bill -

    Didn't/don't have a good reason. Just seemed prudent to put something there.

    As it turned out, the resistor probably made the problem harder to locate. A dead short woulda prolly gotten my attention quicker :eek:

    Jim
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Turns out that it was a good thing you had a resistor as a pull-up - otherwise you would have burned things up! :eek:

    It's OK to have weak pull-ups on pin 4. Some designs leave off pull-ups entirely - not that I think it's a good idea to do so.
     
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