Help with "noise" in Time Delay Circuit

Discussion in 'The Projects Forum' started by Newbee, Jun 10, 2008.

  1. Newbee

    Thread Starter New Member

    Jun 10, 2008
    3
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    I need some assistance. I've wired a simple time delay circuit using the 556 timer based on a 9 VDC power source. The output of the time delay is a relay switch which is intended to relay to a 12 VDC circuit. No big deal - at least that's what I thought.

    The timer works fine and activates the relay switch at the end of the intended time frame. My problem comes in when I attach the time delay circuit to the intended device. The intended device is a 12 VDC circuit that uses 16 gauge wire and a conventional AC plug (like a grounded house plug - sorry I don't know the correct term). When I connect the output of the delay circuit to a 16 gauge wire and house plug, the delay circuit continues to reset after the initial delay and firing of the 12 VDC device.

    It acts as though when connected to the heavier wire (the delay circuit is constructed on a simple breadboard - I wanted to make sure it worked before soldering it on a permanent board) the heavier wire is picking up some "noise" or other interference and shorting the time delay circuit - starting the timer over and over again. If I remove the heavier wire connection, the time delay circuit seems to work just fine.

    Is there some way to transition the input and output (from a bread board or soldered circuit) leads to 16 gauge wire and minimize the risk of the delay circuit from continually resetting? I'm new at this electronic circuits hobby, so I hope that I've explained the problem correctly. Any help would be greatly appreciated.
     
  2. Newbee

    Thread Starter New Member

    Jun 10, 2008
    3
    0
    Oops - I left out a crucial piece of information. The input leads to the delay circuit are also 16 gauge wire with a common household plug. I'm pretty sure this is where the short must be occurring for the delay circuit to keep triggering.

    Basically the input and output leads from the bread board are 16 gauge wires with common household plugs.
     
  3. RiJoRI

    Well-Known Member

    Aug 15, 2007
    536
    26
    A schematic may help us, but first of all, do you have a diode across the coil of the relay?

    --Rich
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Two things, you need a large (50uf or bigger) decoupling capacitor across the 555 (or 556 in this case), and the second is 555's have a pin that is designed to desensitize the chippie to noise (pin 5 in the case of the 555). You need to add a capacitor to it, something in the 0.1uf range. Either or both will probably fix your problem.
     
  5. Newbee

    Thread Starter New Member

    Jun 10, 2008
    3
    0
    RiJorI,

    Thanks for the response, I did place a diode in parallel with the coil of the relay switch (I think I used a 40001?). Here is a schematic of the timer that I built - I have to give credit where credit is due - I obtained the schematic from Hiviz.com. The only difference is that I'm connecting directly to the output of the second timer pin 9 (as opposed to the SCR output).

    http://hiviz.com/tools/triggers/DELAY.GIF

    Bill,
    Judging by the capacitors that are in the circuit - I'm guessing that I don't have a large enough capacitor in the right place. Could you please indicate where the 50uf decoupling capacitor should go? Like my name implies - I'm a newbee.

    I appreciate everybody's help on this one.
     
  6. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    [​IMG]

    OK, Pin 5 is the filter pin on the 555, and the equivalent pins on the 556 are 3 and 11.

    That filter cap is on the power supply pins, a coil won't do it.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Bill, what on earth are the numbers on the inside of the rightmost 555 depiction?

    I find them very confusing!
     
  8. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    Translation for a 556 pin numbers into equivalent 555 pin numbers. In other words, when you have the 555 practically memorized it allows easy reading for a 556, at least for me. I've used a LOT more 555's than 556's.

    You will note I included the can version of the 555. This is because I started with them. :p

    I lifted a section from my pinouts template on PaintCAD to post them here.
     
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