Help with timing circuit

Discussion in 'The Projects Forum' started by SR46, Apr 14, 2013.

  1. SR46

    Thread Starter New Member

    Apr 14, 2013
    4
    0
    Hi,

    I have a really urgent problem with a circuit I am building for a project. To summarise it, it is a target that is made from two conductive layers with a foam insert between. When an object hits the mat, the two conductive layers touch and light a light. The 14538 monostable circuit holds this light for 2 seconds. I have 5 areas on the mat and the other layer of the mat just has the one common. Therefore I have 5 LED blocks that will be lit.

    I have built the circuit and initial testing shows that it works, however, after putting it together I face this problem where in which when i touch the ground wire to an area wire, ALL the lights turn on for 2 seconds instead of the specific light. However, when I just use a short piece of wire to and tap the screws on the terminal block for LED 1 and ground, the specific light comes on and works properly. However, this is a bit hit and miss and sometimes all lights come on and sometimes the specific light works. Where do I start to trouble shoot this? Images are below to highlight my problem.

    When touching these two long wires together that lead to the mat, it causes all lights to turn on:
    [​IMG]
    [​IMG]

    However, when I use a loop, it sometimes work how it should like so. However, one thing I have noted (not sure if it effects it) but if i push the wire into the screw on the terminal quite hard, it is more likely to turn all lights on:
    (hard to see but only the blue light is on)
    [​IMG]
     
  2. SR46

    Thread Starter New Member

    Apr 14, 2013
    4
    0
    Now I have checked all the tracks in the circuit, and ensured solder hasn't gone over adjacent tracks because, as I said, when using a small wire between ground and the specific area, it lights the single light properly. But other times it doesn't and that is what makes no sense to me.
     
  3. #12

    Expert

    Nov 30, 2010
    16,318
    6,818
    Post the schematic that you used to build it.
     
  4. SR46

    Thread Starter New Member

    Apr 14, 2013
    4
    0
  5. SR46

    Thread Starter New Member

    Apr 14, 2013
    4
    0
    After more testing and realising that it is the long wire that seems to be affecting it as the short wires seem to work fine. I am lead to believe that it could be cross talk? How could I fix this?
     
  6. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,869
    1,393
    Often, problems of this type turn out to be electrical noise, sometimes from the power supply. Be sure that your power supply is properly filtered and regulated. In addition, add decoupling caps across the power pins on each IC.
     
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