7555 timer board

Discussion in 'The Projects Forum' started by tresguey, Apr 17, 2014.

  1. tresguey

    Thread Starter Member

    Apr 22, 2013
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    I built this board and after putting it together it doesn't seem to work. It is a simple 555 timer circuit. As I am new I just wanted to see if anyone can see a flaw in the board. It looks good to me.

    http://oshpark.com/shared_projects/cogcWq5z
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Post a schematic, and closeup photos of your assembled board, and you will get better help.
     
  3. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    The traces are hard to see, but I think this is what it looks like, if you add connection from pin 7 to top of R1. Only the bare minimum was etched away, so component side is + battery & back is - battery. Might be a clip to hold CR2032 battery inplace.
     
    Last edited: Apr 18, 2014
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    If Bernard's diagram accurately represents the PCB it will not work because the sensing sytsem is complete (pins 2 and 6) but there is no discharge mechanism for the timing cap C1.

    Normally there would be a connection from between the two resistors (VR1 and R1) to the discharge pin 7.
    :)

    It is also a good idea to put a small resistor (220 ohm) between the top of Vr1 and the + power pin. That protects from overcurrent fault if the pot VR1 is turned to zero ohms.
     
  5. ScottWang

    Moderator

    Aug 23, 2012
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    The common point of VR1 and R1 already were connected to the pin 7.
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    Upload the schematic to the forum.
    If you mean that it doesn't work is the LED didn't flash, you should check the LED first, and the value of R2, and measure all the wires.
     
  7. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    One question I have, are the ground connections of C1, pin 1, & LED plated thru holes or soldered in jumpers. As THE_RB pointed out, a resistor in series with VR1 would be adviseable, or it might already be part of VR1.
     
  8. tresguey

    Thread Starter Member

    Apr 22, 2013
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    Here is the schematic from KiCad


    View attachment Blinky.sch


    The ground connections are plated through holes to ground on the back side of the board.

    The LED does work I did de-solder and checked it. When the battery is placed in the circuit it gets drained quick.
     
    Last edited: Apr 21, 2014
  9. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    Many of us don't have kicad. Can you print a pdf of the schematic, and post that?
     
  10. tresguey

    Thread Starter Member

    Apr 22, 2013
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  11. ScottWang

    Moderator

    Aug 23, 2012
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    Vcc to Pin 7 can use 1K, R1 can be replace by a 10K+VR500K, 100K+VR500K or 220K + VR500K, or 470K+VR100K, etc..., it depends on how large the range you like to adjust.
     
  12. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    A circuit built to the schematic should work so long as the voltage across the LED does not exceed the voltage out of pin 3, but really there should be a .1uF cap across the 555 power pins and a .01uF cap from pin 5 to ground.
     
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