Variable 555 Timer Circuits (Monostable)

Discussion in 'The Projects Forum' started by Dorfmandesign, May 10, 2011.

  1. Dorfmandesign

    Thread Starter New Member

    May 10, 2011
    3
    0
    I have been designing a circuit with a 555 timer set in a monostable capacity.

    The 555 timer triggers a 5v DPDT sub mini relay, which in turn triggers a power isolated DC motor. Additionally, the circuit includes variable resistors to change the run time, as well as a reset button to stop motor function if needed. Both of these circuits are powered by separate 9v batteries.

    When put together on the breadboard, the system works perfectly. However, when translated to PCB, it will only turn on and off (on when i press the trigger, and will only turn off when i press the reset, but it will stay off until the trigger is pressed again). So essentially, this giant circuit is an on/off switch.

    Something is clearly wrong in my jump to PCB, but I do not know what to look for. Included are the schematics as well as the PCB layout. I have checked connections as well as polarity of items that require it.

    Thank you in advance for any assistance.

    View attachment Turntable Unit_pcb.pdf

    View attachment Turntable Unit - Schematic_schem.pdf

    Parts list;
    (2) - 1k resistors
    (1) - 220 resistor
    (1) - 1000uf electrolytic capacitor
    (1) - .01uf ceramic capacitor
    (2) - 9v Batteries
    (2) - Normally Open Momentary switch
    (1) - 5v DPDT Sub Mini Relay
    (1) - 555 timer IC
    (2) - 10k sweepable resistors (103 series)
    (1) - 3mm LED
    (1) - DC motor
    (1) - On/Off Switch
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    Welcome to AAC! I've converted your images into something easier for other folks to look at.

    Wiring errors are always possible. The other thing is filter caps, you need a 0.1µF and a 220µF on Vcc, as well at a 0.1µF on pin 5. Motors are notoriously noisy parts. Seems like I've had to design several lately for folks here. Looking at your PCB I see several issues.

    [​IMG] [​IMG]

    Besides the lack of filter caps I also don't see a swamping diode for the relay coil. Anything you deal with coils (like relays) you should have one similar to these circuits. Pin 2 on your schematic needs a pull up resistor, you never leave this input floating (most likely problem).

    [​IMG]

    [​IMG]
     
    Last edited: May 10, 2011
  3. Dorfmandesign

    Thread Starter New Member

    May 10, 2011
    3
    0
    Thank you for the response.

    Is this how the filters would be hooked up?

    Turntable Unit - SchematicV2_schem.jpg

    I should mention that on my breadboard it works perfectly.

    I've built two different PCBs, with different parts, and both work as described above.
     
    Last edited: May 10, 2011
  4. Dorfmandesign

    Thread Starter New Member

    May 10, 2011
    3
    0
    Does this mean the 1k resistor on the trigger is on the wrong location, or that an additional resistor is needed? If so, what value in ohms would I use?

    Thank you.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    You do realize your filter caps have a dead short across them? They should be across the power supply lines.

    You are fighting an intermittent because you are making a rookie mistake. Inputs (especially comparator inputs) should not be left open (IE, floating). You need a pull up resistor, just because you got lucky and it seemed to work doesn't obliviate this. The exact value of the resistor isn't important.

    All 555 monostable circuits, almost without exception, need a pullup resistor to Vcc on pin 2. Without it the comparator gets to freewheel, or pick its own value if you will.

    555 circuits tend to be specialty of mine. :)

    Bill's Index

    LEDs, 555s, Flashers, and Light Chasers

    The 555 Projects

    555 Monostable
     
    Last edited: May 10, 2011
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