555 monostable questions!!!

Discussion in 'The Projects Forum' started by riz, Feb 21, 2013.

  1. riz

    Thread Starter New Member

    Feb 20, 2013
    8
    0
    Hello everyone,
    These days I am working on a project in which I have to design a circuit board to activate 4 pneumatic valves. I have made the circuit on circuit wizard software and simulation works fine for it. So I was looking at some circuit diagrams and some of them had relays in it. Just wondering when do we use relays in a circuit? Also the project i am working on is something like this there are 4 pneumatic valves connected to the airbags in a car and as someone presses a button, airbag goes off instantly. Just like that you press another button second airbag goes off. For the programming of valves I have to use four 555 monostable circuit. So can anyone also tell whether relay would be required here or not? (I want airbags go off instantly as the button is pushed down).

    Also I am going to use a variable resistor connected in series with a 1k ohm resistor to program my circuit correctly. So my question is that how do I select the appropriate resistor? Any criteria or calculations other than T=1.1RC?


    Also I have designed this circuit in the software called Circuit Wizard and it works fine in the simulation. In real I want to connect four such circuits connected in parallel to power supply .
    [​IMG]

    I think I have described my problem clearly, so please do reply to me as i am worried about this project :S
    Thank you
    Waiting for replies!
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    You did not wire the timing part correctly.
    See this post made by Bill_Marsden for more info:
    555 Monostable

    Bertus
     
  3. riz

    Thread Starter New Member

    Feb 20, 2013
    8
    0
    ahan thanks for the reply!
    Other than timing part can you please tell me if everything is correct?
    like the components used and is 555 monostable circuit really suitable for this task? as mentioned in my previous post.
    Please do reply me!
    Thanks
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    Now I see you are using airbags.
    Is the circuit going to be used in a car?
    Automotive modifications are a no no over here.

    Bertus
     
  5. riz

    Thread Starter New Member

    Feb 20, 2013
    8
    0
    hi there! no this car is only an exhibit for reusable airbag system! the engine is already stripped off!
    The valves to be used are solenoid ones.
    So what do you think? is monostable 555 suitable here? like I want that mechanism in which someone presses a button and an airbag goes off immediately!
     
  6. BarryBon

    New Member

    Feb 13, 2013
    1
    0
    Hi Riz,
    The 555 can only source 100ma. Your circuit as drawn will supply a little over 10v through the coil of your solenoid. If the coil of the solenoid has a resistance of 20 ohms, pin 3 of the 555 will see a little over 500ma. That's why you would need to use a relay. The 555 picks the relay with less than 100ma so all the big current stays on the solenoid side. You can probably find lots of diagrams of 555 circuits and relays. Good luck.
    Barrybon
     
    Last edited: Feb 21, 2013
  7. sheldons

    Well-Known Member

    Oct 26, 2011
    616
    101
    to prevent damage to your 555 -heres a suitable output stage for use with your solenoid,also you will need a decent power supply when you get everything up and running....12v battery or mains power supply
     
  8. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    Hello riz.
    The diode D1 LED? in your circuit must have a series current limiting resistor.
    With a 12 VDC supply and an LED with a 2.1 volt drop you could use a 1k resisitor.
    And you still need a reversed biased diode across the solenoid as shown in Sheldons' circuit.
     
  9. mecha

    New Member

    Nov 10, 2012
    18
    0
    "The diode D1 LED? in your circuit must have a series current limiting resistor."

    ditto that. The flyback diode (across the inductive load, ie solenoid) and the display led are quite different things, two separate circuits off pin 3.

    Regarding the relay, the point is this - the solenoid could draw 100 to 1000 times the power the 555 can deliver. When you try to suck too much power through a 555, the magic smoke comes out.

    What you need is an output stage - basically a power amplifier. Depending on current draw, a TIP 120 or similar darlington will get you out of trouble. You'd need a transistor to drive the relay - probably- anyway.

    Given the number of 'tips' offered about your circuit, my observation is that if Circuit Wizard says this circuit will work, throw it away or ask for your money back - it should be called Circuit Dunce!
     
  10. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Why do you need any electronics? Simply connect the solenoid to a battery via a switch.
     
  11. riz

    Thread Starter New Member

    Feb 20, 2013
    8
    0
    ahan i have 12v battery with me. if i use this arrangement after my pin 3 would it be ok then? i could see a number of resistors with different values such as 220k, 1k,10k so do i have to use the exact rating of resistors as shown?if not then how do i know the correct rating of resistors i am using? and is Q1 also a transistor? Is there any specific name or some code such as BC(some number) like that? also is there any criteria for transistor,diode selections?
    Waiting for your reply
     
    Last edited: Feb 23, 2013
  12. riz

    Thread Starter New Member

    Feb 20, 2013
    8
    0
    ahan i dont know! i was so excited that my circuit was working fine! it even lit up my LED in simulation and i was wondering how could that be possible without having a resistor in series!
     
  13. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,365
    Because simulators don't do a good job simulating smoke.
     
  14. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    Are the air bags you are using electrically or mechanically triggered?
    Are you going to trigger them all at the same time?
    What is the purpose of this...a display or a training exercise?
    Is a person going to trigger the airbags at a certain time? If not, what?
    And to repeat Mr. Chips question, why do you need a 555 circuit at all? Why can't you just use a simple switch?
     
  15. sheldons

    Well-Known Member

    Oct 26, 2011
    616
    101
    you can use any suitable n channel power mosfet in the op stage iv given you and you need to stick to the values shown...also your 555 timer circuit is incorrect
     
  16. sheldons

    Well-Known Member

    Oct 26, 2011
    616
    101
    as an alternative,you can use edge triggered monostables-made from either discrete transistor stages or gates as shown here....
     
  17. riz

    Thread Starter New Member

    Feb 20, 2013
    8
    0

    no i dont want airbags go off at same time.
    the purpose of this is more like a display like you press the button and an airbag will go off!
    well 555 is required because the valves to used will need programming for time!
     
  18. riz

    Thread Starter New Member

    Feb 20, 2013
    8
    0
    ahan! thanks for the reply!
    how is my 555 timer circuit incorrect? can you please tell me the reason so i can correct it?
     
  19. sheldons

    Well-Known Member

    Oct 26, 2011
    616
    101
    you havent got the connections to the 555 timer correct-pin 7,pin 6....this schematic will give you an idea connection wise plus if you download the 555 data sheet it shows you all you need to know connection wise etc
     
    riz likes this.
  20. riz

    Thread Starter New Member

    Feb 20, 2013
    8
    0
    Thank you so much for correcting me :D Thank you once again :)

    So I have made the circuit again, i havent added that transistor bit as i will add it tomorrow then post it here :D

    [​IMG]
     
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