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  1. #1
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    Getting trigger signal for 555 timer from elevator button.

    I want to turn a load like light on for a certain time when one of elevator buttons pressed say button 1 and while the button pressed the load should be on.

    In the figure below circuit 1 describe how the controller of the elevator read button .

    I tried two circuits to get low signal to trigger 555 timer .

    I tried circuit 2 with 10k for R1 and R2 and i faced a problem . *the led of pc817 always on an this tell the controller the button always pressed.

    So i tried to change R1 and R2 to 1M the problem disappeared. But another problem happened when the motor of elevator start this trigger the 555 timer :( I am using decoupling caps 0.1u and 10u.

    So i tried to add C1 to trigger pin with different values 0.1u ,1u,10u,47u and 100u the problem still exist.

    So i went to try the circuit 3. The problem when the elevator motor start run still exist . So i tried to add 1 uf cap to trigger pin the problem disappeared.

    Does the last circuit i used practical circuit and will work properly? is there another approach better than what i tried do what i need?

    Note:i cant use the controller because the controller board of the elevator is ready made ^^

    Regards

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  2. #2
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    Re: Getting trigger signal for 555 timer from elevator button.

    Hi,

    There are many solutins.
    I'd go this way:
    * use a 10k pullup on TRIG pin to 12V
    * use a diode from TRIG (A) to the pushbutton (C)

    Klaus
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    Re: Getting trigger signal for 555 timer from elevator button.

    Quote Originally Posted by KlausST View Post
    Hi,

    There are many solutins.
    I'd go this way:
    * use a 10k pullup on TRIG pin to 12V
    * use a diode from TRIG (A) to the pushbutton (C)

    Klaus

    I tried your suggestion i have the same problem when motor start run , The 555 timer triggered.



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    Re: Getting trigger signal for 555 timer from elevator button.

    Hi,

    then itīs most probably an EMC problem, not a problem of the schematic.

    Use: (urgent)
    * PCB with solid GND plane
    * proper decoupling capacitors on ech supply pin of each IC
    * EMC filters on every PCB IO

    maybe additionally:
    * shielded wiring
    * shielding case

    Klaus
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    Re: Getting trigger signal for 555 timer from elevator button.

    Could you give me an example on emc filter for pcb io ?

    Thank you for your appreciated effort :)



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    Re: Getting trigger signal for 555 timer from elevator button.

    Hi,

    there are millions. For a million of different requirements.

    But the most important - IMHO - is a proper PCB layout.
    The next important is an analysis of what really causes the error.

    ****

    Not only frequency, voltage, current ... but also HF environment, wiring, cable type, cable length ... and so on have influence.

    ***
    Filters:
    R, RC in many cases is sufficient.
    But from your description, I assume there may be some common mode noise or GROUND loops.

    It makes no sense to give an example without knowing details about the application.

    Klaus
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    Re: Getting trigger signal for 555 timer from elevator button.

    "I tried two circuits to get low signal to trigger 555 timer .
    I tried circuit 2 with 10k for R1 and R2 and i faced a problem . *the led of pc817 always on an this tell the controller the button always pressed."


    OF Course!! R1 and R2 will draw a current, and that current will flow thru the optocoupler because it is IN SERIES with the divider.

    Increasing the divider's resistance will "starve" the optocoupler of current, and will therefore have very poor noise immunity.

    Put the optocoupler IN PARALLEL with R1, R2.

    Edit: your last circuit leaves the 555 trigger input floating, and therefore will also suffer from poor noise immunity.
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    Re: Getting trigger signal for 555 timer from elevator button.

    Reading your first post worries me a little in that you seem to be just trying various values of your components to see what happens.
    What you need to do is to learn how to read the data sheets of the devices you are using (in this case the 555) and understand them. This is not easy at first but does get better: but if you don't then you will always be setting yourself up to fail.
    Next you need to *design* your circuit. That means determining the operational parameters of the circuit and then calculating time constants (for the RC part of the circuit), current flows, voltages etc.. Don't guess.
    Getting started is not easy and asking for help is essential. So is learning how to 'fault find' which means you need to measure what is happening in your circuit and comparing that to what you expected from your design. Then you can work out what is wrong with the design and make the necessary corrections.
    As you gain experience, you will start "getting a feel" for what is happening in your circuits and also what sort of values are needed in various places, but that does take time.
    Susan



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