icm7555 discharge and threshhold

Discussion in 'The Projects Forum' started by The Engineer, Nov 7, 2014.

  1. The Engineer

    Thread Starter Member

    Sep 3, 2014
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  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    R4 and C4 control the duration of the pulse for the transmitter. It is a typical monostable configuration for a 555-family device (here labeled Ra and C):
    upload_2014-11-7_7-30-51.png
    Source: LM555 datasheet

    IC1 provides the trigger through C3. R3 is a pull-up.

    John
     
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  3. The Engineer

    Thread Starter Member

    Sep 3, 2014
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    thankyou :)
    I realise that in monostable position the total frequency time is defined using the formula "1.1 (R1 * C1)",
    also for the ne555 used for IC2 in astable mode the frequency is defined using 1/(Ln(2)*C1*)*(R1+R2),
    and the on time (duty cycle) is defined using ln(2)*(R1+R2)*C1, with the off time being ln(2)*R2*C1 (because it is discharging only through R2),

    However would you be able to fill me in on info regarding how the CR combination circuit actually does this ? how does it hold on to the signal for x amount of time ? any links to useful websites would be great !
     
  4. jpanhalt

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  5. Dodgydave

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    Jun 22, 2012
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    the capacitor will charge upto 2/3 of the supply,then it will be discharged by pin 7, until the next trigger pulse on pin 2, the time taken to charge upto 2/3 is derived from the values of R4C4.
     
  6. jpanhalt

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    Jan 18, 2008
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    I don't believe that is completely accurate, as it ignores the effect of the Control Voltage, pin 5 on the Threshold. Using that voltage to create a VCO and adjust PWM is well documented.
    upload_2014-11-11_17-8-15.png


    John
     
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