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i need help with my monostable circuit!!

Discussion in 'General Electronics Chat' started by toh liu wang, May 3, 2017.

  1. GromTag

    GromTag Active Member

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    Regards to device by manufacturer that may differ by each 555 timer made. This is an block Diagram, there should be numerous types online to check in regards to design all tho not all manufacturers may display the actual block diagram accordingly or even be close in design to this depiction.

    This example and info based on what I have observed via the LM555 Timer that I acquired for an project.

    The VCC pin 8 within devices applicable range, depending on the voltage will change the resistor divider within the chip resulting in a different voltage requirement for the trigger via the Comparator #1.

    The resistor divider depending on voltage at VCC will also determine the control voltage at
    pin 5 Comparator #2 inverting input.

    Comparator #1 function.

    Internal resistor divider voltage from main supply sets the non inverting input of comparator #1.

    The trigger pin 2 produces an inverted operation of comparator #1 upon the trigger going from high to low from being above the required voltage level based on supply voltage to an low causes the comparator #1 output to go from low to high during the trigger being briefly held low starting the set function of thr flip flop. And at the same time the flip flop deactivated the discharge transistor allowing the capacitor to charge slowly from the resistors current limiting.

    This is of a few issues that can cause the output to stay on if the trigger is held low continuously.
    The comparator #1 output stays high holding the set function of the flip flop. The flip flop reset pin 4 going low can override this. However if the trigger is still low when reset is taken back to high, the output will switch on again and stay on for the capacitor charge time to continuous. If the trigger is taken high after this, the remaining capacitor charge up time if any left is used then output switch off.

    Comparator #2.

    Internal resistor divider sets the voltage at comparator #2 inverting pin control voltage.

    Comparator #2 pin 5 control voltage in monostable mode requires the capacitor to stabilize the voltage reference to comparator #2 inverting input.

    :External note:
    The control voltage can be altered only to produce other functions as to
    why it is available as an externally interactive pin.
    However that applies a different direction and will not be discussed
    as I am not aware of those functions as I only acquired
    an 555 for the monostable basic function.
    Online data is available for astable and numerous other
    configurations that result in alterations to the pin 5 and others set up.

    The voltage into pin 6 from the charging capacitor must exceed pin 5 control voltage to result an change in comparator #2 output within the IC. The charge of timing capacitor causes the non inverting pin to go above the inverting pin resulting in an low to high output from comparator #2. This causes an reset signal to the internal flip flop. Output switches off and the discharge transistor switches on via an likely inverted Q feature from the flip flop.

    The discharge transistor holds the capacitor low continuously until the trigger is taken from high to momentary low again.


    Electrical Characteristics included for an reference only.

    And as stated an leaking capacitor would never charge up to the required voltage, this can be determined via testing pin 5 voltage and then after some time elapsed after triggering the timer to start check the voltage on the timing capacitor to see if it is near or above, if above and the device is still on then something elsewhere is going wrong with the IC.
     

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  2. ci139

    ci139 Active Member

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    qc.png
    if R5 is less there might be problems + i don't know much about snubbers !

    R3 C3 -- ensures a proper startup for 555ic -- the longer the C9,C8,C7 charge the greater should be R3,C3
    C4 R4 -- may be not required
    ups! -- D1 , D3 (not connected to p2) maybe they and shouldn't
    PS! -- in real the L1 may be somewhat greater but if the motor runs the noise introduced may be even grater !!!
    -- just a "quick random pileup" of components -- the main problem likely was with pin-4 network
     
    Last edited: May 16, 2017
  3. toh liu wang

    toh liu wang Member

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    This is my connection now for the circuit. It doesn't turn off! However when i remove both the resistor at pin 7 and capacitor at pin 8, the led turns off after 26 seconds.
     

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  4. dave

    Dave New Member

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  5. ci139

    ci139 Active Member

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    pb~cpy.png
    LT Spice download
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    You show a 150 ohms resistor from pin 7 to ground which is completely wrong. Then the timing capacitor can never charge.
    What is that huge capacitor from pin 8 to ground? Maybe it is 60 years old?
     
  7. toh liu wang

    toh liu wang Member

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    So do i remove the resistor from pin 7?
     
  8. ci139

    ci139 Active Member

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    Last edited: May 17, 2017
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Do you see your 150 ohm resistor connected wrongly so that it is shorting the timing capacitor to ground so it cannot charge and do the timing?
    Yes, the 150 ohm resistor in series with pin 7 is not needed and is not shown on any datasheet for a 555. Remove it.
     
  10. ci139

    ci139 Active Member

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    maybe
    FF280PE_.png
     
  11. toh liu wang

    toh liu wang Member

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    so it is not needed? Thanks.
     
  12. toh liu wang

    toh liu wang Member

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    Yes, the motor power requirement is still unknown.
     
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The spec's for the 1.2V to 3V motor says it draws more than 4A when stalled or when it starts. The max current of the 12V motor is 1.3A.
     
  14. toh liu wang

    toh liu wang Member

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    What about this motor? This motor can run when the button switch is pressed. It turns off after 26 seconds. IMG_4282[1].JPG
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Does the 555 IC get hot when driving that new motor?
    While it is driving the motor, measure the power supply voltage and the motor voltage. Tell us.
     
  16. ci139

    ci139 Active Member

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  17. toh liu wang

    toh liu wang Member

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    The 555IC didnt get hot
     
  18. toh liu wang

    toh liu wang Member

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    does that mean the 555ic can drive this particular motor?
     
  19. alec_t

    alec_t Well-Known Member Most Helpful Member

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    I don't see a back-emf spike supression diode across the motor? Without one your 555 will likely die.
     
  20. ci139

    ci139 Active Member

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    #157 -- it might

    but i guess it -- the 555 -- is designed to drive magnetic relays at most

    back in the past millennium i made a test comparing
    [​IMG]
    and
    [​IMG] 14-pin thing dn-lf
    e.g. к555ла3 (russian) and SN74LS00 (western)
    • к555ла3 (russian) survived couple and more of seconds @ mistakenly set Vcc +9V -- worked after that as if nothing ever happened
    • SN74LS00 (western) died near Vcc +6V !!! -- ??? i wonder the then datasheet spec.-d Vcc ±5% , the Ti-s Abs.max is +7V for Vcc -- so it supposed to be surviving the +6V ???
    so it much depends on the manufacturer how much extra safe operating area there actually is outside that of specified
     
  21. GromTag

    GromTag Active Member

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    Current limit requirements with TI chips through the inputs body diode, transistor or separate are not clarified very well. Looks better on paper.
    This obsolete open collector HEX Inverter schematics show some conclusion of possible others in the thumbnail.

    Unless the internal junctions could be kept at specified temperature or cooler preventing breakdown. Not likely to happen directly ..... Liquid Nitrogen externally would crack the plastic housing and be an monetary waste to use for an illogical reason in the first place (that stuff is cold). Also to why I'm not going towards TI any longer. TI chips as with the issues that have been going on with them in use and testing.

    One must be delicate with them, for they are not meant to be used with intent of using them! :)
     

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