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How to amplify 555 monostable 2 volts output signal(edge triggered)

Discussion in 'Troubleshooting and Repair' started by Nexsir, Dec 10, 2019.

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  1. Nexsir

    Nexsir

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    Dec 10, 2019
    Hi friends,
    I've been for sometimes working on constructiong a PI metal detector where I need to drive an IRF840 mosfet by 555 monostable output (edge triggered mode).The output is 1.8v while IRF840 gate needs to receive above 10v to run satisfactory.Do you know solution for my problem?In ciruits which an stable 555 output drives mosfet gate BC557 can be used to do the duty;However it doesn't seem to work with 555 monostable or edge triggered output because it's said it causes pulse length problem and BC557 emiter - collector will be open when base receives positive pulse.
    After weeks of hard work I'm going to be disappointed because I cant find a solution.So any help will be highly appreciated.
    Looking forward your gentle suggestion.
    (I'm no a native English speaker.So I hope I'd introduce the problem clearly)
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
  3. Alec_t

    Alec_t

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    Jul 7, 2015
    Please post the schematic of your detector.
     
  4. Nexsir

    Nexsir

    4
    0
    Dec 10, 2019
    Here is the schematic. Before connecting the coil the voltage of gate-soure is satisfactory(12v), but it falls below 2v as soon as coil is connected to circuit /// I add two changes to the schematic. I add a 10 k ohm resistor between gate-source pins of mosfet against Miller effect. And 556 stable output connected to pin5 of CD4066.
     
    Last edited: Dec 15, 2019
  5. Nexsir

    Nexsir

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    Dec 10, 2019
    IMG_۲۰۱۹۱۲۱۴_۱۷۳۳۳۲.png
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    This is not a schematic but a pcb layout. Please do not expect us to reconstruct the schematic from the layout.
     
  7. Nexsir

    Nexsir

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    0
    Dec 10, 2019
    Sorry, you're right.just a mistake.Here's schematic.
     

    Attached Files:

  8. Harald Kapp

    Harald Kapp Moderator Moderator

    9,632
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    Nov 17, 2011
    The circuit in question is in the upper left part, D1B and Q1, right?
    You are using an IRF840 in place of the IRF740 in the schematic?
    Bothe have a Vgsth of 2 V ... 4 V, so at 1.8 V they will be off, at 10 V they will be on.

    Where do you measure the 1.8 V? Vgs is measured between gate and source, not necessarily between gate and ground (0 V). In this circuit the source of Q1 is tied to -12 V, therefore a voltage of 1.8 B at the gate (measured to ground) is equivalent to a gate-source voltage Vgs = 1.8 V + 12 V = 13.8 V and Q1 is turned on.

    The 556 is operated with supply voltages +E and -E. In the lower left corner of the schematic we see +E = +12 V and -E = -12 V + 2*0.6 V = -10.8 V (2 * 0.6 V is the voltage drop across diodes VD8 and VD9. This makes for a total operating voltage of Vop = E+ - E- = 22.8 V
    The 556 datasheet (e.g.here) states a max. operating voltage of 18 V. This circuit is a bad design and may work with some 556 chips that tolerate the much too high operating voltage but not with others. The problem you face is imho not driving the Mosfet but operating the circuit within the limits of its components. Your 556 may already be dead (with some luck it is not).Try operating the circuit from +9 V and -9 V.
     
  9. Alec_t

    Alec_t

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    733
    Jul 7, 2015
    The 556 output will be about +11V. The FET source is at -10.8V. Hence Vgs will be about 21.8V, which exceeds the absolute maximum rating of the IRF740 and the IRF840. I smell smoke.
     
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