555 timer pulse voltage

Discussion in 'General Electronics Chat' started by evanwidloski, Oct 8, 2011.

  1. evanwidloski

    Thread Starter New Member

    Oct 8, 2011
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    I read somewhere that the output pulse of a 555 timer was 1.7 volts less than it's power. So does this mean that with a 9volt it will give an output pulse with 7.3 volts?

    If so, will a 2n7000 MOSFET be fully activated with this pulse?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    The output of a BJT (transistorized) 555 timer will be ~ 1.3v less than Vcc under a light load. You can make it go higher by using a pull-up resistor, but that will increase the current drain on your battery.

    You can use a CMOS 555 timer. It works very similarly to the bjt 555. It's output goes nearly to the supply rail; but it's current is more limited than the bjt version.

    Since you're using a 9v battery, you really should use the CMOS version instead. It will make your battery last longer.
     
  3. evanwidloski

    Thread Starter New Member

    Oct 8, 2011
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    So the CMOS version has an entirely different configuration?

    This thing would run for at most 2 hrs.
     
  4. SgtWookie

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    The CMOS version is Complementary Metal Oxide Semiconductor. It uses much less current than the bjt version. It is very different inside, but it functions similarly to the bjt version.

    It would help if you would post a schematic of what you're considering presently.
     
  5. evanwidloski

    Thread Starter New Member

    Oct 8, 2011
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    This is a monostable circuit
    [​IMG]

    The unlabeled capacitor and resistor values are 50k ohm and 100 uf.

    The part labeled "device" will control the MOSFET which will start and stop a stopwatch.
     
  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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  7. evanwidloski

    Thread Starter New Member

    Oct 8, 2011
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    Bump ,for the schematics I posted
     
  8. SgtWookie

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    Sure, that should work fine.

    Just so you know, 50k is not a standard value of resistance, but you could use 47k or 51k, or two 100k resistors in parallel to get 50k.
     
  9. evanwidloski

    Thread Starter New Member

    Oct 8, 2011
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    Is there anything wrong with adding an led ( with a resistor of course) in parallel with the MOSFET, so I can see the pulse?
     
  10. SgtWookie

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    Do you mean connected to the output of the 555?

    I will suggest that you don't do that, as a CMOS 555 timer has weak current sourcing ability (10mA).

    Connect the cathode of the LED to the 2N7000's drain instead, and connect it's anode to a resistor, the other end of the resistor to +V.
     
  11. Wendy

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    Mar 24, 2008
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    If you really want to you can use the drivers shown in this tutorial. A CMOS could do it by itself in theory, but the drive from it is unpredictable. Of course, one of the things I suggest often is to try it and see. This is how we learn, and LEDs do not need the maximum current they are rated for.

    CMOS 555 Long Duration Red LED Flasher
     
  12. evanwidloski

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    Oct 8, 2011
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    Could this damage the ic?

    The device being switched by the MOSFET is a battery powered stopwatch, so I have to use a different v+ for the led. Can the stopwatch and the led share the same drain but have different power supplies?

    Also should I connect the ground of the stopwatch and the 555 circuit?
     
  13. SgtWookie

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    I missed this somehow.
    I have no idea of the power requirements of your stopwatch. Is this a substitute to closing a switch on the stopwatch? What is the voltage present if the switch is not pressed?

    You need to tell us what the voltage is across the stopwatch connections.

    The more negative side will need to be connected to the MOSFET source terminal; and the MOSFET source terminal will need to be connected to the 555 ground terminal.

    The more positive watch terminal will need to be connected to the MOSFET drain.

    If you really want to see the LED light up, you will either need to settle for a lower brightness for the LED by using a larger resistor which will let less current flow through the LED, or you will need to use another 2N7000 to sink current from the LED.
     
  14. Wendy

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    Mar 24, 2008
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    It probably will not damage the IC, but without an exact schematic it is speculation.

    Case in point...

    CMOS 555 Long Duration Minimum Parts LED Flasher

    One thing to be aware of, if you overdrive a chip like this the thing that suffers is the voltage swing, it will not necessarily go to the voltages you want.
     
  15. evanwidloski

    Thread Starter New Member

    Oct 8, 2011
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    Im on vacation so I can't test the stopwatch.

    But assuming that the button is connected to ground, can the led share drain but be connected to it's own power supply? Will I need a diode?
     
  16. SgtWookie

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    Tell us when you get back from vacation. We'll wait.
     
  17. KMoffett

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    Many years ago I triggered an array of battery stopwatches using CD4066 analog switches wired across the stopwatches' start/stop button pads.

    ken
     
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