555 and 24 volts

Discussion in 'The Projects Forum' started by nerdegutta, Jan 10, 2010.

  1. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Hi.

    I'm currently working on some NE555-timer circuit. I've wired it like the attachment, monostable.

    And I need 24v out of pin 3. How can I do that. Use a BC557 or BC547?

    Please advice.
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Are you planning to only one power supply, 24V, for both the 555 and you load from pin 3? If so, since the absolute maximum is 18V, you need to provide a regulator to drop the 24V to ~15V for the 555. Then you will need a level shift circuit to provide a 15v to 24V output to you load. What are you controlling off of pin 3?

    Ken
     
  3. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Hi.

    It may sound weird, but I'm driving 320 LEDs. Maybe 24v is a bit overkill. I think it can be done with only 12v, or less.

    I was thinking of two power sources, one for the 555 and another for the LEDs. That's why I mentioned the transistors. I think an NPN, that opens to let more voltage pass thru when a positive is applied to the base.

    Am I way off?
     
  4. sceadwian

    New Member

    Jun 1, 2009
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    You could use a mosfet, a 555 should drive an N channel FET pretty well from 12 volts, and then you can use anything you want on the Source/drain voltage side of things. Would require a split power supply though, or at least a regulator to the 555. If the 555 is driving a FET it will use very little power so the regulator requirements for dissipation would be pretty low.
     
  5. AdamPrulhiere

    New Member

    Nov 12, 2009
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    you could use an opamp to get the 24V but youll have to pay attention to how much current you need to supply. opamps can only supply ~25mA. you could also use the 555 with a BJT
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    If you're thinking you could use an opamp to produce 24v from 12v, that is not correct.
     
  7. AdamPrulhiere

    New Member

    Nov 12, 2009
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    as long as he has 24V to supply to the opamp it is. i thought the issue was that the 555 couldnt provide 24V at the output
     
  8. sceadwian

    New Member

    Jun 1, 2009
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    Why would you use an opamp as a power supply? Just use a linear regulator to the 555 and the 555 would switch a FET. The 555 itself since it's output is no effectivly unloaded will draw very little power so the linear regulator won't heat up very much at all, might not even require heatsinking. Then the only limit you have on the load side is a voltage that won't cause the Mosfet to go into it's saturation region. But 12 volts should keep most mosfets fully on through most practical drain voltages.
     
  9. Wendy

    Moderator

    Mar 24, 2008
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    The 555 needs 12 to 15V, you can use drivers something like these...

    [​IMG] [​IMG]
    .......... .......................Figure 1............................................. ..................Figure 2

    Figure 1 is a constant current source, the transistors regulate the current based on R2 and R3, which can be ¼W resistors. You may need to tweak these resistors, it should work out between 16-20ma, and the transistors may get hot, they are taking all the wattage.

    Figure 2 is simpler, the MOSFETs simply switch to ground. You'll need to calculate R2 and R3 to match your resistors.

    You will need to calculate the quantity of LEDs based on their specs.

    Have you read this?

    LEDs, 555s, Flashers, and Light Chasers
     
    Last edited: Jan 10, 2010
  10. CDRIVE

    Senior Member

    Jul 1, 2008
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    There's nothing that's new or unconventional about this. U2 can be either 5V or 12V.
     
  11. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    Thank You. Now I have some schematics to work with.

    Mr Mardsen: What type is the transistor in Your schematic?

    CDRIVE: Same to You.

    Can I use BC557, BC547?
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    In Bill Marsden's left schematic, the NPN transistor could be a BC547, a 2N/PN2222, or a variety of other NPN transistors that have an Ic rating of twice that required.

    In the right schematic, the MOSFET could be any number of N-channel enhanced mode power MOSFETs that have a Vdss rating of 30v to perhaps 100v, and rated for twice the current required.
     
  13. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    OK, thanks. I'll try with my BC547. Could I also use a 2N3904? Maybe I should compare the datasheets? That sounds like a good idea.

    Thank you for your time and effort.
     
  14. CDRIVE

    Senior Member

    Jul 1, 2008
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    You will note that I used high side switching while Bill used low side switching as I didn't know which you wanted to use. Anyway, my schematic can use a 2N2222 for Q1 (NPN) and a TIP42G for Q2 (PNP). Both are available from Radio Shack.
     
  15. Wendy

    Moderator

    Mar 24, 2008
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    Note, you will have to adjust these schematics to fit your application somewhat. The color of the LED matters, for example. It will determine how many you can put into a chain.
     
  16. thyristor

    Active Member

    Dec 27, 2009
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  17. CDRIVE

    Senior Member

    Jul 1, 2008
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  18. nerdegutta

    Thread Starter Moderator

    Dec 15, 2009
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    You are absolutely correct!

    I'm sorry. I attached the wrong schematic. I have wired it with monostable configuration. That is, when I press the button, it's sending a signal to pin three just one time. NOT repeatedly.

    I'm sorry.
     
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