555 3V LED Flasher circuit Help?

Discussion in 'General Electronics Chat' started by mrayvon, Dec 24, 2010.

  1. mrayvon

    Thread Starter New Member

    Dec 24, 2010
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    Hey! I want to blink 6 3mm LEDs at 10 Hz using a 555 timer. I've built a circuit in a simulator that seems to be operational, but I wanted to run it by someone more experienced before I start buying components. Does anything seem off? I'm an artist primarily and a bit of an amateur when it comes to electronics.

    Heres what I have so far:
    [​IMG]

    Thanks for your time and thoughts!
     
  2. #12

    Expert

    Nov 30, 2010
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    I don't think you can get 54 milliamps out of a 555 chip with a 3 volt supply. Exactly which 555 do you have in mind?
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    Welcome to ACC!

    The schematic will not work.

    You can use a CMOS 555, such as a Radio ShacK TLC555 or a 7555, because they are rated for a lowest voltage spec of 2.0VDC, but that configuration will not work. A conventional 555 has a lowest power supply voltage spec of 4.5VDC.

    One of the problems with a CMOS 555 is that the current drive is dramatically reduced, and the problem gets worse at low voltages. At 3VDC you are lucky to get 20 ma drive for a CMOS 555.

    I have created a series of projects using a CMOS 555 at 3.0VDC, they use a much reduced pulse width. You might be able to lift the core transistor drivers if the 50% duty cycle is important, or use the design as I made them.

    Other issues with your design, 10 Hz is a terrible frequency to use. In some people it can trigger and epileptic attack (no fooling). You can't see the flash rate clearly (though you can clearly see it is flashing). You probably want to slow it down 5 hertz or so.

    The other issue is the LEDs themselves. What is their Vf (forward dropping voltage)? Most modern red LEDs drop 2.5VDC, much older types drop 1.5V (but they are very old, very dim, and very rare). If they drop 2.5VDC, this means your current through the LEDs is 5ma. Most LEDs are at their best brightness at 20ma.

    Like I've said, I've come up with several solutions that may work for you.

    I've written about 5 articles for the AAC book volume 6, Experiments, that you see on top of the page.

    Chapter 8: 555 TIMER CIRCUITS

    Pay special attention to the articles with "Long Duration CMOS 555 ... Flasher" in the title.

    I also have another article that covers what you want to do on Chapter 6 and 7, using lessons learned from the above experiments. You probably also want to cover Chapters 1 and 2, as they go into the specifics of LEDs.

    LEDs, 555s, Flashers, and Light Chasers

    Figure 6.1 comes closest to what you seem to be after, Oscillator #3.

    [​IMG]

    Repeat R4 and D1 (do NOT parallel the LEDs!) off the collector of Q2, and drop R3 to 1KΩ. The two transistor driver is compatible with your 555 design (as long as the 555 is a CMOS type).

    Other reading material.

    Bill's Index

    The 555 Projects

    My Cookbook
     
    Last edited: Dec 24, 2010
  4. #12

    Expert

    Nov 30, 2010
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    I was looking at ICM7555
    rated at 3 volts, 100ma, but not both at the same time.
     
  5. iONic

    AAC Fanatic!

    Nov 16, 2007
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    According to your circuit it appears that you want the 6 LED's all on at the same time. Is this what you want?

    Pin 3 of the 555 could drive a transistor that can run the 6 LED's, but that means you would need between 75mA and 120mA from the 3V supply, depending on how much current you choose.. I's this 3V supply batteries? AA's? AAA's?
     
    Last edited: Dec 25, 2010
  6. Wendy

    Moderator

    Mar 24, 2008
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    Something I found in my experiments is you can have the high current pulses, but it is advantageous to keep them narrow. If you can keep the duty cycle small the power supply filter cap takes over from the power supply, the batteries charge the capacitor slowly, and the capacitor dumps its charge through the LED. This dramatically extends the life of the batteries. This was from AAA batteries.
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    The datasheet for the ICM7555 shows that its output high current with a 3V supply is only about 1.5mA in your circuit. So each LED will be very dim at only 0.25mA and much less when the battery voltage drops as it is used.
     
  8. Wendy

    Moderator

    Mar 24, 2008
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    Which is why I am showing the drivers. Actually 1.5ma is a low ball value, it can be more, but not by much. The manufacturer guarantees at least 1.5ma. I've used this to drive an LED directly, though it is not something I normally recommend.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    With a 5V supply that is much higher than 3V the datasheet for the ICM7555 guarantees an output high current of only 0.8mA and the voltage drop is as high as 1V.
    I quoted the typical 1.5mA output when the supply is about 3V that is shown on a graph.
     
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