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Flashing LED with 555 timer

Former Member

Hello all,

 

I am trying to create a flashing 3 LED circuit with a 9v battery. I want a specific flash time of 20ms and a duty cycle of 1.3 seconds. My question is what size should R1, R2, and C1 be in order to achieve that timing. I found the equations for figuring it out, I was hoping maybe someone would rather take the challenge of giving me the answer instead of me going back to school to relearn algebra. Any help is very much appreciated.

 

Thanks.

  • 0

    Hi Rob, Without a schematic to reference no one will be able to figure those values for you. Just in case you are looking for a good 555 reference here is an online book one of the guys showed me a while back.

     

    http://www.talkingelectronics.com/projects/50%20-%20555%20Circuits/50%20-%20555%20Circuits.html#9A

     

    You can vary the percentage of on to off time with this circuit. You will have to experiment to get it to the frequency where you want it.

    John 

  • 0 in reply to jw0752

    I agree with .

     

    We would really need to see your proposed circuit.

  • 0 in reply to jw0752

    image

    Thank you for the quick reply Mr. John and Mr. Paul.

     

    Here is the circuit that I came up with. I am shooting for a 20ms time high with a 1.3 Second cycle time. I found an online calculator at 555 Astable Circuit Calculator

    but its very confusing to me at this point. When I change one value it seems to change others throwing me way off. I can't keep up with it. If any of you gentlemen have a better suggestion for me on this circuit I'm listening. Also thanks for the link Mr. John that will be very helpful in my future projects. Also a quick correction. It will be powered with a 9v Battery, not 5 volts as pictured above.

  • 0 in reply to Former Member

    Hi Rob.

     

    Thanks for your diagram.

     

    I have also visited the page you referenced. The calculator (annoyingly) works based on submitted component values rather than timings, the reverse to what you really need. An interesting point I did take from this page is as follows:

    In this type of circuit, the duty cycle can never be 50% or lower.

     

    The following page goes on to say: http://www.daycounter.com/Calculators/NE555-Calculator.phtml

    Note that the minimum duty cycle is 50 percent, so if you need a smaller duty cycle you need to use an inverter on the output. 

    I don't really have much 555 Timer experience, but from what i have read, this would seem to apply to you.

     

    At this point I would probably put an extra 30p into the project and use an AtTiny85 instead. image

  • 0 in reply to Former Member

    Hi Rob,

    paulellison is correct that what you want to do might be easier to accomplish with a microprocessor. Also for the 5 volt power you are showing on your schematic you will probably not be able to drive 3 LEDs in series. Most LEDs have junction voltages that are high enough so three in a series would be at or higher than 5 volts. The duty cycle you are trying to achieve is 2% at a frequency of approximately 0.77 Hz. The best that the circuit that I posted before can do is 5%. You can drive three LEDs by using a higher supply voltage. I believe that the 555 for example will work with up to 18 volts on some varieties.

    John

  • 0

    You could get half of this to chase the tail of the other, but you would need to provide for a chain-initiating step:

     

    https://www.fairchildsemi.com/datasheets/MM/MM74C221.pdf

     

    You could use a counter and a magnitude detector (equality gate) to generate your desired waveform.

  • 0

    Wow! Thank you gentlemen very much for taking the time to throw me those tips. Sometime tomorrow I will work some breadboard magic and let you all know what I came up with. All of the links were very helpful in helping me get a better grasp on exactly what I'm trying to do.

  • 0

    Hi Rob, Just for the fun of it I mocked up your idea using an Arduino Duemilanove. I will attach a video but keep in mind that my camera has a slow scan rate and as such it misses the 20 uS flash occasionally. In real life the 3 LEDs flash for 20 uS and then are off for 1.3 seconds before they flash again. The camera misses every third of fourth flash. I have taken the simplest of Arduino sketches called "Blink" and modified it to your specifications of 20 uS on and 1.3 Seconds off. The Arduino outputs this signal on pin 12 in this case and I am using the signal to trigger an NTD4906 gate level N channel MOSFET. I am supplying 9 volts to the LEDs and the FET so there is enough voltage to drive them in series and there is also a 220 ohm current limiting resistor in series. I do not know the intention of your project whether you need this effect or if you are just trying to learn about the 555.

     

     

    John