Could i get a second pair of eyes on my 555 strobe?

Discussion in 'The Projects Forum' started by dest, Aug 15, 2013.

  1. dest

    Thread Starter New Member

    Jan 14, 2013
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    Hi, i put together my first ic based circuit, a 555 strobe. If i could i would like to have more experienced eyes look over it to make sure it is not on some kind of slow buildup to destruction or some other horror scenario i can imagine.


    Glad i bought 555 timers in bulk. I burned out three trying to figure out how to do my first 555 circuit. XD

    Linked below is the circuit simulation.

    http://www.falstad.com/circuit/#$+1....0+0.0+0.0 o+5+32+0+35+10.0+9.765625E-5+0+-1


    I built the circuit as shown and it works exactly as simulated, is flashing away beside me. Aside from general worries about it catching fire or exploding, i read at one point that you need to have a diode going in parallel with r2. I have it in the simulation and the actual circuit, but when i remove it from simulation it makes 0 apparent difference.

    Was i misinformed/ read it wrong and don't need that diode?

    Also, all my resistors are 1/4 w. I know that's fine for the current going through the led, but is there a way to calculate the correct wattage for R1 and R2 on the timing circuit?
     
  2. wayneh

    Expert

    Sep 9, 2010
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    PLease provide an image of your circuit. Many folks here won't follow a link that goes somewhere else.
     
    #12 likes this.
  3. SgtWookie

    Expert

    Jul 17, 2007
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    While it's great to have such documentation available, the best thing to do is post an image of your schematic, most preferably in .png format; as no additional software besides a browser is necessary to view it. The Falstad simulator requires that people have a Java-enabled browser; and many don't (particularly on "smart" cell phones).

    For your values of resistance and how you have the output wired, you really don't need the diode. R1 is so large compared to R2 that there isn't much difference between R1, and R1+R2.

    Yes there is - don't have time to write it at the moment.
     
  4. dest

    Thread Starter New Member

    Jan 14, 2013
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    Ah thanks!

    Apologies for the link, png of the circuit attached below.


    From what you said about the diode, it is correctly placed and under other circumstances would serve a purpose. If so will keep it in the circuit for the sake of best practices.


    In terms of the wattage of the resistors in the timing circuit. After a day of operation everything is still cool to the touch.

    While i do not know the exact formula for it , i checked what power each resistor would dissipate if it were on its own in a 12 v circuit http://www.ohmslawcalculator.com/ohms_law_calculator.php and they all checked out as perfectly fine for 1/4 watt resistors. So i think i am good.

    Many thanks for taking the time to reply to me, much appreciated. Once this thing is sealed inside a model kit i wont have a lot of room to make changes. XD
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Sorry I did not have time yesterday evening.

    Basically, you want to get the wattage requirement using Ohm's Law, and then multiply that by at least 1.6 (2 is a number commonly used), and then use resistors that are rated for at least that wattage. That way, you are not operating the components near their maximum ratings, which is never a good idea. Use components that are rated for significantly more power than they will be expected to dissipate in your circuit, and your circuit will be much more reliable.
     
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    You don't need that diode, that is only used to get flash ratios in the other extreme (which you don't need).

    If you remove the diode you get cap charge through 304.5k and cap discharge through 4.5k, functionally the same as now (with the diode) that has roughly 300k to 4.5k ratio.

    You also need to add the 0.1uF noise filter cap on the CTRL pin (pin5?) to ground.
     
  7. dest

    Thread Starter New Member

    Jan 14, 2013
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    Thanks for the advice guys!

    Will buy a pack of them, guessing they are the ceramic disc ones (104).

    Also , a page i was reading, linked for convenience

    http://pfnicholls.com/electronics/555_timer.html

    mentioned having a 47uF capacitor running from vcc to 0v as a decoupling capacitor. Since the flasher will just be one of several led circuits running off the same power rails is it fair to say i should add it too?
     
  8. nerdegutta

    Moderator

    Dec 15, 2009
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  9. dest

    Thread Starter New Member

    Jan 14, 2013
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    So from reading those and some other sites, what i am taking from this is i need a .1uF from pin 5 to 0v and i need a .1uF ceramic from vcc to ground with a 22 uF electrolytic in parallel to that, placing the ceramic closer on the board.

    http://www.interfacebus.com/555-timer-ic-operation.html


    Honestly this is all starting to slide above my comfortable knowledge level hahahaha. I did not expect that there would be so many complex issues to a strobing led. Which is simple ignorance on my part.

    Thanks for your continued patience.
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    That sounds good. :)
     
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