astable 555 troubleshooting

Discussion in 'Homework Help' started by rjyusmc2005, Jan 22, 2016.

  1. rjyusmc2005

    Thread Starter New Member

    Jan 22, 2016
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    I'm having trouble with a simple astable 555 circuit. I've rewired it multiple times and get the same results each time so I'm confident it is not a wiring issue. I'm using a NE555P if that makes a difference. The measured values of the components are as follows: R1 is 8.15k, R2 is 9.97k & Cext is 0.0102uF. My calculated duty cycle is 64.5% but I'm only getting about 24% on time. The frequency should be about 5.13kHz and I'm getting 1.84kHz when I measure the output on pin 3. I've attached a picture of my breadboarded circuit as well as the diagram I wired it off of. I have tested the cap to ensure it is not shorted or open and it is not. I have also tried adding a cap from pin 5 to ground and that did not have any impact on the output. Any suggestions as to what the problem could be would be greatly appreciated.
     
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  2. WBahn

    Moderator

    Mar 31, 2012
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    Put the cap (recommend a 0.01 uF cap) back on pin 5 and add a 0.1 uF ceramic cap across pins 8 and 1 to bypass the supply at the chip. It probably would be a good idea to put a 1 uF electrolytic cap in parallel with the 0.1 uF cap. I'm just throwing that out, so you might want to look at the product datasheet to see the actual recommendations, but I think these will be close.
     
  3. RichardO

    Well-Known Member

    May 4, 2013
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    No really an answer to our specific question but...

    One of the problems that WBahn pointed to was caused by the symbol for the 555 not showing all of the pins on the package. This is a common mistake that is made _way_ too often. It is good practice to always check the schematic to verify that all of the symbols have all of the pins accounted for. Before a design is complete, you must understand all of the pin functions and having them all in the schematic helps.
     
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Are you measuring between pin 3 and ground or pin3 and (+) power supply rail? You should get 64% to 75% range when you use ground and pin3. Also, what is pin3 connected to? Big loads (over 50mA can change the output voltage and duty cycle of a 555).
     
  5. rjyusmc2005

    Thread Starter New Member

    Jan 22, 2016
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    I was measuring pin 3 to ground. I didn't have pin 3 connected to anything.

    I found another ne555p today and as soon as I hooked it up I started getting the expected results. Turns out the ne555p I was using initially was defective. Went to 4 different radio shack stores before finding one that had them in stock. They have really gone downhill in their hobby electronics parts.
     
  6. hp1729

    Well-Known Member

    Nov 23, 2015
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  7. hp1729

    Well-Known Member

    Nov 23, 2015
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    Just to consider all the possibilities ... how well used is the breadboard? I have to replace mine every few years as contacts get dirty or spread out too far. When rebuilding it did you use a different spot on the breadboard?
     
  8. hp1729

    Well-Known Member

    Nov 23, 2015
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    What voltage are you running it at?
     
  9. rjyusmc2005

    Thread Starter New Member

    Jan 22, 2016
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    I did indeed change locations multiple times while building it. My breadboard is fairly new, <1yr old.
    I'm running a 15v supply which is within the specs for the 555 I'm using.
     
  10. hp1729

    Well-Known Member

    Nov 23, 2015
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    That eliminates both of my ideas.
     
  11. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Can we assume you are using an oscilloscope? If so, can you post a screen shot of the oscilloscope display?
     
  12. hp1729

    Well-Known Member

    Nov 23, 2015
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    Re: Radio Shack
    Agree. There is just too little profit in this small change stuff.
     
  13. WBahn

    Moderator

    Mar 31, 2012
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    It's not that it is small change stuff -- otherwise they would have never sold the selection of things they did. It's that the market for this particular small change stuff has all-but vanished in the retail world. It's not even that the profit is particularly bad -- the mark up on components like these are enormous. But the tiny market means insufficient revenue from the sales of it.
     
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