are there any problems with having lots of resistors in series

Discussion in 'The Projects Forum' started by lotusmoon, Jul 10, 2013.

  1. lotusmoon

    Thread Starter Member

    Jun 14, 2013
    203
    4
    I am using a 555 timer circuit similar to this one but using it to drive an LED flasher - http://forum.allaboutcircuits.com/showthread.php?t=84399
    I will have 12 separate frequencies available on it all run to a twelve way switch.
    I wish to get the frequencies very accurate so for each frequency i may have up to three fixed resistors - {1-10r) (10-100r)(100-1k)(1k-10k)ect
    this means that for the last frequency in the line I could have up to 36 resistors in series. Are there any problems with this?
     
    Last edited: Jul 10, 2013
  2. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    You said that you want to drive an LED flasher, but the LED flasher is for our eyes to look, do you really need the accurate frequencies?
    How accurate you want?
    What's the frequencies?
    it's ok, or you want to spend more money to buy small pcb type Potentiometer, there are many kinds of types, you can choose the type you like from internet or local stores.

    [​IMG]
     
  3. lotusmoon

    Thread Starter Member

    Jun 14, 2013
    203
    4
    Thank you for this info and pictures
     
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
    4,004
    1,525

    If your desire is to have "very accurate frequencies", your wasting your time with the 555. They shift frequency with variance in heat and other outside influences. Your better off to start with a crystal oscillator and electronically divide it to your desired frequencies. http://en.wikipedia.org/wiki/Crystal_oscillator
     
    Metalmann likes this.
  5. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    +1

    You'd also be wasting time if you were not using high precision resistors, but those would be a waste since the 555 itself will drift as noted.
     
  6. lotusmoon

    Thread Starter Member

    Jun 14, 2013
    203
    4
    I was going to use a frequency counter to set the frequencies what kind of % drift is there in a 555 circuit.?

    thats very interesting about the crystal oscillator, do you know where could I find a schematic and information how to do this?
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    Someone will know, but the timing resistors drift as well, particularly as temperature changes.
     
  8. lotusmoon

    Thread Starter Member

    Jun 14, 2013
    203
    4
    Thank you I guess the only way to know is for me to build one and test it to see the % variance.
     
    Last edited: Jul 10, 2013
  9. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    No, these things are known and predictable. Resistors have temperature coefficients and specified precision ranges. The 555 will have a specification. OK, it may do better than the spec, but that's not something you want to rely on.

    There's a reason that guitar tuners, for instance, use quartz crystal oscillators. A 555 would never allow adequate accuracy.
     
  10. lotusmoon

    Thread Starter Member

    Jun 14, 2013
    203
    4
    I have looked at the specification and that variation is fine for me. thank you
     
  11. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    Yes.. its just plain ugly and implies an unprofessional design. :p
     
  12. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    lotusmoon likes this.
  13. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    A couple of percent variance is typical with a lot of caps in 555 circuits.

    Just the temperature rise in the cap in the time from first being switched on to being running for a few minutes can give you 2% freq variation. Room temp changes or temp changes inside your enclosure can make that worse still.

    Do you know the actual frequencies you need?
     
    lotusmoon likes this.
  14. WBahn

    Moderator

    Mar 31, 2012
    17,720
    4,788
    You've got a fundamental problem in that let's say that I want to create a resistance of 2487Ω using a 2kΩ, a 400Ω, an 80Ω, and a 7Ω resistor. Let's say that I want it to be accurate to ±1Ω. So my last resistor can be a 10% resistor. Doesn't sound too bad. But the 80Ω has to be a 1%, the 400Ω has to be a 0.1%, and the 2kΩ has to be a 0.01%. And even that won't quite do it as it would give me an uncertainty of roughly ±2Ω.

    If the 555 has short-term variability of a percent or two, then you are gaining nothing by using multiple resistors instead of a single 1% resistor per frequency (or perhaps 0.5% since you would still have twelve in series for the slowest frequency).

    I think I am picturing the circuit you have in mind for your switching. Are you putting a resistor (a series of three, in your case) between each pair of adjacent contacts and the using the wiper as one end of your adjustable resistor and one of the end contacts as the other? I think you can do much better with a slight modification so that each frequency selection uses a single, independent resistor. Just put each frequency's resistor on a separate pole of the switch and tie the other end of all resistors together. Now you can change one frequency without affecting all of the frequencies lower than it.
     
    lotusmoon likes this.
  15. lotusmoon

    Thread Starter Member

    Jun 14, 2013
    203
    4
    Hi I have been trying to make a new thread for a few days now but it shows up server error each time. is there something i can do to achieve this?
     
  16. WBahn

    Moderator

    Mar 31, 2012
    17,720
    4,788
    Don't you think it might be useful if you were to describe what the error says?
     
  17. lotusmoon

    Thread Starter Member

    Jun 14, 2013
    203
    4
    thank you for all of these new posts lost of great information.
    The error I am getting when trying to start a new thread is --
    Server error
    The website encountered an error while retrieving http://forum.allaboutcircuits.com/newthread.php?do=postthread&f=6. It may be down for maintenance or configured incorrectly.
    Here are some suggestions:
    Reload this web page later.
    HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfil the request.
     
  18. lotusmoon

    Thread Starter Member

    Jun 14, 2013
    203
    4
    I am just experimenting at this stage. I will build this circuit and give it time to warm up and then fix and test the frequencies when it is warm. The accuracy is not a problem for me but I would like to be able to give a % variance to the people I am giving these to.
     
    Last edited: Jul 12, 2013
  19. Metalmann

    Active Member

    Dec 8, 2012
    700
    223


    Thank you, I just thought those 555s were more accurate than that.

    Always helps to read from the Pros!:cool:
     
  20. WBahn

    Moderator

    Mar 31, 2012
    17,720
    4,788
    The first thing you need to do is determine how accurate you need your frequencies. Don't be qualitative about it. It does cut it to say, "I need it to be very accurate," or "I need it to be as accurate as possible." You need to say something like, "I need it to be accurate to within 0.01%, " or "I need it to be within 1Hz from 100Hz to 10kHz."

    And it is important that you determine how accurate you NEED it to be, not how accurate you would LIKE it to be.
     
Loading...