555 timer question

Discussion in 'General Electronics Chat' started by lokeycmos, Dec 30, 2011.

  1. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    i built the attached circuit(everything to the left of pin 3) no transistor or FET. this is for a demonstration im doing on frequency and duty cycle on a scope(for my youtube channel) which 10k pot is for frequency and which is for duty cycle? i hooked up my frequency and duty cycle meter to the output of pin 3. it seems like both pots adjust both the frequency and the duty cycle. thank you in advance!
     
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  2. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    The one on top is for adjusting the frequency, and the one between pin 7 and 2 is for adjusting duty cycle, I believe. However, I wouldn't run the circuit the way it is set up now with an inductive load. you have no back emf protection and it could damage your chip. As long as you're only using it to show how PWM works on a scope, though, you should be okay.

    Good luck!
    Regards,
    Der Strom
     
  3. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    is there a reason that both pots adust both frequency and duty cycle?
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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  5. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    It's difficult not to with only a single 555. Adjusting the frequency will usually change the duty cycle slightly, and vise versa. You can usually get around this by either using two 555 timers, or a 555 timer and a 393 op amp. If you only have one 555, I would recommend this circuit:

    [​IMG]

    It's not perfect, but it works better than the one you have.

    If you find a 393 (or a 358--they have the same pinout and function), you can make this circuit here:

    393-555_PWM.png

    I've used that one on many occasions and it works like a charm. Frequency and duty cycle adjustment is completely independent of one another, and I must say, this is one of my favorite PWM circuits.

    I hope this helps!
    Regards
     
  6. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    what are the values of parts in your eagle cad schematic?
     
  7. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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  8. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    is there any common household electronics that would have a 393 in them?
     
  9. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Oops, sorry. I guess I forgot to include the parts list.

    Yes, RMCybernetics is where I got the design for the 393 and 555 circuit. I believe a 741 would work, but it may not work as well as a 393. It is older, and I believe it is less precise and efficient. Also, the pinout would be different from the 393, so you would have to look at the datasheet to find the right pins to use.

    I have found many 358s on computer motherboards, video came console boards, TV circuit boards, and several others. They also shouldn't cost very much--you can probably buy them online for very cheap.

    Any equivalent of the 393 or 358 should work fine. I once found a BA6993. I googled the part number, and it was almost exactly the same as the 393. It had the same pinout and everything, and I was able to substitute it in for my circuit that required the 393.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    Your original post schematic was actually this. I show this to show how the math plays out...

    [​IMG]
     
  11. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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  12. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    i was able to find an AX358 in an old computer power supply. i built this circuit but it seems unstable. i left out the diode and mosfet. its connected directly to frequency meter and oscilloscope.

    http://www.rmcybernetics.com/projects/DIY_Devices/homemade_pulse_controller.htm

    the output seems to cut in and out. its almost like a loose connection, but when i put my hand near it to wiggle a wire then it cuts back on before i even touch it. couple questions: is the AX358 sutible for this? also, where does the output from the 555 come from? i always thought the output was pin 3. its not connected to anything. the AX358 has 2 opamps in it, do the unused pins have to be grouned out to function correctly?
     
  13. DerStrom8

    Well-Known Member

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    Yes, you can use the 339. The 339 is only a quad version of the 393. Watch the pin numbers, though. They'll definitely be different.

    The AX358 is a CMOS operational amplifier. Being CMOS, it requires very little current, so any stray capacitance will affect the output. Try to find a non-CMOS version, or use the 339. It should work a lot better, and will not be as sensitive to nearby body parts (like your hand).

    Regards
     
  14. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    After looking at the specs of the AX358, I think it is an LM358 clone, which is bipolar technology, not CMOS. If it is sensitive to hand proximity, it is almost certainly oscillating. Do you have a 100nF cap from Vcc to ground, with short leads, connected as close to the IC pins as possible? if not, that is the likely cause of your oscillation.
    The 358 is an op amp. It will work as a comparator, but it is much slower than an LM393 or an LM339. Keep in mind that 393s and 339s need output pullup resistors, because they have open collector outputs.
    Also - you need to connect the +input of the unused op amp to ground, and the output to the -input. Otherwise, it may oscillate, or otherwise wreak havoc with the section you are using.
     
  15. DerStrom8

    Well-Known Member

    Feb 20, 2011
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    Hmm, strange. For some reason I was thinking I read in the datasheet that it was CMOS.... You do seem to be right, though. Thanks Ron.
     
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