Potentiometer question

Discussion in 'General Electronics Chat' started by archangeljess, Jan 16, 2014.

  1. archangeljess

    Thread Starter Member

    Dec 27, 2013
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    I found a schematic for fading LEDs. One of its components is a 10k 1/2 watt linear rotary potentiometer. When I inquired about this component, my local electronics store doesn't sell this one. They only have "ordinary" potentiometer. My question is, can I use any potentiometer instead of a linear rotary potentiometer?

    Thanks
    Jess
     
  2. pwdixon

    Member

    Oct 11, 2012
    488
    56
    The circuit will work but the brightness against pot position will be non-linear it rather depends what you are trying to do if you are just setting a light level by eye then you may be easily able to do what you want. It just means the intensity adjustment might be a bit sensitive on one end of the scale.
     
  3. ScottWang

    Moderator

    Aug 23, 2012
    4,855
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    What is the circuit?
    If you using a NE555 PWM brightness control circuit, the VR(potentiometer) is not a problem.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,145
    3,055
    If you share the schematic, we can offer many alternatives. Your local shop must surely have some linear pots, maybe at 5K or 50K? You can probably adjust your schematic to whatever parts are on hand.

    You can also, in a bind, change the behavior of a pot by using additional external resistors.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
    2,375
    Did you ask if by 'Ordinary' they mean Logarithmic types, volume control etc?
    Max.
     
  6. #12

    Expert

    Nov 30, 2010
    16,321
    6,818
    10K, linear, 1/2 watt is as ordinary as they get.
    Place flashlight next to clerk's ear, watch other ear for change in brightness.
     
    PackratKing and MrChips like this.
  7. archangeljess

    Thread Starter Member

    Dec 27, 2013
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    This is the circuit I am talking about. I am very new to electronics so I do not know everything about many components. The store just tells me they don't have it. As it turns out, I'm gonna be shopping for "ordinary" pots.

    Flexible_LED_Pulsing_Circuit_6.png

    Here's the link to the while article I am referring :
    http://www.pcbheaven.com/userpages/Flexible_555_LED_Pulsing_Circuit/
     
  8. archangeljess

    Thread Starter Member

    Dec 27, 2013
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    By the way, since the schematic that I am using also uses a 555 timer IC, I am wondering why any 555 timer would emit too much heat. I have a police light flasher circuit which is working fine until i began experimenting on it. Any thoughts on what might have been the problem? Since I began working on electronics I have 3 fried 555 timers on my stash. Which demonstrates how stupid I am with circuits. But I am not giving up any time soon.

    Thanks for the help.

    Jess
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
    2,375
    In component jargon, these are described as 'Trimmer Potentiometers' , the circuit symbol also bears this out, in practice these are almost always 'Linear' types.
    On the 555 heating, make sure you do not exceed the current rating of the output.
    Max.
     
  10. archangeljess

    Thread Starter Member

    Dec 27, 2013
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    I guess they mean volume control. That's what they are selling.
     
  11. ScottWang

    Moderator

    Aug 23, 2012
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    The Pd of ne555 is 600mW, if the power is 12V, then the
    I = W/V = 600mW/12V= 50mA, so the drawing current should be less than 50mA, if you want to use for some more then you can using a heatsink or adding a bjt or mosfet to drive the Load.
     
  12. archangeljess

    Thread Starter Member

    Dec 27, 2013
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    0
    Thanks for the information. May I refer you to another post so you could better understand what I mean.

    http://forum.allaboutcircuits.com/showthread.php?p=686890

    I am building a LED Police Flashing lights. I want to add more LEDs to the circuit and maybe a separate set of LEDs that flashes independent of what I already have made.

    Thanks

    Jess
     
  13. ScottWang

    Moderator

    Aug 23, 2012
    4,855
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    You can see the attached for reference.

    The Vcc for the 555 is too high, usually we just using it up to +15V to the GND, but you used +18V, that is the rated voltage, it can't be work for long.

    And the value of R2(1K), normally we using it with Vcc=5V, it is too less at here, when you offering the Vcc is >=12V then the R2 don't use 1K, it should be increasing more, we have to considering when the Vout(pin 3) is Low then the current will flowing through R2 to Vce of bjt inside of the Vout, The current will be I_R2 = (18V-Vce)/R2 = 17.8V/1K = 17.8mA,

    The CD4017 can be offer -3.5mA, but that is at the +25℃, the calculation values of R3,R4 are 4.7K at +25℃, but we have to concerned the CD4017 may not keeping on that degree, so I reducing to 2.2K, the calculation value of R5, R6 close to 440Ω, but I don't want them to drawing too much current, and the 555 is working in a low frequency, so R5,R6 can be using a higher values.

    [​IMG]
     
  14. archangeljess

    Thread Starter Member

    Dec 27, 2013
    34
    0
    Thanks ScottWang. I appreciate your help. My power supply is not 9 volt as indicated in the schematic but 12 volts. How many LEDs will I be able to attach to this circuit?

    What I just want to do is have 12 red LED, 12 blue LED and 12 white LED.
    The blinking that I want to achieve is like this sequence:
    blue flashes twice, red flashes twice...
    white always flashing...

    Would this be even possible?
    What is the 820 in the resistors?

    Thanks.

    Jess
     
    Last edited: Jan 17, 2014
  15. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    If you want to reducing the current of NE555 then you can following as what I changed.

    The new circuit is using the darlington pairs to driving the LEDs, if you still want to using 2N3904 is ok, but you will need some more.

    I'm calculating the current of LED, the Vce of TIP122 around 0.9~1.1V and I used 0.9V, the LED drawing current about 30mA, but the average current of blue and red LEDs <15mA, the white LEDs about 15mA, they will depends on the duty cycle of LED.

    The Ic of 2N3904 are 200mA, if you want to use it, you just count it as I = 200mA/3 = 66.7mA, and calculate the numbers of LED and 2N3904.

    [​IMG]
     
    Last edited: Jan 18, 2014
  16. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    In post #15 there are no resistors on the bases of the transistors.
    You will need them, otherwise the transistors and/or the chips will blow.

    Bertus
     
  17. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    Thanks.
    I forgot they are not ULN2003.
     
  18. archangeljess

    Thread Starter Member

    Dec 27, 2013
    34
    0
    Thanks for all your help. I'll be working on ScottWang's suggestions this friday. I'm sorry I didn't have much time replying to my thread. I've been busy last week.

    I'll post some pictures or video if I have the time.

    Thanks.


    Jess
     
  19. Metalmann

    Active Member

    Dec 8, 2012
    700
    223
    "Since I began working on electronics I have 3 fried 555 timers on my stash."



    Don't feel so bad, I've murdered quite a few more than that.:D
     
  20. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    I just tried to use Triac circuit to dimmer the bulb which can't using that way, it was burned out and cost me about US$ 12.
     
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