Power LED and Potentiometer Circuit

Discussion in 'The Projects Forum' started by TheAmateur, Oct 15, 2012.

  1. TheAmateur

    Thread Starter New Member

    Oct 13, 2012
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    Hi all,
    Electronics noob here. I want to put together a simple circuit that (at the moment) consists of a power LED (one like this ideally), a potentiometer and a 9V battery.

    The application is for a play - ideally we'd have the LED on an actor's chest, with a wire running to the battery and potentiometer, which are in his/her pocket (so they can vary the brightness of the light from their pocket).

    The theater can be kept fairly dark, but I'd like the LED to be bright enough make it very clear to the audience that the brightness is changing, as well as illuminate the actor's face a little if possible.

    Unfortunately, I'm no Michael Faraday... so I'm a bit confused as to how it should all work. I have brought with me a list of questions for you fine folk.


    1. Would a circuit with a power LED, potentiometer and 9V battery work sufficiently for my purposes, or do you see me needing more robustness?
    2. What power LED intensity would you guys recommend?
    3. Would the LED be uncomfortably hot if e.g. I taped it to my TShirt?
    4. Do you know of any methods to protect the soldering joins from the wires to the parts? I'm conscious of any bit of tugging possibly pulling a wire off.
    5. How quickly (roughly) would the 9V battery drain?

    I'm based in Ireland, so I'll probably order from Radionics.
     
    Last edited: Oct 15, 2012
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The shown led can be a 1 Watt or 3 Watt powerled.
    The powerleds must be cooled with a heatsink.
    If they are not cooled the lifetime will be VERY short.

    A 9 Volts battery will not live long with that as a load.

    Better use a LARGE battery with a buckpuck as a regulator:
    http://www.ledsupply.com/buckpuck.php?gclid=CO7Hncve-LACFU2htgod7hXzlQ

    Bertus
     
  3. TheAmateur

    Thread Starter New Member

    Oct 13, 2012
    13
    0
    I see. The problem with a larger battery will be that the actor would have to carry it around for the duration of the play in a pocket. Also, there's not much of a budget unfortunately (student production), and we're looking to build 4 of these circuits for 4 actors, so four of those buckpucks are a bit too pricey.
    What if I was to build my own current regulating circuit á la http://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/?ALLSTEPS#step7 ?
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The main problem with the shown circuit will be heat.
    The over voltage in that current source will produce heat.

    The buckpuck uses very fast switching, to reduce the heat.

    Here is a switching regulator on the website of one of our forum members THE_RB :
    http://www.romanblack.com/smps/a05.htm

    For correct advice we will need more details on the leds.

    Bertus
     
    Last edited: Oct 15, 2012
  5. wayneh

    Expert

    Sep 9, 2010
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    Use a lower power LED. A "generic" LED would be plenty bright for this, IMHO, and won't risk burning your actors. You might want to put it behind a diffuser.

    You can use a 9V battery for a normal LED. You'll need a pot that covers a low resistance range - just a few 100Ω - and you'll absolutely need a fixed resistor in series with it. Otherwise you'll toast the LED when the pot gets turned to near zero resistance.

    There are all sorts of "better" ways to do this, in terms of efficiency, degree of control, low parts cost and so on, but I think this simple approach will work for you if you can find the right pots. You may have to trim the pots to get the control you want. This means placing resistors in series or parallel with it to change the range of values you get as you turn the knob.

    The problem is that the LED varies current and brightness over a narrow range of voltage.
     
  6. TheAmateur

    Thread Starter New Member

    Oct 13, 2012
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    Ah, yes I hadn't even thought of this! Ok then, I'll shoot for a lower-power LED. What about something like this: 0.5W White LED? Or is that still too high-power?
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Have you looked at generic 20ma LEDs, the kind in really cheap flashlights? Are they just too dim?

    A pot is a lousy control mechanism for a LED, though it can control other circuitry that will control the LED.

    LEDs, 555s, Flashers, and Light Chasers

    Chapter 5 shows one way to do it. There are many others.

    Chapter 1 and 2 are tutorials on LED basics.
     
  8. stanman11

    Member

    Nov 23, 2010
    230
    4
    you can get a pack of 100 LEDs for 99 cents off ebay from china or 2 bucks from a usa seller.
    a pocket isnt a good place for a pot or variable resistor. maybe you can get a wristband or some sort of braclet and attach it
     
  9. wayneh

    Expert

    Sep 9, 2010
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    Would a "throbbing" circuit get rid of the need for the actor to control the light? It would solve a lot of issues if you don't have to have that live control.

    Oh, and I also bought cheap LEDs from China via e-bay. They're crazy bright, many fold brighter than one I paid much more for, from Radio Shack. Just one LED will light up the wall of a dark room.
     
  10. bertus

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  11. TheAmateur

    Thread Starter New Member

    Oct 13, 2012
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    Sorry for taking so long to get back.
    Cheap flashlight-level brightness would probably be fine, I reckon. When we initially looked into the idea, we were using our phones as lights, and the brightness was fine (and a typical phone screen isn't that bright imo). In fact, someone else suggested I write a "quick" Android app that would vary the brightness and just get the actors to stick their phones to themselves!

    The idea is that the brightness is linked to each actor's emotional state, hence why leaving them with control over the light is prettymuch unavoidable. And I don't want to run a wire from them offstage and let someone else control from behind the curtain, for obvious reasons.

    Sounds good. Would someone mind linking me to some of these generic 20mA LEDs? I think I'm looking up the wrong kinds. I don't really have a good idea of how bright a 20mA LED actually is.
     
  12. MrChips

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    Oct 2, 2009
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  13. MrChips

    Moderator

    Oct 2, 2009
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    Actually, there is a simple solution that can be compact and will not generate much heat and wasted power.

    Use a high efficiency 20mA LED. It will be bright enough.
    What colour LED do you want?

    You can use a microprocessor chip with two small push buttons, one for UP and the other for DOWN to control the light intensity. If you wish, you can have three or more push buttons to select preset brightness. How many different brightness levels would you need?
    This is not difficult to make and many of us here can do it for you.

    If you still prefer continuous control with a pot, this is still an option.
    You can do it easily with a 555 timer chip. Your battery choice is still up to you. You can use a compact 3 to 3.6V lithium cell or if you prefer, a 9V battery. Again, no heat is generated or energy wasted.

    If you really want to get fancy, each light can be remotely controlled by a stage hand using an RF or IR transmitter. But that is getting complicated.

    Again, while you may not have the knowledge to do this yourself there are many here who would love something like this to keep them busy. I have students who always need ideas for design projects. When do you need this?
     
  14. wayneh

    Expert

    Sep 9, 2010
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    The ones I got were from an e-bay listing similar to this one. I got mine some years ago and unfortunately that supplier is no longer active on e-bay and may have gone into the sign business.

    Anyway, I recall looking for the highest mcd rating I could find for 5mm cheap LEDs. I doubt the ratings are accurate and they may just reflect how focused the beam is (as opposed to total light output). But anyway, they're very bright and work well. Just don't get 'em wet! (The leads rust easily)

    Let's think about using just a pot to control brightness. The targeted max current should be 20mA, which could happen at one end of the pot using a fresh 9V battery. Let's assume the Vf of the LED is 3V, meaning we need to drop 6V across our current-limiting resistor. By Ohm's law, ∆V = I•R, 6V = 0.02A • R and thus R = 300Ω. Standard values are 270 or 330.

    So the LED is crazy bright at one end of the pot. To go very dim, you need to limit current to about 1mA or less. By the same analysis as above, you need R = 6000. In other words, your pot needs to add about 5K, and 5K is a standard value. A 10K would also be worth trying.

    In between these extremes, it would be nice to have a smooth, linear reduction in brightness as you turn the dial. All the action is at the low-current end, though, and you won't see much brightness difference between 20mA and 10mA. Without doing the math, I think this means a log-taper pot would be a better choice than a linear taper.
     
    Last edited: Oct 17, 2012
  15. TheAmateur

    Thread Starter New Member

    Oct 13, 2012
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    Well that is kind of the problem now, the play opens on the 2nd of Nov! It's not a huge deal if we don't have the lights, but it would be a very nice touch.
    So if you guys think it's worth a shot picking up parts along the lines of what wayneh has mentioned below, I'll go ahead and do that. If it works, great. If not, well nothing ventured nothing gained I suppose.

    Thanks for that. So, if I've understood correctly, if I put a 10k pot and 300Ω resistor in between my battery and the LED, it might work. In that case, here are the parts I'm thinking of ordering:
    LED
    Potentiometer
    330Ω Resistor
    These seem about right?
     
  16. MrChips

    Moderator

    Oct 2, 2009
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    With the short time frame, go with the resistor and pot solution.
    Some details are in order:

    1) Do you wish the LED to go off completely? With a 10kΩ pot in series the LED will still be lit but dim. You have to wire the pot as a potential divider.

    2) That pot is rated at only 50mW. Go for a minimum of 0.25W.

    2) A 330Ω series resistor will limit the current to about 20mA. I would go with a 220Ω resistor for 30mA and more brightness.
     
  17. wayneh

    Expert

    Sep 9, 2010
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    The LED looks great. Note that it does have a narrow beam of 15°, so you might want a diffuser over it. The datasheet says it has a Vf of 3.1 at 20mA, can tolerate a max of 30mA, and has roughly linear brightness increase versus current. In other words, it's a normal modern LED.

    The resistor could be a bit smaller (220 or 270Ω) if you want to ensure brightness at the higher end. Three reasons: 1) Vf is a bit more than 3.0 in my example, 2) the battery's true voltage will rarely be a full 9V. They often sag to 7V or so fairly quickly. Do you know about the specific battery you plan to use? and 3) The LED can tolerate well over 20mA, particularly if it's not intended for a long service life. For instance it would live a long time at 25mA. You might want to get a few different values for experimenting.

    I've done some calculations this morning to satisfy myself that you want a log taper - you do. A linear taper would be too touchy at low brightness. Beyond that, it comes down to cost and how well it fits in with your build. Your choice. For ease of use by the actors, I think you want full travel to be no more than 1 rotation, ie. not a multiturn pot.

    Do you need the light to go off completely? There are pots with a switch at the end, like the power knob on an old radio.

    Gaaa!!! Mr. Chips beat me on the draw.
     
  18. wayneh

    Expert

    Sep 9, 2010
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    For the record, here's the analysis I did comparing a linear pot with a log pot, one that gives 10% of nominal resistance at 50% rotation. (Some give 20% of nominal at 50% rotation.) This has been nagging at me a long time, so I finally broke down and went through it.

    Since brightness is roughly proportional to current, the better choice here is the log pot, to get closer to a straight line relationship.

    Two things worth noting: Real pots don't behave like theoretical ones. But adding a resistor or two externally can give all sorts of curves. So if you don't like the 10K pot you get, it is possible to tweak the way it behaves to better suit your needs.

    [​IMG]

    One more thing, power dissipation in the pot never exceeds 29mW in this scenario. A 50mW rating is probably OK but marginal. It's isn't as large a safety factor as you might like, such as 2 or 3X.
     
    Last edited: Oct 18, 2012
  19. MrChips

    Moderator

    Oct 2, 2009
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    You have two ways of connecting the 220Ω resistor.

    1) +ve battery goes to 220Ω resistor, resistor to top end of pot (terminal-3).
    Bottom end of pot (terminal-1) goes to -ve battery. LED anode connects to wiper of pot (terminal-2), LED cathode connects to pot terminal-1 or -ve battery.

    2) +ve battery goes to top end of pot (terminal-3).
    -ve battery goes to bottom end of pot (terminal-1).
    Wiper of pot (terminal-2) goes to 220Ω resistor to LED anode.
    LED cathode connects or pot terminal-1 or -ve battery.

    I tested this with a 10kΩ linear pot and the response is ok.

    If you want to use a log pot, choose a reverse log taper, if you can find one.
    If you choose a normal log taper, reverse terminal-1 and terrminal-3. The problem with this is turning the pot clockwise will turn LED brightness from high to off.
     
    Last edited: Oct 18, 2012
  20. wayneh

    Expert

    Sep 9, 2010
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    My analysis applied to the 3rd choice: LED, resistor and pot all in series. No 2nd branch to ground.

    Following the approach suggested by MrChips allows for a more certain turn-off of the LED but is otherwise quite similar as far as the curve profiles. The log taper provides a more linear output.

    This chart is for a 5K pot, which is better than the 10K when you use this circuit. Even a 1K would work but is less linear.

    IMHO, this looks like pretty darn good control of an LED using a pot. Why does everybody say this doesn't work?
    [​IMG]
     
    Last edited: Oct 18, 2012
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