Low buck projects- Let's see 'em! - Page 64 - SailNet Community
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post #631 of 1420 Old 05-02-2013
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Re: Low buck projects- Let's see 'em!

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Originally Posted by bristol299bob View Post
thumbs, do tell.

btw looks great!
To understand what resistor needs to be added, you need to understand how an LED works.

For a conventional lamp, the current in the circuit will be determined by the resistance of the lamp, and the voltage you apply to it. It's ohms law. I=E/R

Also, a conventional lamp will drop all the source Voltage. If you connect the lamp to 12 V, the lamp will drop 12 V. Even if you connect a 6 V lamp to a 12 V source, the 6 V lamp will drop all 12 volts. It won't last long, because you are exceeding it's design limitations, but it will drop the 12 V.

An LED is a totally different animal. While it does drop a voltage, that voltage is fixed, regardless of what voltage you connect to it. Not only that, but an LED has no real resistance when on, behaving much like a closed switch or piece of wire.

It is for that reason that whenever you connect an LED, you must add a resistor in series. Without a resistor the LED will act like a short circuit, and the current will be high. The LED will burn out, the fuse will blow, or both. The resistor limits (and determines) the current through the LED.

To determine the resistor size, you must know some things about the LED's you will use, and how those LED's are connected. LED's vary a little, but most LEDs drop between 2 and 3 volts, and need a current of 15 to 20 mA to be nice and bright but not burn out.

For my flashlight, I actually connected the batteries and measured the LED voltage drop and current using a digital multimeter. Measuring the voltage drop was as easy as putting the meter across an LED while on. Mine measured 3V.

Last edited by AllThumbs; 05-02-2013 at 11:03 AM.
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post #632 of 1420 Old 05-02-2013
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Re: Low buck projects- Let's see 'em!

Here is my flashlights circuit (except there were 24 LED's, all in parallel:



The LED's each dropped 3 V, and since they were all in parallel, the whole bank dropped 3V. The 3 AAA batteries added up to 4.5V. Since the LEDs, drop 3V, the rest of the voltage (1.5V) drops across the series resistor. Without the resistor, the 1.5 volts would try to drop across the circuit wires. (like shorting out a 1.5V source).

I determined my LED current by measuring the total current and dividing by 24 (the number of LED's). My circuit was drawing 360 mA, which meant each LED was drawing 15 mA.
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post #633 of 1420 Old 05-02-2013
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Re: Low buck projects- Let's see 'em!

Here is the modified circuit of 9 LED's:



Since I was connecting my LED's to 12 volts, I needed to take advantage of that. I decided to connect them 3 in parallel, in series with 2 other sets of 3 in parallel.

Each bank of 3 in parallel will drop 3 V, so the 3 sets in series will drop 9V. I need a resistor to drop the last 3.

I want each LED to pass 15 mA of current, so the total circuit current needs to be 45 mA. This means the resistor needs to be sized to "set" the circuit current to 45 mA. Since ohms law applies to the resistor, R = E/I. The resistor voltage is 3V, the resistor current needs to be 45 mA, therefore:

R = 3/.045 = 66.7 ohms. I chose 68 ohms, as that is what I had.

Clear as mud?

Oh, and unlike conventional bulbs, the polarity of an LED matters. If it doesn't work, try reversing the polarity.
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post #634 of 1420 Old 05-02-2013
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Re: Low buck projects- Let's see 'em!

BTW, if you don't want to use this low buck idea on your coloured navigation lights then don't.

For me, the red one looks red, and the green one looks green. It's a trailer sailer, and this is the first time its had any navigation lights at all since I have owned it. Your mileage may vary, use common sense.

Either way, this project might fit other lighting applications around your boat.

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post #635 of 1420 Old 05-02-2013
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Re: Low buck projects- Let's see 'em!

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Your mileage may vary, use common sense.
Agree with both. Also, be careful that the light doesn't actually shine through a wider, or narrower arc than it should. But, as you said, for a trailer sailer, that won't be in the dark much, probably fine. Great trick for getting lots of lights at a low cost. My house battery has been screaming at me for a few years to try such a mod.

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post #636 of 1420 Old 05-02-2013
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Re: Low buck projects- Let's see 'em!

If you had made the limit $200.00 instead of $100.00, I could've shown you a boat my son and I built a couple of years ago. But I guess we missed the cut...

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post #637 of 1420 Old 05-02-2013
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Re: Low buck projects- Let's see 'em!

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BTW, if you don't want to use this low buck idea on your coloured navigation lights then don't...
I won't use it. I was just warning others to be careful about what they do, because it will cause problems with many types of LEDs, especially the cheap ones.
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...Your mileage may vary, use common sense...
Common sense may not be sufficient in this case. "Common sense" says you put white light through a green filter and it looks green. But a closer look at the spectral distribution of inexpensive LEDs reveals that they're not "white" in the way that incandescent lights are. LED's emit a few narrow bands of wavelength that blend together to approximate white, but when put through a green filter can actually look like a different color than green. This has been shown by many to be a problem, and I was pointing that out.
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...Either way, this project might fit other lighting applications around your boat.
I agree that this could be very nice for applications where color fidelity is not important. Red/green nav lights, not so much.


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post #638 of 1420 Old 05-02-2013
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Re: Low buck projects- Let's see 'em!

I have a bunch of original lights in the cabin. I think many of them have wiring issues (corroded switches, etc.). I'd still like to keep the housings (I like the aesthetic) and replacing the innards and switches with LED lights. A trick like that shown above would be a great way to get LED's in there easily.

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post #639 of 1420 Old 05-02-2013 Thread Starter
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Re: Low buck projects- Let's see 'em!

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If you had made the limit $200.00 instead of $100.00, I could've shown you a boat my son and I built a couple of years ago. But I guess we missed the cut...
Gotcha covered:

Low-Buck Dinghies- Lets See 'em!

let's see it- bonus points for the boat being a father-son project.

It's 5 o'clock somewhere:


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post #640 of 1420 Old 05-03-2013
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Re: Low buck projects- Let's see 'em!

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Gotcha covered:

Low-Buck Dinghies- Lets See 'em!

let's see it- bonus points for the boat being a father-son project.
Well, it doesn't really fit in that thread, either. It isn't a dink. But what the heck; I'll post a few pictures there anyway....

I also have a complete build thread for it on boatdesign.net:

Building a flat bottomed canoe - Boat Design Forums

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