Papercraft Dial Is The Slide-ruler Of Current Limiting Resistors


This paper dial makes selecting current limiting resistors a snap. [Giorgos Lazaridis] came up with the tool, which he describes in detail in the Worklog tab of his writeup. If you want one of your own he also posted a PDF which you can print, cut, and tack together.

At this point we can calculate resistor values for LED circuits without looking at reference material. But it wasn’t always like that. This wheel will be a fantastic tool for those just starting out in hobby electronics who are trying to grasp the theory behind lighting up a simple project. The outer wheel references the source voltage, with the inner being a gauge of forward voltage across the LED(s). Line those two values up and you can read the optimal resistor value in the window seen to the right. But wait, there’s more! As you can see in the video after the break the opposite face of the dial also includes a window which will tell you the power dissipation so that you may choose a properly rated resistor. Slick!

33 thoughts on “Papercraft Dial Is The Slide-ruler Of Current Limiting Resistors

  1. I don’t get it …. why would you build this for such a simple math calculation? I get these things for old times when you couldn’t get a calculator with scientific functions, or for calculating some things based on complex formulas.
    And how are you supposed to understand the theory behind it if the thing just gives you the answer?
    No! today you don’t make a slide ruler. To day you make an on line calculator so that anyone can use.

    1. Obvious you have an internet connection or a calculator. Some people in this world don’t/ can’t have the internet/calculator for whatever reason. This is a great resource for teaching, this isn’t a end all solution, its a tool to get you going. To get the students interested in the project, instead of “boring math” lose interest and the project is a fail. This is a bridge, to make it fun, not boring. Cheers

      1. Ok, ill give you this.

        But for me, thinking:
        1. 5V mirco
        2. Error led, make it red, so 1.8 V
        3. Wanna make this thing bright: 30mA
        4. 5V – 1.8 V = 3.2V
        5. 3.2 / 3 = 1.1 ish
        6 .mutiply by 100 for scaling between amps and 10s of miliamps : 110 ohm resistor
        7. closest value 120 ohm

        can happen in my head in the 5 meter trip between the desk where i solder stuff and the part drawer.
        There’s not point in making this if it takes you more time to find it in whichever drawer you stored it and input the data than it takes to open a calculator app on your computer and make the calculation.

        And why is is with people that complain about math being boring? This kind of math is a trivial too. You don’t see people complaining that the screwdriver is boring, or the pliers are boring. It’s just a tool, learn to use it efficiently.

        I stand my ground: at least make an on line thing. Or you know make a simple 1 file web page with a script that can work off line too.

        1. There are already online calculators for LEDs, and apps for iOS/Android for the same… That’s what I use. So I suggest not wasting your time coding up a script/App unless that’s truly your passion.

          The frequency with which I do LED of calculations isn’t high. And while I can derive the simple equation (did so recently…), I know I am prone to stupid math mistakes – I’m better off working through the math on paper just to be careful…

          This tool is faster than checking on my computer (which I don’t solder in front of), or even checking my ipod touch. Use a highlighter to mark the common LED voltages for different colors, and select the voltage I’m using.

          And, heh, it’s a tool just like that math tool of yours.

        2. Well, obviously you don’t have issues with learning, you’re to far to clever. Fact of the matter its its not about you! Its about getting people involved. Bogdan, do you have a learning disability? Most likely not, for some that have a learning disability,do find this subject difficult to understand at first. Even average people find it difficult at first. This is a tool for people, to get into it(training wheels). Once mastered, you don’t need the training wheels. Besides this slide calculator thingy is great for youth, but great for anyone that wants to learn. Whats seem trivial to you or me, is like magic to someone else. This is a great tool for someone just starting, not for someone that already know it.

          1. Nope. I agree with Bogdan. You can’t put training wheels on everything. At some point s**t gets real and unless this tool is part of a larger and properly thought out educational system that fully streamlines the teaching of electrical engineering, I don’t see it being anything other than a crutch.

            I understand you’ve gotta walk before you can run, but if someone’s intellectually quadriplegic… I mean come on! We’re talking 3rd grade math here!

          2. @ John & Bogdan,
            I don’t get it… why are you mental giants still using text to communicate? It’s time to take off the training wheels and start using long-distance telepathy. Frankly, I’m insulted that I had to dust off the keyboard to come down to your level. I mean come on! We’re talking 3rd grade telepathy here.

          1. Bad analogy. If you watch a power screwdriver, you would understand how the activity works (see the head rotating, screw is turning, etc.) and would be able to duplicate it with a manual screw driver.

            If a person uses this wheel, it may take them 1000 calculations before they can work out the formula. It’s much better to just teach the formula and have them work it out on paper.

      1. Definetelly with you on the 220 and 330 shortcut. Have just ordered a new batch of 100 of each.
        The example I was giving was for a situation out of the ordinary that requires recalculation.

        @pyrobutters Trust me, i have plenty of issues learning in other things. But this is not the problem I see here. The problem is that you are creating a highly dedicated tool for a task that you could accomplish with other trivial tools with the same effort, and, by creating this tool you are hiding the understanding of fundamental knowledge in a filed.

        To use your training wheels metaphor is see this as someone who knows they have to use a bike for a long time in the future thinks: screw learning how to ride the bike the proper way, I can always get from A to B with the training wheels on.

    1. Pilots and race car drivers prefer analog displays (dials) over digital displays, it is quicker to understand a guage is “a little below 3000” than to infer it from “2968”

  2. I don’t do calculations that often so when I do, i second guess my result and love seeing some form of computer verify it.
    Sure knowing the equations by heart and having really good mental math skills are great but I have neither of those things…

  3. [old guy mode]
    It’s slide “rule”, not slide “ruler”.
    [/old guy mode]

    Neat project! I’ve always been attracted to paper calculators. If you have an adhesive laminator, you can run the pieces through and not only will they last longer, the laminating material will make them slide easier.

  4. Einstien is quoted as saying “never remember anything you can look up in a reference manual” or something along those lines, which is probably why he was so smart, don’t waste your brain on the trivial things.
    I spend way too much of my work day as a tech teaching young engineers basic electronics or which is the proper end of the soldering iron to pick up (if you need a hint, you may wanna put some ice on that hand…). and these are EE’s mind you. Learning how to calculate a current limiting resistor may not have much value up front, but learning the concept may not be so bad in a practical application sense. That said, if you are in school for an EE degree, or any engineering for that matter, please take a lab or two or join a robotics club or something with practical applications, they are your friend! And once you’ve learned a few basic things, you may find yourself reaching for just a resistor and this thing sits in your desk drawer more and more often. In fact, if you don’t have a stock certain value resistors laying around, you are either way over thinking things (OCD) or….

  5. Back in Oct. 1995 on the Usenet group sci.electronics the following was posted in regard to using a sliderule to find resistor values.

    From: Dave Slee
    Newsgroups: sci.electronics
    Subject: Re: Sliderules (do they still exist?)
    Date: Wed, 18 Oct 1995 21:52:16 GMT
    Organization: Soil Machine Dynamics Ltd.

    I use all sorts of calculators, PCs, computers and micros, but a slide rule is one of the few tools
    that helps you to see beyond the digits and not worry overly about precision, but worry more about magnitude: bigger, smaller, close to and nearly good enough…

    Take your standard slide rule and doctor the scales. Add some new markings:
    the standard resistor values:

    1, 1.5, 2, 2.2, 3.3, 4.7, 5.6, 6.8, etc. (have I missed a few? so what you get the idea)

    Mark them as little dots on the ratio scales.

    Now a pair of scales on a slide rule work on ratios. If you want a potential divider that has a 3:7
    ratio, put the 3 on one scale against the 7 on the other scale. Now find the pair of dots that come
    closest. These are the standard resistor values that are the best approximation to the required
    ratio. The error in dot spacing gives you some idea of how far you are out.

  6. I love making stuff out of printed paper, especially since I have a craftrobo and don’t have to do any of the fiddly cutting parts myself. :3

    Which reminds me, I was going to make heat tracker cards for BattleTech, with a dial. I should go do that. :D

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