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Three cheap calculators in one!!!

Discussion in 'General Electronics Discussion' started by Juan Diez, Dec 24, 2016.

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  1. Juan Diez

    Juan Diez

    Dec 24, 2016
    Hello guys,

    I´m a mechanical engineer that understand a few about electronics. I´m in sports arbitrage bussines and I´d like to build a calculator from 3 others. Those cheap chinese ones.

    The idea is to use three displays, and two keyboards in order to always divide the number introduced in display 1 by the number introduced in display 2, and watch the result in dislay 3. Without the need of typing the / and = keys. Others features are needed but for now I want to know if this is possible and if is anybody out there who can help me.

    Thanks a lot.
  2. Gryd3


    Jun 25, 2014
    Many of those cheap calculators use purpose built boards with a single IC that is usually covered in a little black bubble. I can imagine quite the head-ache in trying to modify these to make it suit your needs.
    I have two solutions for you, one would require building a calculator from scratch using your own microcontroller, display and keyboard. The other solution is getting your hands on an existing calculator that you could bend to your will like the Texas Instruments graphing calculators. Look for a TI-86 or similar, and ensure you can build your own program into it.

    Of course, you need to share some more of your requirements. What kind of budget do you have, how large/small do you need the calculator, and what kind of features do you want?
    To me, it sounds like you are simply after a calculator or application (Android/iOS, or TI-86) that will let you show the top, bottom, and answer while retaining a method to alter the top or bottom without having to re-type the whole thing in again.
  3. Juan Diez

    Juan Diez

    Dec 24, 2016
    It would be much easier with two keyboards instead of one as in the case of the TI. Much worst with a single tactile screen in Apps. I better follow your construction advice. In the meantime I learn basic stuff about electronics.

    The idea of two keyboards is because I dont want to select in any form the display I want to enter the numbers. Instead, one keyboard for each display and a third display that shows the result. Is like you said, I´d like it to be able to always do the calculations as the numbers are entered.

    The main requirements are.

    The displays, keyboards and calculation things.

    One button for each display that clean them separately. It would be great if after 1sec i.e. of entered the number in each display, you dont have to clean them but just enter the next ones for the next calculation. The idea in arbitrage betting is reduce time between the bets.

    thats it!!

    About the size I prefer it to be as big as possible, those keys and displays of the big and cheap claculators work perfectly. That in order to avoid typing mistakes and clearly see the result.

    About the budget, I dont think it would be expensive as a weekend project. You know, I´m the kind of guy who likes to build stuff from ¨junk¨ and re-used parts, as a mechanical it works real good for me, I just don´t know how that idea fits in electronics.
    Bottom line is that I don´t want to buy real expensive parts to build it, but I can spend a few dozens of bucks as a personal project.

    Thanks a lot
  4. Gryd3


    Jun 25, 2014
    Well... re-using or recycling electronics parts can be a time consuming task.
    Step one is disassembly of course, then you need to ID the parts and either remove them or modify the circuit to do your bidding... This is not always easy and the step required could vary wildly from one calculator to the next. So if you want it to be a weekend project, I would certainly suggest you either buy new parts, or you find/buy equipment you *know* you can hack/salvage.
    I'm confident that the guts from any/all calculators will be useless, but depending on the calculator you find, you may be able to re-use the keypad and/or display. This is going to depend largely on the type of display and the type of keypad used and may require some tinkering and engineering work to connect to a third party device to do what you want. Keep your eyes open for the accounting style calculators that use LED segments for the display instead of those cheap, flat, LCD displays. They will most likely be easier for you to work with.

    Anyway, let's explore buying new parts, and what to expect with costs.
    Your keypad does not seem to require any more buttons than 0-9 and maybe decimal and clear (or backspace) because this is a purpose built machine that only does division.
    Something like this perhaps :
    * can be the decimal, and # can be the backspace (or hold for clear)

    This can control the LEDs you find/buy:
    Maximum of 16 digits per controller.

    LEDs can be found here:
    (Various colors available. Sections of 4 numbers.)

    Then you need a microcontroller, and some miscellaneous resistors and a couple caps perhaps. (Oh! and a power supply)
    Total cost if you want to build it from new parts would probably be in the order of $70-$90 USD *if* you got the parts from adafruit. This would include an Arduino microcontroller, a pair of LED controllers, 6 sets of 4 character LEDs giving you 8 characters per display, and a pair of keypads.

    You can also buy parts from other suppliers if you find them cheaper.

    This is very rough work, but outlines what you would need. The real tricky part for you will be finding something you can reprogram. It would be best if you at least buy a microcontroller, then you can add onto it as you find used/recycled parts.
  5. OBW0549


    Jul 5, 2016
    You cannot. Calculators are designed to do a specific set of functions and are hard-wired to do ONLY those functions. And those functions DO NOT include sharing data with other calculators or driving additional displays.

    If you want to make a device such as you've described, you'll have to build it from scratch using a programmable microcontroller, two keypads, three displays and a power source plus other miscellaneous goodies. Then you'll have to learn how to program, and then program the microcontroller to do what you want. Putting all this together and getting it to work, unless you already are adept at electronics design and construction and microcontroller programming, will be more like a year's task instead of a weekend's task.

    Life is short. Why not do this the easy way? What you've described can be easily done-- in about 30 seconds-- with a spreadsheet app running on a smartphone. One cell for entry of the first number, another cell for entry of the second number, and a third cell to calculate and display the result.

    But doing this by hacking calculators? Forget about it. Ain't gonna happen.
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