I’ll give the background on this sometime, but I have a need for a weird thing and I don’t know if it exists or not. I need a calculator that basically only calculates one formula whenever asked. I put in a number, it applies the formula and spits a number back out at me. I need this to be portable, so not an app on a computer or tablet. Ideally it would be a standalone bit of consumer electronics. If an app is the only solution, I can try to find a cheap Android device somewhere (I think I have some old ones lying around).

Does something like this exist?

Excel on a smartphone?

Javascript on your webserver? Can be accessed from anywhere.

Are programmable, portable, battery (or solar?) powered calculators still a thing?

Save your formula into ‘permanent’ memory in the calculator, then run it against the input number. Hot key button on the calculator to run the calc would make it ‘foolproof’

I doubt that it exists with your specific (unknown) formula programmed into it.

Building one from scratch is possible, with an Arduino, battery pack, keypad, and LCD or OLED display. Perhaps $20 in parts. But probably not what you have in mind..

That was my first thought too. Is this something that other people in the world might want, as well? If so, maybe someone out there has already built such a thing.

If you build one yourself, maybe others would want one too, and you could sell them. You can already get some prebuilt arduino/esp32 button+screen boxes that you can program with whatever you like inside. The programming sounds like it would be just a few lines of code. Though googling for your specific form factor (keypad+lcd) results in a lot of "how to" guides rather than actual prebuilt ones you can just buy. (example, example)



This is an even better idea, unless the use case involves being out of service or being unable to use a touchscreen easily. (Arctic? Snowboarding? Spelunking? Diving? Sailing?) Now I'm really curious about the formula and its use case.

As tahir suggested, Excel on a smartphone seems like the simplest solution to me.

Cell A1: [where you enter your number]
Cell A2: Locked, not changeable without unlocking, contains this formula: =A1*A3
Cell A3: Locked, hidden, contains your Mystery Formula™.

Call up Excel on the phone, the cursor is already on A1, enter your number and the result appears below it. Or use A1, B1, and C1 if you prefer a horizontal layout.

tanstaafl.

Others have suggested a spreadsheet on your smartphone. If you want something lower-tech, you might see if you can find an old-school programmable calculator. I wrote all sorts of junk in my HP-28S when I was in college.

You may be amused to see that you can buy new implementations that are compatible with the old HP calculators.

So…I’m gonna make a quick thread about the background of this use case, then come back here. Lol. Sorry, it’s going to make too much of a tangent!

Ok, so, go read my post and come on back.

The store I manage has an arcade where you can earn tickets from most of the games. Then you redeem those tickets at the redemption counter for prizes (pretty standard stuff). Recently, the company policy changed about selling these redemption items outright (we were always told not to).

The tricky part is figuring out how much to charge the customer for a random item (there’s a couple hundred). However, there’s a pretty straightforward formula to figure it out - one that never changes. We know that the item costs us $1 for every 250 tickets. So we charge $2. The problem is that…well…things need to be made dead-simple when you’re dealing with a bunch of teenage employees. When things are going crazy and there’s dozens of customers to deal with, I don’t want them doing math in their heads.

I want them to be able to punch in the ticket value on the item, and have a cost get spit out. Yes I could easily do this with an excel spreadsheet, but I need something bullet-proof that can’t get messed up in the middle of a shift and result in a phone call to me at home!

The easiest soltion would be just a calculator and the instruction to “divide by 125.” But yesterday I was told our vacuum was broken and this morning I found a hair tie, a 2” screw, an outlet safety cover, and gum in the brush roller. I’m dealing with developing brains here, folks!

When your only tool is a hammer, everything looks like a nail. And since pretty much my only tool is Excel...

How about this: A simple Excel list in alphabetical order in column A, each of the "couple hundred" items. In column B is the formula for the price you charge. Column C is hidden (not printed) showing your cost for the item, and tucked away somewhere is a single cell containing the never changing division or multiplication factor you use to generate the price in column B.

Then on card stock print and laminate the list. You ought to be able to put three groups (6 columns) of items and prices on a single side of letter size paper. Use the largest font that will allow you to do that and get 50 rows on a page, and if you print double sided, on one sheet of paper you will have all the information your employee might need for up to 300 items with no math or electronic devices required.

As your costs per item change, update and reprint the spreadsheet, and if the never-changing "125" ever does change, you change that one cell and the whole sheet updates.

It ain't elegant or high tech, but that's how I'd do it.

tanstaafl.

If you want to DIY it, then a basic Arduino Uno and a touchscreen shield are all that's needed for hardware -- no soldering either.

Here's a similar project, showing what it might look like. They've already done the programming for a "keypad" and "result display" on the touch screen. So you'd just have to remove any code that has to do with the telephone shield part.

https://learn.adafruit.com/arduin-o-phone-arduino-powered-diy-cellphone

Or here's an even better one: A calculator. You simply have to customize the code with your simple formula:

https://www.instructables.com/Arduino-TFT-LCD-Touchscreen-Calculator/

Cheers

Brilliant.

This is of course the way someone would automatically have done it, way back in the days before we got so used to having handheld electronic devices do everything for us.

That's pretty brilliant indeed. And Tony's right, my brain is poisoned by convenience

I even have a spreadsheet of all our redemption products already. It'll be trivial to eliminate all the duplicate ticket prices and set up exactly what you said, and my wife has a laminator!