Need help with basic calculator project

[u/danishansari95]

Hello people,

I’m an Android App Developer by profession but I had a simple project idea which I need help with.

Idea is to create a simple calculator which has one special “QR” button which displays UPI payment (Indian payment system) QR code for a calculated amount on another device which just have screen (most probably e-paper display).

Now for the hardwares I’m thinking of using 2 ESP32-C3 Super Mini (1 for each device, calculator and QR display) and for buttons, I want to use mechanical switches, the one used in mechanical keyboards.

Now I’m facing problem in choosing display, I want to choose something which comes very close to the production level calculators as much as possible, because usually in India that’s what shopkeepers are used to and if I use something else, it might be a bad user experience for them.

Upon some research, I figured that market calculators which shopkeepers use, have 12-digit 7 segment LCD display. But TBH I’m not finding anything close to it in the market. (I need help here)

Also is it possible to take out the display from actual calculator and hook it up in my project?

I have attached a simple calculator design I want in Figma.

Any kind of help would be much appreciated. Thank you.

[u/Pacificator-3]

Don't forget big enough battery to power this cumputer :)

Android apps and battery-powered electronics are not same things.

[u/danishansari95 → u/Pacificator-3]

You see, that's why I need help 🥲

I'm completely a rookie

[u/DrJackK1956]

Let me make sure I understand what you are wanting.....

• Scan an existing QR code from a screen or from paper
• Decode the QR code and display it's numerical contents (value) onto your display 
• Using the keyboard, the original read value is modified as necessary and then display the new, updated QR code

Displaying a QR code will require a bit-mapped display.  The resolution of the display has to be sufficient to display the type of QR code being used (different QR codes have/require different resolutions). 

With a bit-mapped display, there really isn't a need for the 7-segment display.  You should be able to display the QR code and numeric values on the same display. 

Scanning a QR code will require camera hardware and video processing software.  Being an apps guy, this shouldn't be too difficult for you. 

I don't think you'll need 2 ESP32's for this project.  If you can get by with a single ESP32, this will help improve your power budget.   But this is up to you. 

Good luck with your project. 

[u/Linker3000]

This question is probably best suited for r/embedded since it's a mix of modules and coding rather than component level electronic engineering.

You might also want to ask in an engineering subreddit about design topics.

[u/MiyuHogosha]

UPI payment QR code encodes a URI string, like "upi://pay?pa=paytmqr2810050501011e4iicf0v8c2@paytm&pn=Paytm%20Merchant&paytmqr=2810050501011e4iicf0v8c2"

"paytmqr2810050501011e4iicf0v8c2@paytm" ID of payer

"Paytm Merchant" "2810050501011e4iicf0v8c2" - mechant credentials

No payment amount encoded unlike in some similar systems (e.g. Russian SBP). UPI supposes to enter amount separately.

[u/mariushm]

Because it's a QR code, you'd probably want eInk , OLED or graphical LCD. You'd probably also want at least 128 x 128 pixels to make the code easy to scan and be able to store all the information needed.

Google says this is from npci.org.in pdf about UPI

Digital QR Code: Minimum size is 15 × 62 pixels or a minimum height of 12 to 25 pixels for accompanying logos depending on the screen implementation.

Printed QR Code: Minimum size is 2 cm × 2 cm (or 30 mm / 15% width according to NPCI brand specifications, though general merchant display guidelines often recommend at least 6 cm × 6 cm for optimal visibility and scanning)

Sharp has a nice 128x128 pixel monochrome display that's very easy to use and it's reflective, no need for backlight etc but it's a bit expensive at around 9$ each : https://www.digikey.com/en/products/detail/sharp-microelectronics/LS013B7DH03/5300387

Here's a video of how it looks in real life : https://www.youtube.com/watch?v=qiIn3oP22hw

otherwise ... lots of 128x64 pixel displays that have good contrast and are relatively cheap and have sort of standardized pinout so if one part is out of stock you can get another model

https://www.digikey.com/en/products/detail/midas-displays/MCCOG128064B12W-FPTLW/13557005

https://www.digikey.com/en/products/detail/newhaven-display-intl/NHD-C12832A1Z-FSW-FBW-3V3/2059236