How to add more I/o pins to Arduino so I can create a 19 by 19 keypad matrix

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Hi, I would like to create a custom keypad that's 19 by 19 in size. I need 38 pins but I don't want to buy a mega. I heard that there are expansion shields that can add more I/o pins to an Arduino board. They probably work like additional multiplexer. Are there commercially available shields like that?

Thanks!

2

Could use a few of these 10x8 keypad scanner ICs. There are others, like this 8x20 series.

3

I'd hunt up several PS/2 keyboards and use their encoders. 19x19 is a lot of keys!
A standard PC-like keyboard will handle somewhat over 100 keys, into about 2 IOs.

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https://mayhewlabs.com/products/arduino-mux-shield
https://mayhewlabs.com/products/mux-shield-2

Or you could use three of these PCF8575 16-bit port expanders. They cost $8.04 for three, with shipping:

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