Hardware wallets are more expensive than you might think, and most hardware wallets connect to USB. This is a security concern for me. So I added the QR code function to the Ethereum Smallet app so that I can use my Android phone as a hardware wallet.

Learn how to do it.

Install the Ethereum Smallet App on your current Android phone. Please refer to this document for detailed installation instructions.

Install Smallet Key Vault on old Android Phone. Once installed, the phone sets airplane mode to block all external connections. While using as a hardware wallet, this phone does not need to be connected to the network, and it is recommended that you do not turn off airplane mode for security reasons.