If you'd like to earn some cryptocurrency using your Smartphone, I recommend checking out the free app called Pi Network. Pi is a digital asset that may be earned by running a light weight application on your smartphone that will not compromise the performance of your device, and only requires that you open the app once a day to tap a green button and ensure you remain active in order keep earning Pi coins. Furthermore, if you chose to run a node for the Pi Network on your PC you may earn additional coins.
Accumulated Pi coins may be sold on an exchange when they reach Phase 3 of development (autumn/winter of 2020). Currently in Phase 2 now, Pi coins can be mined quickly and easily for free. If they rise in price the way alt coins are expected to rise in 2020, the coins you mine today for free on your phone may increase in value drastically turning you a nice profit!
Pi Network is available for Android and iOS smartphones and they require that you use a referral/invite code upon installation. Installation actually will not proceed without the invite code as the program is still in Beta testing so you may use the code: cpalaniuk
This is a free and easy way to dabble with Cryptocurrency and possibly make a lot of money in the future, depending on how many coins you mine and if the value of Pi rises in the near future as expected.
Good luck and happy mining!