Samsung has made a Bitcoin Mining Rig Out of 40 Old Galaxy S5s 
Their headlining project at Samsungs recent developer conference was a 40-stack of Samsung Galaxy S5s turned into a Bitcoin mining rig. Not only were the phones able to mine Bitcoins, they were able to do it more efficiently than a desktop computer. In fact, eight Galaxy S5s can mine more efficiently than a computer with an Intel Core i7 2600.
Of course this is an experiment, but it serves to see the potential that can have relatively obsolete devices.
Their headlining project at Samsungs recent developer conference was a 40-stack of Samsung Galaxy S5s turned into a Bitcoin mining rig. Not only were the phones able to mine Bitcoins, they were able to do it more efficiently than a desktop computer. In fact, eight Galaxy S5s can mine more efficiently than a computer with an Intel Core i7 2600.
Of course this is an experiment, but it serves to see the potential that can have relatively obsolete devices.