A research firm estimates Amazon.com has sold 3 million Echo voice-controlled home speakers, and that close to half of U.S. Amazon customers are aware of the product. [snip] The device, which sells for $180, also has given Amazon a big role in the emerging field of home automation, as many people use it to control thermostats or lights that work off Wi-Fi. [snip] Launched in late 2014, the Echo slowly picked up speed as word spread about its uncanny abilities — from answering questions to turning on the living-room lights.