We replaced the electronics of a "Chicco first dreams" teddy bear with an ESP8266. As a power source we use an old one-cell power bank. We repurposed the old speaker and drive using just a transistor (using the delta-sigma output implemented by the ESP8266 Audio library).  By cutting out the old Battery case and using a, on perfboard, custom soldered board we managed to squeeze everything in the old case. There is even enough space to include the old board! That means we were able to re-use the power switch and the built in buttons. Hence it's apart from the mini USB connector for charging completely indistinguishable from the old toy. Of course that impression will change once you turn it on, as the teddy will start playing WAP!

The software is written in Arduino and uses the excellent ESP8266 Audio library. We added a beat detection using the Goertzel algorithm which turns the LED strip on, if it detects a bass frequency.

Here are some videos:

• Finished Teddy
  
• Testing the custom board, some parts (voltage regulator) still missing
• Prototype on breadboard
  

Source code and schematics can be found Github