Re: Question for a circuit
There are microcontrollers or modules with bluethooth capability with no need of any additional shields, running as standalone CPUs; therefore I would not consider ATtiny/ATmega for this. With regard to alternatives to this, I am a bit suspicious to recommend the solution more suited to your case since I have my preference, but I have been using ESP8266 and ESP32 for some time, and this device, besides being very small, also has the capacity to communicate, along with bluethooth, via Wifi as well, which is another feature often available on smartphones.