We are trying to make an open-sourced LED Rubick's cube which a child can "instantly" scramble every ~30 seconds with a stand-alone arduino kit.
===> QUESTION Each Rubick's cube needs 20 individual close-range radio-bluetooth-IR receivers that are: small, low power, low cost & low radiation. (each of 20 little cubies has: LEDs, battery, receiver). What do you recommend?
Really only need receivers (not tx) for each cubie
For fitment, the BLE Nano (nRF51822) sold on arduino.cc aka redbear Bluetooth 4.0LE board (18.5 x 21m) just fits (but it is too expensive and big size will force smaller batteries)
The cube will be right next to the arduino so radio distance and noise are not really issues. Some early ideas:
*  NRF51822 bluetooth BLE 4.0 module (no wifi) seems to be among the lowest power consumption receivers out of the box. Any opinions here?
- Via audio spectrum with microphones embedded in each cubie (maybe a nonstarter with WS28xx timing tolerance and ambient noise?)
* Via IR with photoelectric sensor embedded in each cubie (maybe a nonstarter with cube's WS28xx strict timing requirements and interference from Rubick's cube LEDs/ oil from player's hands?). Also cube might fail with more holes. Killed based on recommendationof @Robin2
Tweaked LM393/ LM358 (see jlettens post here)
ESP8266 / ESP32
Radio-bluetooth-wifi is definitely not my forte so appreciate any macro level guidance here!