Since the transmitter (TX) needs to talk to receiver (RX) when triggered, we need to find a way of wireless communication between the two.

There are a number of popular wireless chips. ESP8266 is for WiFi, so not relevant here. The ubiquitous 433MHz RF module is another option. They are very cheap, but take up a lot of space. Those modules are also relatively primitive in terms of functionality, as they don’t have error detection/auto retransmit/different channels. Reliable transmission is very important in this project, so not ideal.

The easy winner in this case would be the hugely popular nRF24L01 “Single chip 2.4 GHz Transceiver”. This chip is found on countless ready-made modules for Arduino projects, with vast amount of documentation and community support. This chip supports a huge amount of RF channels and communication addresses, sophisticated error detection and acknowledgement/retransmit algorithms, and a fast data rate. Those chips are also very reasonably priced, where a ready-to-use module only costs around $2.

Which leads to a fascinating question, how can a complete module costs half as much as a bare chip on mouser?

The answer is, as many has discovered, very simple, the chip on those cheap modules are actually Chinese counterfeits. There are multiple threads about this issue:

https://forum.mysensors.org/topic/1153/we-are-mostly-using-fake-nrf24l01-s-but-worse-fakes-are-emerging

https://forum.mysensors.org/topic/1664/which-are-the-best-nrf24l01-modules/28

https://hackaday.com/2015/02/23/nordic-nrf24l01-real-vs-fake/

https://forum.mysensors.org/topic/6412/nrf24l01-genuine-vs-counterfeit-checked-some-modules

The thing is that a lot of users report that the counterfeit chip works just fine, and that made me curious. I did some digging around internet, and found that most of the counterfeit chip is actually a compatible clone called Si24R1. They could have just called it that and it would have been fine, but relabelling them as nRF24L01 is where they crossed the line. 

If you dig around a bit, you can find Si24R1 for sale for as little as $0.375 each, that’s less than a tenth of the price of a genuine nRF chip! Pretty remarkable, and now you know how they make money on those cheap modules. Anyone I bought half a dozen of those Si24R1 chips for testing on prototypes to see how they really work.