Learn how to use cheap 433MHz radio transmitter and receiver modules to send data between Arduinos, such as might be used to create a wireless puzzle prop "beacon" that can send messages to trigger remote effects, or be triggered itself from a control system.
Hardware Used:
https://www.ebay.co.uk/itm/New-1-5PCS-315-433-Mhz-RF-Transmitter-Receiver-Wireless-Module-Arduino-ARMMCU/272472455163
Software Used:
http://www.airspayce.com/mikem/arduino/VirtualWire/index.html