Dilshan Jayakody has published a new build:
When working with expensive speaker system, speaker protector is an essential item to avoid any damages to speakers. In this project we build low cost speaker protector by using NEC’s μPC1237 IC. The circuit in this project is based on μPC1237 datasheet and it is specifically modified to work with 24V AC power source and with 24V relay.
Project info at Dilshan Jayakody’s blog.