Comparators are a very handy circuit block that everyone should know. The basic idea is that you have a reference voltage, and as your input goes above or below that voltage, the output of the comparator goes high or low. In the video I give an example clapper circuit that toggles an LED on and off based on the volume of sound in the room. You can extend the concept to use comparators to toggle any device you want whenever a voltage exceeds a certain threshold.