Recommended · for beginners
Comparator tutorial & clapper circuit
Why people recommend it
An Afrotechmods tutorial on using an op-amp as a comparator, including a clapper circuit, often linked when someone needs a simple threshold detector without a microcontroller.
Watch for: Pairs with his peak detector video
In their words
EDIT 2017-03-07 I went search in direction mentioned in comments and found this video.
MightyPolo on electronics.stackexchange.com, Mar 2017. CC BY-SA.