Peak detectors are used when you have a rapidly changing AC input signal, and you want to obtain the peak voltage the signal reaches. Peak detectors are really simple to make – just a diode and a capacitor in their simplest form. In the video I give an example where I use a peak detector to hold on to a peak audio level for several seconds. This video builds on circuits I built in my amplifier and comparator tutorials so make sure you watch those first.
Bonus: Here is an online circuit simulation of a peak detector using an op-amp. This technique incorporates what is sometimes called an “active diode”, “precision rectifier” or “superdiode”.