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I'm looking to complete one of my first electronics projects so I was looking for inspiration from youtube Great Scott and this project caught my eye.

In short, his project converts the stereo signal to mono, amplifies it then compares it using a voltage divider network to light up rows of LED's based upon the voltage of the audio signal.

My question is looking at the circuit to me it looks like the OP amp can be used as a non-inverting amp for the audio signal. But then should this signal be used with a comparator and the voltage divider network to drive the MOSFET's rather than the OP amp circuit that is shown?

I didn't know much about OP amps before but this doesn't look like how they should properly be used?

enter image description here

and Youtube video.

Voltage Spike
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    People frequently misuse op-amps as comparators. – Peter Bennett Aug 02 '19 at 21:46
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    This looks like a waste of time and effort when the LM3914/-5/-6 series of dot / bar LED meters have been around for 40 years. They do linear (voltmeter) or logarithmic (audio), can be daisy chained for multiples of 10 LEDs, etc. There are thousands of tutorials on these devices on the web. – Transistor Aug 02 '19 at 22:24
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    You should have current-limiting resistors in series with each LED. I noticed in the video that he used an adjustable DC-DC converter set to the forward voltage of the LEDs - this is very bad practice as the forward voltage of a LED depends on its colour, temperature, exact chemical composition, phase of the moon, etc. – Peter Bennett Aug 02 '19 at 23:11
  • @PeterBennett There do exist LED's that contains an internal resistor that you just attach to 5 V and call it a day. So on a schematic it is literally 5V, LED, GND, similar to the schematic above. Maybe he used them, maybe he did not. – Harry Svensson Aug 03 '19 at 01:42
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    @PeterBennett "misuse" is such a strong word :-). I'd suggest use / press into service / utilise ( nuanced version of use in this context), ... . ie use of an OA as a comparator is entirely legitimate when the specs adequately the application requirements. – Russell McMahon Aug 03 '19 at 02:10
  • @HarrySvensson Idally [tm] his cct would show part numbers, and if they have an internal R it is exceptional enough to either show it on the diagram or mention it in a note. If I had to bet your life on it I'd say there are no explicit resistors internal or external. – Russell McMahon Aug 03 '19 at 02:12
  • @HarrySvensson: Yes, I know - but Great Scott was adjusting the LED power supply to 2.something volts, which sounds like he was using bare green LEDs. His Amazon and EBay links are to plain LEDs (but the ones at Amazon do include 2K resistors as well as the LEDs.) – Peter Bennett Aug 03 '19 at 02:13

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