Help in designing a VU meter circuit w/ an electret mic. - Page 1
Forum section: electronics / projects

author: K-Experiment
author_rank: Contributor
timestamp: April 29, 2020, 01:04:12 pm
content: I'm still a beginner in designing my own circuits.can you help me with the design of the circuit if there is something to change?also, I have some questions like:Can it all operate in 5v? it says in the datasheets that the ic's can operate on 5v but I'm afraid that the voltage is too low and can cause distortions.Do I need the precision full-wave peak detector? I was told that it can help smooth out the audio from the op-amp.

author: fcb
author_rank: Super Contributor
timestamp: April 29, 2020, 01:55:50 pm
content: Assuming there a speaker missing in your design? (hence the LM386 and huge ac coupling capacitor).Let's sort out the front end and get to the bottom of what the overall device is trying to do before setting the rails.

author: K-Experiment
author_rank: Contributor
timestamp: April 29, 2020, 02:32:15 pm
content: There wont be a speaker.it will just measure how noisy the room is or see how loud the music in the background is played.

author: fcb
author_rank: Super Contributor
timestamp: April 29, 2020, 03:31:14 pm
content: Quote from: K-Experiment on April 29, 2020, 02:32:15 pmGet rid of the LM386 and humongous coupling capacitor and replace with another pair of LM358 stages - probably a gain of 50-100 will be sufficient (so each stage needs sqrt(50) to sqrt(100)=7 to 10).Assuming you are trying to make the whole circuit work on a single rail? 5V can be done - 9V might be nicer.

author: K-Experiment
author_rank: Contributor
timestamp: April 29, 2020, 03:48:52 pm
content: Yes I am using a single rail.I currently only have one lm358 and one lm386 but I do have a few lm324s, should I use the lm324?I checked the datasheet for the lm386 and I can set the gain to 50. will that work instead of using a second lm358?

author: fcb
author_rank: Super Contributor
timestamp: April 29, 2020, 04:16:56 pm
content: LM324 (quad) would be much better than an LM386 (single) in this application.Have you got an oscilloscope?

author: fcb
author_rank: Super Contributor
timestamp: April 29, 2020, 04:31:14 pm
content: Half-wave will be fine.VU is a type of reponse, you won't get a proper VU response with the LM3915 - I suspect you are after a VU like response. If you use a simple precision rectifier, and then slug the response - could be as simple as parallel RC network between the rectifier and the LM3915 will give you something like you want.https://www.mouser.com/datasheet/2/405/lm3915-443929.pdfIf you've got a single rail - you'll probably want to create an artifcial ground.

author: K-Experiment
author_rank: Contributor
timestamp: April 30, 2020, 02:10:04 am
content: nope, I don't have an oscilloscopeI designed the precision full wave peak detector in reference to the lm3915 datasheet.and what is an artificial ground?

author: fcb
author_rank: Super Contributor
timestamp: April 30, 2020, 01:22:21 pm
content: You're probably going to struggle during development without a 'scope. And if you don't have a 'scope you probably won't be familar with LTSpice or other simulation suites.Artificial ground - most audio is AC, AC amplification typically requires split rails (eg +/-5V). You can achieve this by creating an 'artificial' or 'virtual' ground at the mid-point between the single rail and 0V. This might be as simple as potential divider (sometimes with a decent size capacitor(s) across one or both resistors), or it might be buffered by an opamp.So your +9V and 0V now become +4.5VA (previous+9V), 0VA, -4.5VA (previous 0V), all your circuitry is now referenced to the artificial/virtual ground called 0VA.

author: K-Experiment
author_rank: Contributor
timestamp: April 30, 2020, 01:27:00 pm
content: so the design changes that I need to make are the op amps, the power supply, and the coupling capacitor.are there any more design changes that you recommend?about the lm324, I read that it isn't a good amplifier for audio because of the high noise and THD.I added a schematic diagram, is it correct?

author: fcb
author_rank: Super Contributor
timestamp: April 30, 2020, 03:34:39 pm
content: LM324 - it's much more nuisanced than that. Noise and distortion are unlikely to be issues if you aren't actually listening to the output of your amplifiers, and if you haven't got an oscilloscope then you are flying blind somewhat.The schematic of the virtual ground looks OK, but it won't source/sink much current, so you'll have to watch your design.

author: K-Experiment
author_rank: Contributor
timestamp: April 30, 2020, 04:41:14 pm
content: Thank you for helping me, I have learned a lot of things, and if you have any more recommendations pls tell me.

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 30, 2020, 05:01:22 pm
content: Because you will be driving a lot of LEDs, your current consumption can be significant for a 9V battery.You have two options:-connect the LM3915 in dot mode.-use 6 AAA batteries in series. Actually AA would be even better, if you have the space.

author: K-Experiment
author_rank: Contributor
timestamp: May 01, 2020, 02:32:58 am
content: alright, anything else?