choosing an amplifier for LM3915N VU meter - Page 1
Forum section: electronics / beginners

author: Adhith
author_rank: Frequent Contributor
timestamp: March 31, 2017, 09:03:58 am
content: hello everyoneI'm have made a VU meter with LM3915N, but i found that the IC needs a much higher audio signal as input rather than the direct audio signal from audio jack. so I'm looking forward to make a small amplifier for the circuit. can any one suggest me a good circuit??

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2017, 09:37:49 am
content: A simple op-amp amplifier will probably do.Post a schematic of your LM3915 circuit.

author: capt bullshot
author_rank: Super Contributor
timestamp: March 31, 2017, 09:38:36 am
content: Just have a look into the original (National Semiconductor) Datasheet. They have a few suggestions of how to use this IC as an audio VU meter. You may want to replace the LF353 etc. by nowadays more common TL072 or whatever ...http://www.experimentalistsanonymous.com/diy/Datasheets/LM3915.pdf

author: Adhith
author_rank: Frequent Contributor
timestamp: March 31, 2017, 12:45:26 pm
content: can you suggest a good op amp circuit??

author: Adhith
author_rank: Frequent Contributor
timestamp: March 31, 2017, 01:04:52 pm
content: I found this circuit which uses LF353 so is this the amplifer circuit??

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 31, 2017, 01:07:18 pm
content: You say: "needs a much higher audio signal as input rather than the direct audio signal from audio jack." But that tells us nothing about what your audio signal actually IS. What "audio signal" are you talking about? What "audio jack"? What is the SOURCE of your audio signal? What is the impedance? Can't really recommend an amplifier circuit without at least knowing something about your audio signal.The LM3915 circuit works fine on "line-level" (on the order of 1V) audio without additional amplification. How many LEDs indication are you getting with your circuit?

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 31, 2017, 01:11:43 pm
content: Quote from: Adhith on March 31, 2017, 01:04:52 pmNo, that is not really an "amplifier" circuit. That is a rectifier circuit (as it says on the title).Since the LM3915 only displays DC, you need something to rectify the audio signal into DC.Perhaps your audio level is OK, but you have no rectifier to convert it to DC.Again, what is the SOURCE of your audio signal? At what level is it?You might find this instructive:

author: Richard Crowley
author_rank: Super Contributor
timestamp: March 31, 2017, 01:18:39 pm
content: If that circuit you show in your first post is actually YOUR circuit (?), then the 1N4148 and the 2.2uF capacitor are a simple signal rectifier. If you need additional signal gain (before the rectifier), then you can use a simple op-amp. We assume you have only a positive power voltage available (what voltage?). So you need a "single-rail" circuit like this...

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2017, 01:38:05 pm
content: Quote from: Adhith on March 31, 2017, 12:45:26 pmAs mentioned above, you need to provide more information.What device is the audio jack connected to? Is it a headphone socket? Is it a line out from a PC sound card?

author: Adhith
author_rank: Frequent Contributor
timestamp: March 31, 2017, 03:06:29 pm
content: thank you everyone for your good support. by audio jack i meant the audio jack of the phone. I'm planning to provide the audio signal directly to the board from the phone via AUX cable. The circuit needs a minimum of 1.3v of audio input which can't be provided by the phone directly. I have done some varieties of this circuit previously for my audio project but at that time i have used amplified signal from my speaker amp. but now I'm not using any amps so, I'm thinking of using any op amps specifically for this circuit. the Lm3915N circuit is powered by a 12v, 1A adapter. I'm using 10 LEd strips for each legs of the IC to make the VU meter & each strip contains 9 LEds.

author: Audioguru
author_rank: Super Contributor
timestamp: April 01, 2017, 12:59:15 am
content: Quote from: Richard Crowley on March 31, 2017, 01:18:39 pmThis circuit is missing an output coupling capacitor so the VU meter will continuously show its output DC voltage.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 01, 2017, 01:45:55 am
content: Quote from: Audioguru on April 01, 2017, 12:59:15 amThat is a part of the complete circuit. Whether a DC blocking capacitor is required is part of the complete circuit integrated design.

author: Adhith
author_rank: Frequent Contributor
timestamp: April 01, 2017, 08:28:51 am
content: hello friendsI'm looking for some specific names of op amps and its circuits which would be ideal for my vu meter. How about LM358, can it be powered by a 12v supply?? .To get a audio signal arroung 1.5v what would be the probable circuit??

author: nugglix
author_rank: Regular Contributor
timestamp: April 01, 2017, 09:35:38 am
content: What about reading the datasheet?

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 01, 2017, 02:17:35 pm
content: Quote from: Adhith on April 01, 2017, 08:28:51 amVirtually ANY opamp would work for your application.QuoteDownload the datasheet. It contains this information.If you need help reading the datasheet, then ask us.QuoteDid you not see my response #7?We are confused why you are asking questions that have already been answered?If you don't understand the answers, ask for help.If you keep asking the same question we will grow tired of answering you.

author: Adhith
author_rank: Frequent Contributor
timestamp: April 01, 2017, 03:56:47 pm
content: sorry for the trouble friends... the reason why I'm asking these questions is that I'm a beginner in electronics so if someone says that 'any OP amps will do' again I'm confused which to choose from. I'm not confident to choose some random circuits from internet which may or may not be a good option. So I thought to rely on experts who knows about which to choose from. Now I'm moving with LM358.Thank you my friends especially 'Richard Crowley' for your valuable comments.

author: Adhith
author_rank: Frequent Contributor
timestamp: April 01, 2017, 04:00:10 pm
content: I have read the datasheet and have got some idea about it

author: Zero999
author_rank: Super Contributor
timestamp: April 01, 2017, 11:26:12 pm
content: You probably don't need any amplification. A precision full-wave rectifier will do. Here's a precision full-wave rectifier which will work from a single supply. It has a gain of 2. You might need to add a low pass filter to the output to get the LM3915 circuit to work properly.