# I need a little help building a simple LED voltmeter

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#### joecool85

##### Member
Hi, I got some ideas from some googling, but it seems I can only find stuff for 12volt setups. Here is what I need.

Green LED on at 1.5volts
Orange LED on at 2.5volts
Red LED on at 4.5volts

If possible I'd like to have it so that while the Orange is on, green stays on, and while the Red is on orange and green stay on. If thats not possible, thats ok though. Also, the volts I gave out can be rounded of +/- .5volts or so. The orange could be rounded -.5/+1

Any ideas?

#### audioguru

##### Well-Known Member
What supply voltage do you want to use for the LED voltmeter? The green LED will need at least 1.8V, the LM324 quad opamp used as comparators will need at least 3V and an LM2931AZ 5.0 little voltage regulator will need at least 5.4V. Then a little 9V battery could be used until its voltage drops to 5.4V and resistor voltage dividers could provide very good accuracy.

#### joecool85

##### Member
Maybe I should explain the whole situation. I'm measuring the voltage coming out of my little gem amp, the one I'm running a mic into and then hooking the output of that into my laptop. The max I've pulled out of it is roughly 5volts. I'm now thinking that even a single LED that would light at around 4.5 volts would be sufficient. That way I would know I'm good as long as I keep the LED unlit. My laptop can take 2Vrms, 5.65Vpp.

#### Nigel Goodwin

##### Super Moderator
joecool85 said:
Maybe I should explain the whole situation. I'm measuring the voltage coming out of my little gem amp, the one I'm running a mic into and then hooking the output of that into my laptop. The max I've pulled out of it is roughly 5volts. I'm now thinking that even a single LED that would light at around 4.5 volts would be sufficient. That way I would know I'm good as long as I keep the LED unlit. My laptop can take 2Vrms, 5.65Vpp.

We told you before, there's no problem whatsoever with the output of that little amp, and if you REALLY want to do something (for no good reason), simply add a resistive attenuator between the amp and the laptop.

Your laptop won't suddenly die is you apply 2.1V RMS, the only possible consequence is that it might clip and become distorted.

#### joecool85

##### Member
Resistive attenuator?

And also, I know it won't kill my laptop, I just want to be able to monitor how many volts are going in. Because if I do that, then I can also plug my guitar straight into that, because it changes the output voltage.

#### audioguru

##### Well-Known Member
A resistive attenuator is like a volume control. It reduces the level.

The LM3914 and LM3915 ICs are DC voltmeters. They need a simple Half-wave Peak Detector circuit which is shown on the LM3915's datasheet so they can properly show audio levels.

You shouldn't plug-in your guitar's output to the same input on your laptop that already has another output connected, you should use a simple mixer circuit to isolate the two outputs.

#### joecool85

##### Member
I don't think you see what I'm saying, let me elaborate.

I have a small amp that I built that I'm using as a preamp. To do recordings right now I plug a mic into the small amp and the small amp into my laptop. I put the mic in front of my regular practice amp, and adjust levels accordingly. What I was getting at was using just the small amp and instead of plugging a mic into it, plugging in a guitar.

#### Nigel Goodwin

##### Super Moderator
joecool85 said:
I don't think you see what I'm saying, let me elaborate.

I have a small amp that I built that I'm using as a preamp. To do recordings right now I plug a mic into the small amp and the small amp into my laptop. I put the mic in front of my regular practice amp, and adjust levels accordingly. What I was getting at was using just the small amp and instead of plugging a mic into it, plugging in a guitar.

If you overdrive the amp it will clip, just as any guitar amp does - if you don't want that to happen, turn down the volume on your guitar!.

If you really want to build something (although there seems no particular justification?) it's called a "VU meter" - you can find plenty of designs on the net. Many use an LM3915, and you require a rectifier circuit to feed it.

#### joecool85

##### Member
I'm not worried about the amp overdriving, I'm worried about how much voltage is going into my laptop.

#### akg

##### New Member
joecool85 said:
I'm not worried about the amp overdriving, I'm worried about how much voltage is going into my laptop.
what's the powersupply for the preamp.?
take this as the max o/p voltage. and u know the max i/p for the LT
then use a simple resistive voltage divider.!!

#### Nigel Goodwin

##### Super Moderator
joecool85 said:
I'm not worried about the amp overdriving, I'm worried about how much voltage is going into my laptop.

And we've repeatedly told you -
IT'S NOT A PROBLEM! you've only got a puny little amp, and (as we've also repeatedly told you), if you are concerned for some reason add a simple resistive attenuator between the amp and laptop.

The circuit you posted already has a pot on the output, that's a resistive attenuator, simply set it at around a quarter or a half.

You really making a big thing about absolutely nothing! - but DON'T plug your 400W pA amp speaker output directly into your laptop, that could cause damage!.

#### joecool85

Sorry, its just that I want to be careful, my laptop cost me $1500 when I bought it new 2 years ago. Also, a bunch of guys at MacRumors.com told me that it was still a bad idea. I trust you guys way more though, since they seem to know NOTHING about electronics. Thanks for being patient with me. The power supply is 9volts on the one I'm using. Now, I know you already said that I shouldn't worry, but to be even safer, if I ran it off lets say, 4.5volts that would be the highest it could possibly get right? The lm386 chip can be supplied on as low as 4 volts, and I have another amp that I built running on 4.5volts and works fine (I don't have it anymore). Would that be better? #### Nigel Goodwin ##### Super Moderator Most Helpful Member joecool85 said: Sorry, its just that I want to be careful, my laptop cost me$1500 when I bought it new 2 years ago. Also, a bunch of guys at MacRumors.com told me that it was still a bad idea. I trust you guys way more though, since they seem to know NOTHING about electronics. Thanks for being patient with me. The power supply is 9volts on the one I'm using. Now, I know you already said that I shouldn't worry, but to be even safer, if I ran it off lets say, 4.5volts that would be the highest it could possibly get right? The lm386 chip can be supplied on as low as 4 volts, and I have another amp that I built running on 4.5volts and works fine (I don't have it anymore). Would that be better?

Just use an attenuator on the output, two similar values resistors will drop the signal by half, if you use 3/4 for the series resistor, and 1/4 for the resistor to ground, then it will drop the signal to 1/4 of the original.

So, use a 4.7K resistor in series, and a 1.2K resistor to ground, this will give you near enough 25% of the signal from the amplifier.

#### audioguru

##### Well-Known Member
Nigel,
Where did you learn math?
My slide-rule says about 20%. :lol: :lol:

#### Nigel Goodwin

##### Super Moderator
audioguru said:
Nigel,
Where did you learn math?
My slide-rule says about 20%. :lol: :lol:

Sorry :lol:

Momentary brain freeze - not that it makes any difference for this application!.

#### mrblingbling81

##### New Member
hi everyone i hope someone can help me im only a beginner. im a stereo fitter so i only know bits and peices of electrical components and i want to build an indicator for 12v and 24v the only catch is when i put 12v to it i want 1 led to light up and when i use 24v both lights light up and it also has to have as less components as posibble maybe even just resisters. haha im sorry i dont want much do I, but i hope someone can help it would very appreciated it has been racking my brain for over a month!

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