# Simple VU meter with Lm3915

I'm making a VU meter with an LM3915 and I would like to be able to plug my computers audio output into the LM3915 directly and get it to register. I've been searching the internet for a design but can seem to get any to work without a preamp. Anyone know of a schematic that would work without a pre amp?

Data Sheets

Russell mentioned probably more ways to solve the problem than you care about, but that's Russell: always gives extensive answers :-)

The Greinacher multiplier he mentioned at the end of his answer may work, but the diodes may mean that you already need a rather high input voltage, I don't know if it will work well on the line output. (The Greinacher is typically used to multiply power supply voltages, not signals. It can be used to get kilovolts from a 110V or 230V line voltage, for instance.)

Russell also mentioned this opamp amplifier:

This is one of the basic opamp circuits, and is easy to understand if you know some basic things about opamps:

• The ideal opamp has infinite amplification
• The output signal is the amplified difference between the inputs
• The ideal opamp has zero input current

The first two items imply that you'll get a finite output signal if both inputs are almost at the same level. And that's what an opamp in a negative feedback system will do: it will try to make the level on the - input (called the inverting input) the same as on the + input (the non-inverting input). So that will be $V_{IN}$, and the current through R1 is

$I_{R1} = \dfrac{V_{IN}}{R1}$

Since there doesn't flow current in the - input the current through R1 is the same as the current through R2, so the voltage over R2 is

$V_{R2} = I_{R1} \times R2$

and therefore

$V_{OUT} = V_{IN} + \dfrac{V_{IN}}{R1} \times R2$

or

$V_{OUT} = (1 + \dfrac{R2}{R1}) \times V_{IN}$

So, if we choose R1 = 1k$\Omega$ and for R2 a 100k$\Omega$ potentiometer, we can vary the amplification between 1x and 101x. This will be a good amplification for a line out signal, and you can set the amplification to the LM3915's full scale.

• My method 2 - using a transformer, probably best suits his overall aims. I think he may want an electronics-less solution so that he can use an existing LM3815 based product. The transformer would allow it to be "just hooked up". Using the cheap transformer that I noted at Jameco as an auto transformer gives a 4:1 stepup with minimal effort. Commented Aug 11, 2011 at 13:07
• @Russell - Yes, but with a transformer the amplification is fixed, and he might want to adjust it to the LM3915's full scale. That's why I mentioned the potentiometer. Commented Aug 11, 2011 at 13:13
• All that. But, the specific challenge was "without a preamp". I agree that an amplifier makes best sense to the electronically inclined. Maybe not so much so to others. // Some of mine are amplifiers. The transformer isn't per se. He could add a pot to a transformer and still have an essentially non-electronic solution. My 4:1 autotransformer is a cheap and known available option. There are of course many other transformers around. Commented Aug 11, 2011 at 14:09
• i have a bunch of lm358 op amps already, would they work?
– Mike
Commented Aug 11, 2011 at 19:06
• @Mike - Yes, an LM358 will do. With a $\pm$5V supply you'll have a limited output voltage swing (like $\pm$3V) but for the LM3915 this is sufficient. Commented Aug 12, 2011 at 5:54

The LM3815 needs a minimum of ~1.3V peak signal to fully drive all LEDs.

Methods:

• 1 Using PC "speaker" output rather than PC "line" output should provide enough signal to work directly

If not

• 2 $3 non electronic solution. A small low cost audio transformer will produce a higher voltage if the available voltage is too low to fully drive the LM3915. (eg this$2.95 Jameco transformer can give 4:1 voltage gain).

Transformers with higher stepup ratios will be available at similar prices.

A potentiometer can be added to a transformer if variable sensitivity is required.

• 3 If necessary an "amplifier" using only diodes and capacitors can be made - see text below.

This is possible because if audio signal level is too low then what is required is a higher voltage DC signal - which can be achieved with a "diode pump".

• 4 If amplified external speakers are used their output could be used to drive the LM3815.

• 5 very cheap and simple amplifier can be made with a single transistor or a single opamp stage. This can be operated from the LM3815 supply.

You should be able to operate an LM3915 VU display directly from a PC speaker connection. You need a ~ 1.3 V audio peak level to drive the IC to maximum output. This corresponds to about 100 mW into an 8 ohm speaker and proportionally less at higher speaker impedances.

You may not be able to operate it from a PC's "Line" audio output. Line output levels are typically about 0.5V peak which is "a bit low". Wikipedia - line levels. Some line outputs are at higher levels eg RCA Sonos system line output.

If your PC only has a line output port and not a speaker output you may be able to connect to the speaker connections of an external speaker system (which will contain its own amplifier). An "amplifier" using only diodes and capacitors can be made - see bwlow.

An LM3915 and other family members have a fixed measuring range of about 1.28 Volt. This is divided into 10 steps. In the LM3915 the steps are logarithmic with 3 dB between steps. Other family members have linear or other responses.

You cannot drive an LM3915 to less than about 1.28V DC and get full output. However, a DC amplifier which amplifies signals by the required amount - probably 2 to 10 times, is extremely simple and easy to build. This could be done with a single transistor amplifier, but using a cheap single supply opamp running from the same supply as the LM3915 would be preferable.

This circuit shows an extremely simple non inverting DC amplifier using half an LM358 (datasheet) or 1/4 of the electrically identical LM324. Gain = 1 + R2/R1. At the gain needed (< 10) the opamp's offset voltages are not a concern.

You can buy small audio transformers or salvage them from an older transistor radio. Or a 600 CT: 600 CT transformer can be used as a 4:1 stepup auto transformer. (Join "top" of one winding to bottom of next to make a single winding effectively. Connect Vin to half of one windiong. Take 4x Vout across whole winding.)(Swap polarity on one winding if phase wrong so no output). (eg this \$2.95 Jameco transformer can give 4:1 voltage gain).

The truly desparate could make a passive DC voltage multiplier using only diodes and capacitors to "amplify" the audio signal up by using a "diode pump" voltage multiplier. eg see Wikipedia on voltage multipliers. Diodes should be Shottky type due to low forward voltage (eg BAT85 and similar - DO34 through hole version. (I won't mention germanium diode except to say that they are now so rare as to not be worth considering, regardless of technical aspects.) Discuss if interested in more detail.