Another Sound Level Meter Circuits



Introduction: Another Sound Level Meter Circuits

This is the additional project for the Sound Level Meter (From hereafter as SLM) circuit making.

I introduced the SLM prototype in the previous instructable as below.

While working on the SLM prototype, I thought several improvements can be applied to make better SLM devices.

Therefore, additional two SLM variants are made by applying the following improvements from the prototype.

- Miniaturing the prototype to reduce the footprint of SLM to locate it anyplace near to the speaker

- For better visual effect, changing LED types (Thick square type or highly bright small LEDs)

- Dividing MIC/pre-amplifier from LM3915 display part to locate electrets MIC conveniently very near to the speaker

I didn't change any circuit components (MIC, pre-amplifier, LM3915 display part) included in the prototype.

These 2nd versions of SLMs are only changing shape, configuration, and assembling methods from the prototype.

As a result, the new SLM will be more convenient to use and fun to look at their operation along with changing audio sounds.

Let's look at more details about how these new SLMs are made.

Step 1: Schematics

Among the two SLM variants, the miniature version's schematics are the exactly same as the prototype version.

But the other variant (I'll call this new device as MIC/Display modules version) uses the same schematics with divided circuit modules and their connection with the USB cable as shown in the picture above.

As the MIC should be located right in front of the speaker, the main SLM circuits (pre-amplifier and LM3915 display module) are also located near the speaker.

When connecting wire between MIC and pre-amplifier becomes longer, the input signal can be weaker, and a long connection wire can receive RF signal as interfering noise.

Also, suitable space should be reserved near the speaker for locating SLM main body.

To remove MIC and pre-amplifier wiring dependency, overall SLM circuits are divided into modules as follows.


- MIC/pre-amplifier circuits are integrated as a single module

- 9V power is supplied via (+), (-) wires of USB cable from display module which including the LM7809 voltage regulator

- Captured weak audio signal by MIC is multiplied by pre-amplifier into several hundred mV to 1~2V and transfer to LM3915 display module via (Data +), (Data -) wires inside the USB cable


By dividing overall SLM circuits into two modules, you can easily locate MIC near to speaker and place the display module in any location you prefer.

Above is the main reason why I'm making the MIC/Display modules version.

For the miniature version, any other explanations will not be necessary as small size is more desirable, and diverse visual effects are more fun to look at.

Step 2: Parts

As new SLMs are using the same schematics as the prototype, the same parts are used for the additional two SLM variants. 

Only the new LEDs shown above need some explanations.

For supporting fun to see a visual effect, the following new LEDs are used.


- I found thick square type Red/Green/Yellow LEDs (width 5mm x length 5mm x height 7mm) on the Internet shop

- I had a high luminosity of small transparent (3mm) Blue/Green/Yellow(Orange)/Red LEDs in my inventory

- For easily exchange LEDs, IC pin type pinheads are used for displaying modules as shown in the picture above


Newly bought thick square type LEDs are supporting more bright and dynamic blinking patterns along with changing sound levels.

Also, small transparent LEDs are showing highly bright visual cues with more diverse colors.

I'm still searching the LED product catalog of the Internet part shop to find more interesting LED parts.

When LEDs are replaced, it seems drastically change the look and feels of SLM operational scheme.

Step 3: Wiring and Soldering

As a separated MIC/pre-amplifier module is made, the original wiring diagram should be modified for making a smaller circuit footprint on PCB.

Also smaller and general-purpose capacitors are used for this project.

For making audio circuits, I had a kind of stereotype such as I should use highly valued parts (Usually they claim their parts as a high-quality audio grade).

But for this project, I bought very general and cheap electrolytic capacitors that were made by not the well-known manufacturer.

Usually, these cheap capacitors are smaller than the so-called audiophile-grade capacitors.

While I'm soldering these parts on PCB, I wonder what kind of results will come out later!

I'll explain the operational result with these cheap capacitors in the later step below.

Also, I'm wondering what kinds of side effects will come out with using the long USB cable between divided modules.

As 9V power supply and audio signals are crossing at the same time with 4 wires inside a single USB cable, any interferences (such as cross-talk) can be happening?

Anyway, the above two concerns are my personal interesting points for this project with new SLM circuits.

Step 4: SLM Operation

Both SLMs are well working same as the prototype I introduced before.

The electrets MIC’s performance of the MIC/Display modules version seems relatively weaker than the miniature version.

Anyway, the MIC/Display modules operation capturing video can be seen in the link below.

As I mentioned above, I worried cheap electrolytic capacitors can degrade the overall performance of the SLM circuit.

Also, a long USB cable can make unexpected side effects such as audio signal cross-talk or high noise because 2 wires of USB are carrying 9V power.

But all the above problems are not happening and as a result the performance of the MIC/Display modules version SLM is quite OK.

When audio quality is an important matter (like audio amplifier), maybe the cheap capacitors are surely degrading the performance of the circuit.

But as audio signal strength and dynamic signal changes are the more important matters for SLM circuits, using cheap capacitors seems quite OK.  

To show another visual effect, I changed thick square type LEDs to 3mm high bright transparent LEDs.

You can see the SLM operation capturing video in the link below.

As the LEDs are too much bright, actual colors (e.g. blue, green, yellow, red) can’t be seen in the video. (Smartphone camera seems too much sensitive to the brightness of LEDs)

But you can see the diverse colors with naked eyes.

So far is the story for the new SLM variants.

Thank you for reading.

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