Sound Level Meter

This is a sound level meter circuit consisting of an electret condenser Mic, pre-amplifier circuit using NE5534/TL071 and LM3915 bar/dot display driver chip.

According to the sound pressure captured by Mic, this device shows sound levels with LEDs.

As the LM3915 requires predefined input voltage levels for correct operation, a pre-amplifier circuit is utilized for amplifying the weak Mic input signal.

For dynamically displaying sound pressure levels with 10 LEDs, the LM3915 bar/dot display driver chip should be configured to support a certain input voltage range such as 60mV ~ 1.25V.

I bought about 10EA of these chips quite a long time ago (Maybe more than 10 years ago!).

Supposedly I had planned to make a battery capacity level meter or audio sound level meter with these chips.

But I forgot the chips are available in my inventory and they are gathering dust for quite a long time.

Now let's make some useful gadgets with this old but good analog IC chip!

This sound level meter (SLM from hereinafter) combines the pre-amplifier and LM3915 circuits I found on the Internet.

The pre-amplifier circuit is posted on the following webpage.

https://www.homemade-circuits.com/op-amp-preamplifier-circuits-for-mics-guitars-pick-ups-as-buffer/

This pre-amplifier circuit can produce RMS 1V output which becomes an adequate signal level to drive the LM3915 display IC.

I had tried many other preamplifier circuits made with diverse transistors and op-amps (NE5532, LM358, TL062/TL082, and so on).

But only the above circuit can produce an adequate signal level for driving LM3915.

Generally, you need to be familiar with the following formulas for properly utilizing the LM3915 IC chip.

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V(REF) = 1.25V*(1+R2/R1)+R2*80uA

I(LED) = 12.5V/R1+V(REF)/2.2Kohm

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As I don’t like mathematics, the above formulas become an obstacle to the best utilization of the LM3915 IC chip before. (That’s why these chips are still in my inventory!)

Anyway, the picture shown in the middle above describes threshold voltage levels that are driving 10 LED output pins of LM3915 IC.

The LM3915 circuit part in the schematics above is extracted from the application note of this chip.

In the chip manufacturer’s applicant note, the above circuit is introduced as a vibration sensor device that displays vibration strength with 10 LEDs.

When noise (or vibration) is applied to a sensor, each LED is turned on from a certain signal voltage level. (threshold voltage)

With the SLM device, the audio signal is input to the SIG pin (5) of LM3915 in the following manner.

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LED1 turned on when input voltage is higher than (>) 60mV, LED2 > 80mV … LED9 > 890mV, LED10 > 1.25V

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As the selected pre-amplifier circuit’s output voltage levels are exactly matched with the above threshold voltage levels of the LM3915 circuit, both circuits can be integrated for making SLM.

When locating the electrets Mic right in front of the speaker, the pre-amplifier output voltages are changed from a minimum of 30mV to a maximum of 1.75V.

If no sound is heard from the speaker, output voltage levels become 30 ~ 50mV range. (No LED turned on)

When a very loud hip-hop song is played, the pre-amplifier output voltage becomes 1.00 ~ 2V. (Turn on all LEDs)

For dynamic visual effect, bar type LED display pattern is used.

The electric condenser Mic is the most important part among all others.

When the quality of Mic is not good, any performing pre-amplifier doesn't help to produce an adequate signal to drive LM3915.

I searched several Mic parts on the Internet and found the followings are available.

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(products/specification): FQ-003, FQ-057, CMP-762, KEIG-4537, CMA-4544PF, POM-2244P

Directional: ----------------  Omni, --- Omni, -- Omni, ------ Uni, ----------- Omni, -------- Omni

Sensitivity (dB): ---------- 48~58, -- - 60dB, - 42dB, ---- -37dB, ------- 46~42dB, ----- 44dB

Output interface: --------- 2Kohm, --- 2.2K, -- 2.2K, ------ 1.8K, --------- 2.2K, --------- 2.2K

Operating voltage: ------ ~10V, ----- ~10V, - ~4.5V, ------- ~9V, ------- ~10V, ---------- ~10V

Signal-to-noise ratio: -- ~58dB, --- ~40dB, ~60dB, ----- ~69dB, ---- ~60dB, --------- ~60dB

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Besides the Mic products listed above, I have an additional part that was salvaged from the condenser Mic supposedly used for Karaoke purposes.

The CMA-4544PF is the one integrated into the Adafruit Mic-Amp break-out board.

I have two Adafruit MAX4466 Mic-Amp boards in my inventory.

Anyway, I bought the Mics listed above from the online part seller shop for the SLM-making project.

Still, I'm testing the performance of each Mic model with the pre-amplifier module.

The output voltage is measured by oscilloscope for checking enough voltage (60mV ~ 1.25V) can be attained.

I'll show the testing result in the other step below.

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The pre-amplifier requires the following parts.

- NE5534, TL071 op-amp IC

- 1/4W 47K, 100K, 10k, 27K, 22K resistors

- Potentiometer 10K

- Electrolytic capacitors 2.2uF, 4.7uF, 10uF 50V

- Film capacitors 100nF 50V

- Pin-header, universal PCB, cables

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10 LED driving circuit board requires the following parts.

- LM3915 dot/Bar display driver

- 1/4W 1M, 1K resistor

- 2.2uF electrolytic capacitor

- 10 LEDs (Green x 4, Yellow x 4, Red x 2)

Let's look at more details about how these parts are wired, soldered, and connected.

When all necessary parts are prepared, SLM making can be started such as positioning the parts on the PCB, wiring, and soldering of parts as shown in the picture above.

For working on a pre-amplifier circuit, a potentiometer type of 10K VR (Variable Resistor) is preferred over a surface mounting VR type.

According to the input signal strength of Mic, the amplification rate of the pre-amplifier should be adjusted by the 10K VR.

This means the 10K VR value should be adjusted for properly activating 10 LEDs.

If you using a potentiometer type VR, (which has a knob) changing the pre-amplifier setting is easier and more convenient.

When an audio source volume level is high (such as a very noisy hip-pop music video is playing), increasing the 10K VR resistor value (such as 5K ~ 7K) can support the more dynamic visual effect of 10 LEDs.

A reversely low sound level of contents (such as documentary and news broadcasting) requires decreasing 10K VR resistance value (such as 1K~5K range) enabling more dynamic LED blinking.

The LM3915 chip and other parts can be wired and soldered as shown in the diagram above for making a display board with 10 LEDs.

For supporting personalized visual effects with 10 LEDs, you can choose a bar or dot-type display pattern.

As I prefer a bar-type display, Pin 9 of LM3915 (MODE) is wiring to the Vcc (12V).

When every part is soldered and wired together, the sound level meter is ready to operate.

Let's look at more details about how each part (electrets Mic, pre-amplifier, and LED display) works together to show sound pressure level from a speaker.

To correctly capture the audio signal, the electrets Mic should be located right in front of the speaker.

When the audio signal is directly wired to LM3915 from a PC or power amplifier, unnecessary noises can be infiltrating the input stage of the audio system.

Therefore, I prefer audio signal (actually its sound pressure from a speaker) should be captured without tempering or embedding to any audio circuits.

As Mic is completely isolated from any other audio signal processing circuits, this sound level meter does not interfere with any audio signals between PC and power amplifier.

But capturing adequate sound pressure level becomes an important matter to make a dynamic visual effect by LM3915.

So an electrets Mic should be located as near as to the speaker.

The followings are testing results with diverse Mic models.

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- KEIG-4537: This is Mic made by a Chinese company. It doesn't perform well and only produces 10mV ~ 70mV signal levels regardless of any volume levels

- FQ-057: As shown in the oscilloscope screen shoot above, this Mic performs well by producing 40mV ~ 1.3V output voltage (from the pre-amplifier circuit)

- POM-2244P: This is a very small profile of Mic and also does not perform well even at the nearest distance from the speaker

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When decreasing the operation voltage of SLM from 12V to 9V, the KEIG-4537 Mic (specification of this Mic rate 9V as maximum operation voltage) also produces similar output to FQ-057.

When a Mic requires a certain operational voltage level such as 1V~10V, applying overvoltage (> 10V) can degrade the performance of the Mic.

Also, each Mic produces specific LED blinking patterns as the sensitivity of Mics is different.

While using the FQ-057 Mic, generally 5~7 LEDs are blinking very dynamically at the same time with relatively a little bit high pitch of sound level.

But half of 5~7 LEDs are turned on completely and another half number of LEDs are dimly turned on.

When using KEIG-4537 Mic, only 3~5 LEDs are completely turned on without any dimly bright LEDs at a similar sound level.

Maybe the physical structure of the internal sound capturing mechanism of FQ-057 is thinner than KEIG-4537 Mic to produce a highly reactive output signal voltage level.

For the actual sound level meter operation, you can click the following link for viewing the video.

https://drive.google.com/file/d/16fYAFcWMJL6mZApDmcsdOUEkID-zU8Uj/view?usp=sharing

According to the power amplifier volume level, ten LEDs are blinking dynamically.

As Mic is closely located right in front of the speaker, it only captures air pressure changes from the speaker.

This is the story of the prototype Sound Level Meter circuit.

I'll show some more variants and further developments from this prototype SLM in other Instructables later.....

Thank you for reading this story.