Visual Sound Level Meter

Have you ever wondered what sound really looks like? That’s exactly what inspired me to create this visual sound level meter. At first glance, it’s just plain cool- watching lights dance and pulse in response to different noises is almost hypnotic. But this project also has a real purpose. For individuals who are deaf or hard of hearing, it can act as a visual alert to their surroundings, turning invisible sounds into something they can perceive. And for music lovers or anyone who tends to turn up the volume a little too high, it provides a clear, visual cue to help protect your ears. This project is easy to build, looks good and has various useful functions. I hope you enjoy it :)

Note: I haven't been able to finish a final product version just yet but I have a working prototypes and PCBs ready to assemble. When I complete my finished version, I will update this Instructable.

I have attached a video of my working breadboard and working online simulation. Sorry for the terrible quality, I used a school chromebook to record.

LED Driver

1. LM3914/15/16

Audio Amplifier

1. TDA7266
2. Microphone

Visual Output

1. LED bar graph

For the LED driver, I chose the LM3915 over the LM3914 and LM3916 due to its sensitivity and logarithmic scale with increments of 3dB(so 1 LED bar graph covers 30dB) making it perfectly suitable for a sound level meter. However, any of the 3 would work completely fine for this project. You will need 1 IC per 10 LEDs/ 1 LED bar graph.

I also used the TDA7266 as my audio amplifier. This particular amplifier is designed to amplify the audio signal to a level high enough to power speakers, thus, it will be more than capable to power the LEDs. Alternatively, most general purpose Op-Amps like the LM385 or LM386 should suffice as well.

Finally, to provide the visual output I decided on the LED bar graph which is generally composed of 10 LEDs and has a compact size with an aesthetic appeal but you can also use a number of LEDs instead.

Below I have attached the datasheets as PDFs but I have simplified the information relevant to this project in the steps below.

The LM3915 is an analogue display driver that takes a input voltage to drive a 10 segment LED bar graph/ 10 LEDs using an internal logarithmic scale which increments by 3dB per step. This is ideal for sound applications as it matches how we perceive loudness.

Below is a list of the pins and their purpose:

Pin 1 and 10-18: These pins connect to the LEDs

Pin 2 and 3: These pins provide power to the IC and complete the circuit

Pin 4 (Ref Low): The bottom of the scale where first LED turns on

Pin 5 (Signal): This pin takes the Input Audio signal

Pin 6 (Ref High): The top of the scale where top LED turns on

Pin 7(Ref Out): Controls LED current changing brightness of the LED

Pin 8(Ref Adj): Sets the reference voltage and controls the sensitivity

Pin 9 (Mode): This controls the sequence of the LEDs, either in DOT mode(one LED turned on sequentially) or BAR mode(LEDs turn on consecutively)

I used the LM3915 datasheet to produce the cascading LM3915 circuit in Circuit Wizard. However, due to its complexity, I have attached an image of my simplified version of the circuit diagram that I created.

1. The potentiometer, connected to pin 5 (Signal pin) is acting as the analogue input signal as the software I used does not have a microphone or audio input available.
2. The potentiometer connected to pin 8 (Ref Adj) controls the sensitivity of the IC.
3. In the circuit diagram supplied in the datasheet (attached on step 2), I skipped the capacitor as it is only required if the wires between the LED and the IC is 6" or more.
4. You can use any voltage between 3V and 25V

I have attached an image of my breadboard that I used to test the circuit.

To ensure it works as expected, I also breadboarded the circuit to test it which is also attached above.

How it works:

1. The Ref High of the first LM3915 is connected to the Ref Low of the second. This means the second LED bar graph will turn on once the first has turned on sequentially.

With a working output, you can then begin diverting your attention to the audio signal amplifier circuit. I used the TDA7266 amplifier which is a powerful amplifier used to drive stereo speakers. Above I have attached the pin and circuit diagram. If you're only using the amplifier to provide the signal to the LM3915, you can use only one side of the amplifier. Alternatively, you can combine the sound level meter with two stereo speakers. I only used it for driving the LM3915, so I only used the top half of the amplifier. These are the pins I used.

1. Pin 1(OUT1+): Output pin for the red (positive) wire of the speaker. This is the one we will join to the input pin of the LM3915
2. Pin 2(OUT1-): Output pin for the black (negative) wire of the speaker. This is not required for the LM3915.
3. Pin 3 and 13(VCC): Power supply (plus volts) connected with two capacitors
4. Pin 4(IN1): Input signal from microphone with 0.22 microfarads capacitor(or a value close enough)
5. Pin 7(ST-BY): This controls whether the amplifier is active or in Standby by using a voltage threshold
6. Pin 9(S-GND): Power supply (0V/ ground)

Circuit wizard didn't have this component available either so instead I breadboarded it directly and have attached an image of my working breadboard as well as the circuit diagram provided in the datasheet.

To combine the microphone with the TDA7266, the microphone requires its own circuit. This consists of a 1.5K resistor connected from the ground voltage to the ground pin of the microphone (shown by the metal connecting the pin to the outer casing of the microphone). The positive pin connects to pin 4 of the TDA7266.

Use a wire from pin 1(OUT1+) of the TDA7266, to pin 5(Signal) of the LM3915. However, make sure you incorporate a 100K resistor from the pin 5(Signal) of the LM3915 to ground, otherwise the audio will have no affect on the LEDs. This is because the resistor provides a reference of 0V for the LM3915 when there is no sound so that it can differentiate the voltage changes when the microphone detects sound.

I haven't had a chance to produce the PCBs but I will produce my own using an acid-etch tank. I used Circuit Wizard to produce my PCB and can't convert it into a Gerber file for JLCPCB etc so I have just attached an image of the entire circuitry. For a better result, I recommend creating separate PCBs, one for the LED bar graphs and one for the LM3915 and TDA7266 and Mic and then stacking them using pin headers/sockets or joining them using wires. This will let you stack them up so you don't have large spaces on the case.

Once you have got your PCB, you can(optional but recommended, especially if producing your own PCB) use a multimeter to carry out a continuity test to test whether the PCB tracks are intact(How to Test Continuity with a Multimeter). Then solder your components and test whether your PCB works.

You will end up with an amazing device allowing you to visualise sound!