IoT Classroom Noise Level Monitor with LED Alert Board and Logged Readings

The IoT Classroom Noise Level Monitor with LED Alert Board and Logged Readings continuously measures classroom sound level, flashes a visible LED warning when the room stays too loud, and logs every reading to a web dashboard. Teachers currently manage noise by voice and feel — there is no shared, objective reference, so 'too loud' means something different to every teacher and every class. The build answers this with an ESP32 reading a MAX4466 electret microphone module, an 8×8 LED matrix alert board the whole class can see, and MQTT logging to a dashboard with a live gauge, a 60-second chart and a timestamped alert log. One builder insight matters for every buyer: the sensor's raw ADC output is linear voltage but decibels are logarithmic, so the firmware maps readings on a log scale and calibrates once against a phone SPL meter at 1 m — without that step the board shows relative loudness bands, not real dB.

The full chain works like this: the MAX4466 electret microphone module turns classroom sound pressure into an analog voltage, the ESP32 samples it on its 12-bit ADC at about 2 Hz and converts each reading to decibels on a log scale — doubling the sound pressure adds 6 dB, so the firmware works in log space, not raw counts. One calibration against a phone SPL meter at 1 m anchors the scale to real dB. The level is checked against a 70 dB design-target threshold with a 3-second persistence filter: a dropped book never fires an alert, sustained chatter does. On alert, the 8×8 LED matrix scrolls 'TOO LOUD' in the red zone and the dashboard logs a timestamped entry. Every 5 seconds the ESP32 publishes the reading over Wi-Fi via MQTT, and the dashboard redraws its live gauge, 60-second chart and alert log. The LED board and threshold logic run fully offline; only cloud logging pauses without Wi-Fi.

1. The MAX4466 electret module converts sound pressure into an analog voltage, with gain adjustable 25×–125× via its onboard potentiometer (datasheet values).
2. The ESP32 samples the module output on its 12-bit ADC (0–4095, datasheet) at about 2 Hz and keeps a rolling 2-second peak window.
3. Firmware maps the window peak to decibels on a log scale, anchored by a one-time buyer calibration against a phone SPL meter at 1 m.
4. The dB value is compared against the 70 dB threshold (design target); the alert fires only after 3 sustained seconds above it (design target).
5. On alert, the 8×8 matrix scrolls 'TOO LOUD' in the red zone, the tri-color strip lights red, and the dashboard records a timestamped entry.
6. Every 5 seconds (design target) the ESP32 publishes the reading over 2.4 GHz Wi-Fi via MQTT; the dashboard redraws its gauge, chart and log.

Tech stack:

• ESP32 (dual-core 240 MHz · 12-bit ADC)
• MAX4466 electret microphone amplifier module
• 8×8 LED matrix (MAX7219) · tri-color zone LEDs
• Arduino IDE (C++ firmware)
• MQTT (cloud logging)
• HTML · CSS · JavaScript (web dashboard)
• Solderless breadboard prototyping
• USB 5 V power

The four capabilities that carry this build are continuous dB measurement, the LED alert board, the persistence-filtered alert logic, and the logged dashboard. Continuous measurement comes from a MAX4466 electret microphone module feeding the ESP32's 12-bit ADC, converted to decibels on a log scale after a calibration against a phone SPL meter at 1 m. The alert board is an 8×8 LED matrix plus a tri-color zone strip: green below 55 dB, amber from 55 to the threshold, red with a scrolling 'TOO LOUD' message above it — the whole class sees the state with no teacher input. Alert logic refuses to fire on transients: the level must stay above the 70 dB design-target threshold for 3 sustained seconds, so a dropped book or a single clap never triggers it while sustained chatter does. Every reading is published over MQTT every 5 seconds to the dashboard, which keeps a live gauge, a 60-second chart and a timestamped alert log.

• [Live dB measurement] A MAX4466 electret microphone module feeds the ESP32's 12-bit ADC; the firmware converts the signal to decibels on a log scale, anchored by a one-time calibration against a phone SPL meter at 1 m.
• [Classroom-visible LED alert board] An 8×8 LED matrix plus tri-color zone strip for the front of the room: green below 55 dB, amber from 55 dB to the threshold, red with a scrolling "TOO LOUD" message above it (bands are design targets).
• [Persistence-filtered alerting] The alert fires only after the level stays above the 70 dB threshold (design target) for 3 sustained seconds, so dropped books and single claps are ignored while real sustained noise triggers it.
• [MQTT cloud logging] The ESP32 publishes each reading over Wi-Fi via MQTT every 5 seconds (design target) to a configurable broker, so the dashboard updates live and the day's readings are logged.
• [Web dashboard] A single-file dashboard shows a live dB gauge, an adjustable threshold slider, a 60-second rolling chart with the threshold line, an LED-board mirror, and a timestamped alert log.
• [Adjustable threshold and persistence] The alert threshold, the 55 dB zone boundary and the 3-second persistence time are firmware constants and dashboard settings, tunable per classroom with no hardware changes.
• [Buzzer + phone alerts (optional)] A buzzer on the alert board and SMS/WhatsApp escalation can be added as an optional extra; they are not part of the base build.

• Assembled and tested prototype (ESP32 + MAX4466 module + LED alert board on breadboard)
• Complete firmware source (Arduino IDE, C++)
• Wiring diagram and component list
• Single-point dB calibration procedure (phone SPL meter method)
• Web dashboard file (live gauge, chart, alert log)
• Project report PDF (background, circuit design, dB-mapping method, calibration, methodology)
• PPT presentation for final review
• Viva Q&A preparation document (ADC sampling, log-scale mapping, MQTT, threshold logic)