Audio Guestbook

Overview

There is the option to use the project with either an ESP32‑A1S or a Teensy 40 equipped with an audio shield.

Dependencies

esp32a1s

• Arduino Audio Tools (a powerful Audio library not only for Arduino)

Warning: The onboard microphones on the ESP32‑A1S cannot be disabled via software. They must be physically disabled (e.g., with hot glue).

teensy

Audio Shield

• PJRC Audio Shield for Teensy 4.0
• Teensy Audio Library

Configuration file

You can use a local configuration file on SD Card "/config.json"

teensy uses wav files esp32a1s only users mp3 files

{
  "useSD": true,
  "deleteInitialRecordings": true,
  "onlyLoop": false,
  "recordDirPath": "/recordings",
  "audioFileName": "/message.mp3", // for teensy use .wav
  "playbackVolume": 0.8,
  "beepVolume": 0.8,
  "recordDuration": 12000, // ms
  "playBusyFirst": 5000, // ms
  "beepDuration": 500, // ms
  "ringDuration": 60000, // ms
  "ringAfterDuration": 3600000, // ms
  "countFilePath": "/call_counts.json",

  // only for teensy
  "recordWav": true, // save raw audio files with wav header

  // only for esp32a1s
  "deepSleepAfterDuration": 5000, // ms

  "wifiSsid": "MyNetwork",
  "wifiPassword": "MyPassword"
}

⚠️ Deep‑sleep support is experimental – enable only for testing

teensy

prepare wav files

Only 16 bit PCM, 44100 Hz WAV files are supported. When mono files are played, both output ports transmit a copy of the single sound. Of course, stereo WAV files play with the left channel on port 0 and the right channel on port 1.

ffmpeg -i input.ext -acodec pcm_s16le -ac 2 -channel_layout stereo -ar 44100 output.wav

The following two projects were used as templates:

• PlayfulTechnology – audio‑guestbook (audio‑guestbook.ino)
• coldkevin – audio‑guestbook

esp32a1s

prepare mp3 files

ffmpeg -i input.ext -ar 44100 -ac 2 -b:a 96k -c:a libmp3lame -compression_level 2 output.mp3

Wi‑Fi

If wifiSsid and wifiPassword are defined, the device starts its own Wi‑Fi Access Point to serve the files on sd card.

Rotary Dial

• If a number is dialed during the initial busy tone, the corresponding file is searched for in the /phone/ folder.
  Example: Number 1312 → /phone/1312.wav is played.
  If the file 1312.wav is not found, the default file is played.

Pulse signal for rotary dial:

• Yellow and green wires. A pulse signal occurs every 100 ms.
• When dialing, brown and white are connected. During idle the brown and white lines are open. Each pulse closes the brown‑white pair for ~100 ms, producing the pulse train

Handset lifted:

• Black/white and yellow have a connection.

Procedure and Useful Examples

Recording examples

• Arduino Audio Tools - streams-audiokit-sd-audiokit
• Arduino Audio Tools - streams-audiokit-sd_wav

Input mixer and effects

• Arduino Audio Tools - streams-generator_inputmixer-audiokit

Notes

The ESP32‑A1S module uses the ES8388 audio codec.
The two “LINEIN” pins (LINEINR and LINEINL) are the line‑in inputs of the codec and expect the typical analog audio line‑level signal—that is, not a microphone signal but a line‑level signal.

What does this mean in practice?

Feature	Typical Value / Expectation
Voltage level	≈ 0.5 V RMS (~ 1 V peak‑to‑peak); up to about 2 V p‑p is still acceptable for the ES8388.
Signal type	Single‑ended (single‑ended); the codec provides a small internal bias so the signal is centered around 0 V (ground).
Impedance	Input impedance ≈ 10 kΩ (typical for line‑in stages).
Coupling	AC‑coupled (capacitor at the input), so a DC offset from the source device is not an issue.
Frequency range	Approximately 20 Hz – 20 kHz (the audio bandwidth of the ES8388).

In short: the LINEIN port expects a standard audio line‑level signal (as you would get from a CD player, a mixer output, or another amplifier output). A microphone signal would be too weak; a very strong speaker‑level signal (≥ 5 V p‑p) could overload the input.

If you want to connect the module to an external audio source, make sure the output signal falls within the voltage range mentioned above and, if necessary, AC‑couple it with a capacitor (e.g., 10 µF) to block any DC components.