Audio Jack Wiring

The beacon’s audio comes out of a 3.5 mm jack so you can listen on any wired headphone. The circuit is tiny but has one trap.

The signal path

GPIO 25 (DAC1) → 100 Ω → 10 µF → jack TIP
                                → jack SLEEVE (GND)

• GPIO 25 is the ESP32’s native 8-bit DAC channel 1 (0-3.3 V).
• The 100 Ω resistor limits current.
• The 10 µF capacitor AC-couples the signal: it blocks the DC bias so the headphone sees only the audio waveform. Without it, headphones click and distort.

The jack

Use a 3.5 mm TRRS jack (PJ-320A or similar):

• Tip: audio (from the capacitor).
• Ring 1: tie to Tip for mono.
• Ring 2 (MIC): not connected.
• Sleeve: ground.

A standard TRS (3-pole) jack also works: Tip = audio, Ring = NC, Sleeve = GND.

Mid-rail parking

The firmware calls dacWrite(128) at silence. This parks the output at mid-rail (about 1.65 V) so the DAC doesn’t click when audio starts. The AC-coupling capacitor blocks that DC anyway.

Wiring mistakes

• Missing capacitor: loud clicks at the start of every tone.
• Tip and sleeve swapped: no sound (or hum).
• Using a 5 V audio source: never connect anything to the jack except headphones.
• Shorting Tip to GND: the 100 Ω protects the DAC, but check the wiring before powering.

Testing

• Play a SEARCH scan: you should hear the pitch rise with signal.
• Play a BEACON cycle: you should hear Morse clicks synchronized with the RF keying.
• Volume 0: near-silence (not absolute silence; the DAC floor is audible with sensitive earphones).

Headphones

Any wired headphone, 16-600 Ω, works. Bluetooth headphones do not: the jack is analog.