What is on it
Six parts and no chip. A Huaneng MLT-8530 magnetic buzzer, an S8050 transistor that switches it, a 1 kΩ from SIGNAL to the transistor, an SS14 diode across the buzzer, and a red LED with its own 1 kΩ that lights while the transistor conducts.
Six parts
The square black part is the buzzer, a Huaneng MLT-8530. It is 8.5 mm square and 3 mm tall, held down by a pad under each cut corner. Two of those pads are its leads; the other two are dummies the maker adds so it sits flat.
Everything else drives it. The transistor at the lower right switches the buzzer's current; the resistor below it sets how hard your pin drives the transistor; the diode at the top right protects the transistor each time it switches off; the LED and its resistor under the buzzer light while the transistor is on.
A coil, not a crystal
Inside the buzzer is a coil and a thin steel disc with a magnet behind it. Current in the coil pulls the disc in; no current lets it spring back. Pull and release it a few thousand times a second and it sounds.
That makes it a magnetic buzzer. The part number, the maker's specification (it is titled "Electro-Magnetic Buzzer") and the 16 Ω coil all say so. A piezo buzzer is a ceramic disc that bends with voltage and takes almost no current. This one takes up to 95 mA, which is more than a microcontroller pin should give, and that is why the board has a transistor.
The LED tells you about the transistor
The LED is not a power light. It hangs from VCC to the transistor, beside the buzzer, so it lights only while the transistor conducts. During a tone the transistor is on for half of every cycle, thousands of times a second, and the LED glows at about half brightness. If it glows fully and steadily, SIGNAL is stuck HIGH.
The LED is red on the boards as shipped. The parts list names a green one of the same size; the fitted one is red.
Nothing else
No oscillator, no chip, no regulator, and no resistor from the transistor's base to GND. What the buzzer does is exactly what your pin does, a transistor stage later.
When it does not work
No. It is an electromagnetic buzzer: a coil of about 16 Ω and a thin steel disc, in an 8.5 mm square case. A piezo takes almost no current; this coil takes up to 95 mA while it plays, which is why there is a transistor on the board. Older pages for this block called it a piezo disc, and that was wrong.
The LED sits on the transistor, so it shows the transistor is on. A steady LED with no sound means SIGNAL is being held HIGH, which a passive buzzer cannot turn into sound. Send it a wave with tone() instead, and make sure the sketch leaves SIGNAL LOW when it is quiet.
The + printed on its top, at the lower right, is the pad joined to VCC; the diagonally opposite pad goes to the transistor. The other two corner pads are dummies that only hold it down. It is soldered at the factory; there is nothing to turn.
From a small port on the side of the buzzer's case, not the round mark on top. Leave the side clear: pressed against a wall or covered with tape, it is noticeably quieter.
Why HIGH is silent on this buzzer, and what an active one does instead.
Passive means you make the wave →Edit this page — content/books/passive-buzzer/what-is-on-it.mdx
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