A capsule and one transistor
An electret capsule, eight small parts and a four-pin header. One S8050 transistor does all the amplifying: there is no op-amp, no comparator and no LED on this board, and the output is a swinging voltage, not an on-off signal or a loudness level.
Five jobs
Pick a job and its parts are ringed on the board and coloured in the circuit. The circuit is the net list's, and the positions come from the placement file the board was built from.
The capsule is an electret microphone: a membrane that carries a fixed electric charge, a metal plate behind it, and a tiny transistor inside the can that turns the membrane's movement into a changing current. That inner transistor is why a microphone this small needs a supply at all.
Powering it is three parts. A 10 kΩ from VCC supplies the capsule, and a 51 kΩ and a 100 nF run from its output to GND.
Passing it on is one 100 nF capacitor, which carries the changes in the capsule's voltage to the amplifier and blocks the steady part.
Amplifying it is an S8050, a small NPN transistor, with a 510 kΩ from VCC to its base. Its emitter goes straight to GND, with no resistor. This arrangement is called common emitter, and it gives a lot of gain from one part.
The output is a 1 kΩ from VCC to the transistor's collector, and a 100 nF from there to GND. The collector is SIGNAL.
What is not here
The page this book replaces said the board was a capsule and an op-amp. It is not: there is no chip on it at all, only one three-legged transistor. It also said the output was an envelope, a level that follows loudness. There is no envelope detector either. SIGNAL is the sound itself, amplified and filtered, swinging above and below a resting level.
There is no comparator and no LED. Modules that have them give an on-off output with a trimmer to set the level; this one leaves that decision to your sketch.
Reading the transistor the right way up
A transistor is a valve for current. A small current into the base lets a much larger one flow from collector to emitter. The S8050's datasheet grades how much larger, hFE, from 120 to 400 at its test current.
Here the collector current flows down through the 1 kΩ. The more current, the more voltage the 1 kΩ drops, and the lower SIGNAL sits. So the output is upside down: when the sound pushes the base current up, SIGNAL goes down. For a microphone that does not matter; the swing is the same size either way. The resting level is what it means for the voltage you read in silence.
When it does not work
No. The net list for this board has one transistor, an S8050 in a SOT-23 package, and no integrated circuit at all. The old page described a different kind of microphone module. Everything in this book is read off this board's own net list and parts list.
No. Some microphone modules carry a trimmer and a comparator that give an on-off output. This one has neither: SIGNAL is analog only, and your sketch sets the threshold, which is more flexible and is what the clap switch does.
Its output lead goes to the square pad and its ground lead to the round one; on most capsules the ground lead is the one joined to the metal can. This capsule's own datasheet could not be found, so check the replacement's markings before soldering.
How the capsule gets its supply, and the capacitor that passes the sound on and nothing else.
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