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Now, this circuit works like this:

Turning the circuit on, R drives the base of the transistor Q
Q turns on and drives current into the primary of the transformer. The current is limited by the limited available base current.
The created magnetic field drives the secondary of the transformer.
The secondary voltage wants to grows large. But the tiny stray capacitance on the output resists the change, although very small, against the rise of the output end and so in return the voltage on the other end of the transformer goes down, pulling the base of the transistor low.
Diode D prevents the base voltage to fall more than 0.7V below ground, which in return pushes the output end of the secondary high.
The transistor turns off and so the magnetic field starts to reduce.
The base voltage rises again and Q turns on and the cycle repeats.
不一定是发光二极管。发光二极管只是为了验证有没有起振