1. Your description is incorrect, but it's not all wrong. Don't use absolute ideas to understand analog circuits. 90 12, PNP transistor, when the voltage difference between the emitter and base of 90 12 exceeds +0.6V (pay attention to the direction of voltage and current), 90 12 starts to conduct, please note that it only starts to conduct, depending on the current. The current between the emitter and the base determines the current between the emitter and the collector, which is called the amplification gain of the transistor and has a linear relationship. That is to say, the current between EB and EC is a fixed multiple relationship: Av, 90 12 should be about 80 to 100 times, and each manufacturer will have process manufacturing errors in each batch. I have used 90 12 many times at work, and it is also a buzzer. )

2. When P0212 is at a low level, 90 12 is turned on, and the buzzer starts to sound, and the loudness is determined by R 14. The greater the R 14, the smaller the EB current, the smaller the EC current and the lower the buzzer sound. On the contrary, the smaller R 14, the greater EB current, the greater EC current and the higher buzz.

When P02 12 is at high level or S6 is off, because the voltage on R 13 is 3.3V on the transmitter, no voltage drop or current will be generated, and 90 12 will be turned off and the buzzer will not sound. The meaning of R 13: Pull up a resistor to a high level, which we call a pull-up resistor. The purpose of this is to make it easy to control, accurate and reliable, and will not embarrass 90 12 (you will understand this later).

3: PWM pulse width modulation, which should refer to square wave here.

DAC: Converts digital signals into analog signals.

Then PWMDAC means to give a digital pulse and convert it into an analog signal (voltage value); Or give an analog signal and convert it into a numerical value.

Then the switch line 4 means that TP2 has a square wave, which is filtered by simple RC low-pass to TP 1. If I guess correctly, TP 1 should be connected to the DAC port of MCU or an external independent DAC module to convert digital signals into analog signals. Of course, it may also be the number of module revolutions, but because the cut-off frequencies of R 15 and C6 are very high, it is judged that RC only wants to filter out the burr on the square wave, instead of filtering the whole square wave into a straight line and turning it into a stable voltage. So it should be digital-to-analog conversion.

Little brother: By the way, in this picture, the emitter above 90 12 is referred to as the E pole, the middle left is referred to as the B pole, and the following is referred to as the C pole.