Please help me, my boss asked me to analyze this circuit in the afternoon meeting.

pqa

Please help me analyze this circuit. I have a meeting this afternoon to prepare an analysis report, but I still don’t understand it very well. I am so anxious. Please help me. I will be grateful for your help. I will report back in the future. 1) Why do we need to use three transistors in series to drive the relay? How are the three transistors turned on? 2) The base of the first transistor Q18 is also connected in series with a coupling capacitor. What is the function of this capacitor? 3) I thought transistors Q18 and Q19 were switching transistors before, but later I checked the chip information and found that they all have internal bias resistors. This confuses me. I don’t know whether they are amplification or switching. 4) What are the functions of the two grounds? Why are there resistors and capacitors in the middle?

wenyangzeng

U3-48 will close if there is a falling edge pulse relay.

littleming1028

Analysis: (1-3) Resistors and capacitors form a bandpass filter amplifier (4) The circuit ground is coupled to the earth through resistors and capacitors to prevent interference

ienglgge

After Q18 is turned on, its collector becomes high, Q19 base becomes high, Q19 is turned on, its collector becomes low, Q17 base becomes low, turned on, and the relay is closed. A capacitor is connected in series with the base of Q18 to filter out the DC component. If the external pin voltage remains unchanged, Q18 will not be turned on.