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Hello everyone, in this lesson we will learn the circuit application of triode

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Through the study of the previous lessons, we have learned the function of the triode in the circuit,

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the conduction condition of the triode, and the identification of the type of the triode

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Next, let's take a look at the application of the triode in the actual circuit

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For example, the following circuit diagram

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The ultimate goal is to make this LED light light up

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First we need to analyze the circuit

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Under what circumstances can the small LED light be turned on?

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The small LED lights can light up after forming a loop

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The negative pole of the LED small light is connected to a triode Q38

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When Q38 is cut off, the small LED light does not form a loop, so it will not light up

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When Q38 is turned on, the small LED light forms a loop and can be turned on

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So we get the first condition, Q38 to be turned on

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So under what circumstances will Q38 be turned on?

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We need to first look at what kind of triode Q38 is

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We first add a bar in front of the small arrow in the circuit symbol

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At this time, the symbol between BE becomes a diode

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The B pole is connected to the positive pole of the diode,

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and the E pole is connected to the negative pole of the diode.

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Positive is P, negative is N

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Then Q38 is an NPN transistor

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The NPN transistor is turned on at a high level, so the B pole of Q38 should be at a high level

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So how can the B pole of Q38 become high level?

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Let's move on to the circuit

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The B pole of Q38 is connected to Q36

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When Q36 is turned on, it will connect 3 volts to the B pole of Q38, which is high level

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After Q36 is cut off, the 3 volt voltage is disconnected,

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and the B pole of Q38 has no pull-up voltage and becomes a low level

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Therefore, the B pole of Q38 should be high level, and Q36 should be turned on.

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So under what circumstances can Q36 be turned on?

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It's still the same, let's first identify what kind of triode Q36 is

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Add a bar before the small arrow, and the circuit symbol of a diode becomes between BE

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The B pole is connected to the negative pole of the diode, which is N

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The E pole is connected to the positive pole of the diode, which is P

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Then Q36 is a PNP transistor

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PNP type triode is turned on at low level and cut off at high level

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If Q36 is to be turned on, its B pole should be low

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Under what circumstances can the B pole of Q36 become low level?

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We continue to analyze

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It can be seen that it is connected to a pull-up voltage and a signal

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If this signal is not grounded, then the B pole of Q36 is directly pulled up by 3.3 volts

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The voltage of the B pole is 3.3 volts

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B pole voltage 3.3 volts, E pole voltage 3.3 volts

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B is equal to E, Q36 ends

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When this signal is grounded, R606 and R607 form a series voltage divider

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Two 10K resistors are connected in series to divide the voltage of 3.3 volts,

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and the divided voltage is about 1.65 volts

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Then the voltage at the B pole becomes 1.65 volts

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E pole is 3.3 volts, B pole is 1.65 volts

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B pole is smaller than E pole by more than 0.7 volts, then Q36 can be turned on

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So we come to the conclusion that if the small LED lights want to light up, the previous signal must be grounded.