RXD outputs a high level, how can we make it output a low level to turn on Q2?

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RXD outputs a high level, how can we make it output a low level to turn on Q2? [Copy link]

 

Help, there is a problem with the debugging circuit.

maychang

Where does the RXD signal come from?

If the signal is a pin on the microcontroller, then instructions can be used to change the state of the pin.

chunyang

Just add an NPN transistor common emitter circuit to form an inverter. However, it is better to talk about the requirements of the subsequent circuit, which may not be required by the Q2 circuit.

chunyang

The author's description is ambiguous and can easily be misunderstood.

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maychang posted on 2020-9-9 16:48 Where does the RXD signal come from? If the signal is a pin on the microcontroller, then the state of the pin can be changed using instructions.

It is an IO port on the microcontroller. Can it be short-circuited directly to the ground?

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chunyang posted on 2020-9-9 16:48 Just add an NPN transistor common emitter circuit to form an inverter. However, it is better to talk about the requirements of the subsequent circuit, which may not be required for the Q2 circuit.

The collector of Q2 is connected to a relay coil, and I want the relay to work.

chunyang

A black spot appeared in front of my eyes. Published on 2020-9-9 16:54 It is an IO port on the microcontroller. Can it be short-circuited directly to the ground?

What is your purpose of shorting to ground?

chunyang

A black screen appeared in front of my eyes. Published on 2020-9-9 16:55 The collector of Q2 is connected to a relay coil. I want the relay to work.

Then Q2 is turned on when the IO output is low. Explain the relationship between the IO output state and whether the relay is energized.

chunyang

Since you are "completely in the dark", please be patient and explain the problem clearly.

maychang

chunyang posted on 2020-9-9 16:51 The description of the original poster is ambiguous and can easily be misunderstood.

I guess you want the LED to light up when the microcontroller receives serial port data, and turn off when there is no data transmission.

If this is the case, then just use an NPN tube.

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chunyang posted on 2020-9-9 17:02 What is your purpose of short-circuiting to the ground?

Can a short connection to ground make its output low? I don't quite understand.

maychang

A black spot appeared in front of my eyes. Published on 2020-9-9 16:54 It is an IO port on the microcontroller. Can it be short-circuited directly to the ground?

Of course, the left end of R17 can be short-circuited to ground, but in this case the microcontroller will not be able to control the light-emitting diode.

Instead of shorting the left end of R17 to ground, it is better to short the upper end of R13 to the power supply and save R17 and Q2.

The pins of the microcontroller generally cannot be short-circuited to ground.

maychang

A black screen appeared in front of my eyes. Posted on 2020-9-9 17:47 Can a short connection to the ground make its output low? I don’t quite understand.

It would be better for the original poster to clearly state what the purpose of this circuit is, whether it is lighting up a light-emitting diode or operating a relay, do you want a certain pin of the microcontroller to control it?

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The MCU IO port outputs a high level by default. Now I want the U1-RXD port to output a low level to power the relay. (The MCU program is directly burned into it, and I don't understand the program).

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maychang posted on 2020-9-9 18:01 It is better for the OP to make it clear what the purpose of this circuit is, whether it is to light up the LED or to operate the relay, or to hope that a certain pin of the microcontroller will...

Hi, I have attached a slightly more complete picture, please take a look.

maychang

A black spot appeared in front of my eyes. Published on 2020-9-10 09:54 Hello, I have posted a slightly more complete picture. Take a look.

"I've posted a slightly more complete picture, take a look"

I have read that the I/O ports of some microcontroller models are at a high level by default after power-on.

maychang

A black spot appeared in front of my eyes. Published on 2020-9-10 09:54 Hello, I have posted a slightly more complete picture. Take a look.

"Now I want U1-RXD port to give a low level to power the relay"

Only instructions can change a certain I/O port of the microcontroller; it cannot be changed from an external circuit.

maychang

A black spot appeared in front of my eyes. Published on 2020-9-10 09:54 Hello, I have posted a slightly more complete picture. Take a look.

If it is a 51 series single-chip microcomputer, the I/O port is allowed to short-circuit to the ground when outputting a high level, but this is not necessarily the case for other types of single-chip microcomputers.

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maychang published on 2020-9-10 10:07 "Now I want U1-RXD port to give a low level to power the relay." Only instructions can change a certain I/O port of the microcontroller...

OK, thank you for the guidance.

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maychang posted on 2020-9-10 10:08 If it is a 51 series microcontroller, the I/O port is allowed to short-circuit to the ground when it outputs a high level. This is not necessarily the case for other models of microcontrollers.

It's not the 51 series, so is there any way to make the relay work without changing the circuit and program?