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enter image description here

I am trying to debug the circuit above. The transmitter pin from the MCU is connected to the NSBC114EDP6's (SMD marking code 7A) input pin 2, and the output from pin 3 is connected to another MCU receiver pin. 

I have scoped the input and output data from the NSBC114EDP6; there is no voltage difference. I have attached the scope image below:

enter image description here

channel_1 (yellow) - input NSBC114EDP6
channel_2 (green) - output NSBC114EDP6

I would like to know what the purpose of the NSBC114EDP6 is.

ocrdu
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anbu b
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    Something must be wrong in the measurement or description, or something is broken. If all is as described, the scope channel 2 should be inverted compared to channel 1. – Klas-Kenny Oct 07 '22 at 06:51
  • @Klas-Kenny Wouldn't that rather be a circuit with 1 NPN and 1 PNP? This is 2 NPN and if I'm reading the schematic correctly, one NPN drives the base of another NPN. How would that invert the signal? Isn't this just acting as a non-inverting amplifier? – Lundin Oct 07 '22 at 08:07
  • Anbu did you have a look at this about [schematics](https://electronics.stackexchange.com/a/28255/173919), in particular "basic layout and flow" section? – jonathanjo Oct 07 '22 at 08:44
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    @Lundin You are right, I misread the schematic. I assumed it was one transistor on the left side of the symbol and the other transistor on the right side (ie. transistor to the right not used). But sure enough, that's not the case. – Klas-Kenny Oct 07 '22 at 10:56
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    @Klas-Kenny Electrical engineering prank on April fools: draw your NPN upside down so that others mistake it for a PNP ;) – Lundin Oct 07 '22 at 11:02
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    And as we can tell from all the confusion, drawing IC containing transistors/MOSFETS as some "black square" isn't helpful. Always draw out the actual transistor symbols explicitly. I have some RL anecdote about when three different engineers reviewing a schematic all failed to account for a MOSFET substrate diode, since the symbol was drawn as a black box, which in turn caused an accident. – Lundin Oct 07 '22 at 11:05
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    **"The STM32F103xx low-density performance line family operates from a 2.0 to 3.6 V power supply".** https://www.st.com/en/microcontrollers-microprocessors/stm32f103c6.html Where did you get 5V logic levels from? – Lundin Oct 07 '22 at 11:15
  • Where did your circuit come from? Are you sure you have the details right? – jonathanjo Oct 07 '22 at 12:11

2 Answers2

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There is something wrong to begin with. The MCU output is measured 5V logic levels and it can't be 5V logic levels out. There should be voltage difference.

What the circuit is supposed to be doing is level converting 3.3V push pull logic level output to 5V open collector output levels, 5V being provided via diode and weak 10k pull-up resistor.

Justme
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  • Actually... this STM32 part doesn't look like it is able to give 5V out to begin with. https://www.st.com/en/microcontrollers-microprocessors/stm32f103c6.html – Lundin Oct 07 '22 at 11:13
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    @Lunding That's why I said the MCU output level can't be 5V. – Justme Oct 07 '22 at 11:37
  • Yep I close voted as unclear. No point in speculating further until the OP can clarify what they are actually doing. – Lundin Oct 07 '22 at 11:49
  • STM32 is dummy mcu. tx and rx 2 controller are working 5v mcu – anbu b Oct 10 '22 at 08:27
  • @anbub Don't put incorrect information into questions because you get your answers based on information being correct. Edit it in then. – Justme Oct 10 '22 at 08:45
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The transistor is able to drive 100 mA, 250 mW (depending on thermal conditions), and 50V.

The CPU output pin cannot: see this question about its drive capabilities.

jonathanjo
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