Serial communication problem.

[CDRIVE]

Is there a reason that you have ch1 and ch2 overlapping each other? Please separate the baseline traces so they're not overlapping each other. It's also good practice to assign Ch1 to the Input and Ch2 to the Output.

Chris

 

[HellasTechn]

Ok sir.
theese are from the bc547.
Picture 1 = input pin while not connected to the transistor.
Picture 2 = with collector to gnd
Picture 3 = with emitter to gnd.

Channel 1 = input signal (at base of transistor)
Channel 2 = output signal

Notice that when connected to the base of the transistor, the input signal drops about 0.4Volt

 

[CDRIVE]

We have quite a few EE's here that posses a far greater understanding of the physics involved with bipolar transistors than I do. Hopefully one of them will give you a precise and detailed explanation.

I can tell you two things though.
(1) If you reduce R2 to 100Ω you will see very different results when you invert Q1. The output won't swing below ~ 3V. If R2 is reduced further to 47Ω the output probably won't swing below 4.5V.
(2) Inverting Q1 can eventually destroy the Base - Emitter junction. This is because the max reverse Base - Emitter voltage is being exceeded.

Chris

 

[CDRIVE]

Constantine, I've done some simulations since my last post. I believe that later today I can clearly explain the scope readings you're experiencing. Right now I'm long overdue for a haircut.

Chris

 

[HellasTechn]

The output won't swing below ~ 3V. If R2 is reduced further to 47Ω the output probably won't swing below 4.5V.

Thats is because the transistor will not be fully turned on. will probably be within the "active region" if i that is the correct term in english. Am i wrong ?

Constantine, I've done some simulations since my last post. I believe that later today I can clearly explain the scope readings you're experiencing. Right now I'm long overdue for a haircut.

I look forword to that sir Chris. I have to let you know though that i will be away for work for the next 6 days without internet acces. So please take your time.

This is a mystery that i really want to solve. What does not make sense to me is why when the transistor is connected correctly then the microcontroller can not read correctly. I think there may be a mistake in the code. i will check that again and when i get the chance i will scope the actual signals going in and out of the transistor and microcontroller from the actual rs232 source.

That will give us a better picture about what really goes on in there.

I will need time for that though for the reason stated above.

 

[CDRIVE]

Here's some plots with an inverted NPN that explain a lot.

Chris

 

[Harald Kapp]

Channel 1 = input signal (at base of transistor)

This can't be. The base of the transistor will not rise much above ~0.6 V ... 0.7 V. At that voltage the base-emitter diode becomes conducting and the resulting base emitter current will drop the rest of the input voltage across the series resistor from signal source to base. Your scope signal shows ~ 5 V signal level, thus you surely are measuring at the signal source or the left side of the series resistor, not at the base of the transistor.

when connected to the base of the transistor, the input signal drops about 0.4Volt

Your signal source is very probably far from an ideal voltage source. It will have some internal resistance.
When the input voltage is high, the transistor is conducting. The input current to the circuit is approx. 400µA as calculated from:
(5 V from the signal source - 0.6 V base emitter voltage -0,4 V voltage drop observed)/10 kΩ.
A current 0f 400 µA and a voltage drop of 0.4 V correspond to an internal resistance of the source of ~ 1 kΩ.
You can check this by replacing the 10 kΩ series resistir by e.g. a 1 kΩ series resistor. The voltage drop will be much higher due to the higher input current. Expect about 2 V ... 2.5 V drop in that scenario.

Picture 2 = with collector to gnd
Picture 3 = with emitter to gnd.

When you operate an NPN transistor with collector to ground, it will seem to work regardless of the mixed up connections. This is called reverse mode of operation.
However, the current gain is greatly reduced. An easy explanation you may read up here.

 

[CDRIVE]

This can't be. The base of the transistor will not rise much above ~0.6 V ... 0.7 V. At that voltage the base-emitter diode becomes conducting and the resulting base emitter current will drop the rest of the input voltage across the series resistor from signal source to base. Your scope signal shows ~ 5 V signal level, thus you surely are measuring at the signal source or the left side of the series resistor, not at the base of the transistor.

Harald, that's an issue that I never questioned him about because I assumed he was scoping the base signal at the input end of the base resistor, as you surmised.

That said I really do appreciate your expertise. Particularly regarding Bipolar Reverse Mode Of Operation. I'm learning about this over 51 years too late but better late than never.

Thank you!
Chris

 

[HellasTechn]

hus you surely are measuring at the signal source or the left side of the series resistor, not at the base of the transistor.

Yes thats right. What i ment was that this is the signal i am feeding to base through the resistor.

When you operate an NPN transistor with collector to ground, it will seem to work regardless of the mixed up connections. This is called reverse mode of operation.
However, the current gain is greatly reduced. An easy explanation you may read up here.

Like i suggested here:
I also had no idea there was such thing as reverse mode. But then again is it the first or the last thing i have no idea about ? I am a complete noob compared to all you.

How can that be explained ? maybe when i tie the emitter to vcc and collector to gnd, the transistor works in "reverse-active" mode ?
like explained here ? https://learn.sparkfun.com/tutorials/transistors

 

[CDRIVE]

Since Reverse Active Mode is an inexplicable revelation to this old dog too, I thought some sandbox time was is order. Here's some (DC Analysis) spice plots on a generic NPN.

Enjoy.
Chris

 

[HellasTechn]

I came across this video on youtube that explains about transistors. I have a question.

At about 7:30 on the video he shows current flow in a pnp transistor and from what i understand, he is suggesting that current flows from emmiter to collector. Is my understanding wrong or is it something else ?

 

[Harald Kapp]

The direction of current flow is a constant source of misunderstanding.
There is the "technical" direction of current, for historical reasons from the positive pole to the negative pole.
The physical direction of current, the movement of the electrons, is from the negative pole to the positive pole.
It is common in electrical engineering to use the technical direction. Therefore, as the emitter of a PNP transistor is usually connected to a point which is more positive than the collector, current flows from emitter to collector.
In an NPN transistor the emitter is usually at a lower potential than the collector, so current flows from collector to emitter.

 

[HellasTechn]

I suspected that but to be honest i thought that unless differently specified, we use conventional current flow.

 

[Harald Kapp]

There's no contradiction in here. Cpnventional or technical current flow from '+' to '-' matches the description of the current flow within an NPN or PNP transistor.

 

[HellasTechn]

I have one question concerning serial terminal software for PC. Like the TERMITE, but i need one that will be able to repeatedly transmit the sentence i write...

The TERMITE is very good but will only transmit the text i write once, then i have to write it again and press enter.
Is there some software that can be set to transmit continiouesly until interrupted by user ?

 

[CDRIVE]

When you say "until interrupted by the user" what does that mean? Are you referring to a response returned by the receiving device or simply a "Repeat" option on your terminal software? If the latter it'll need a timer function that repeats the message typed every 'n' seconds until disabled manually.

Chris

 

[HellasTechn]

"Repeat" option

A simple repeat option

like a checkbox that i can deselect when i want transmission to stop repeating.

 

[CDRIVE]

This and its associate topics has me at a point where I've lost track of where it all started. We've moved through Picaxe, PICs, MAX232's and all of it permutations. I think GSM was somewhere in this mix too.

Sooo.. are looking for a general purpose terminal app or would a utility that's "application specific" be more applicable?

Chris

 

[HellasTechn]

I know it turned ito a spaggeti bowl ! What i need is a general purpose terminal app.

Termite serial terminal is a n excellent tool, what i need more is to be able to write the text and the software to keep repeating it until i stop it.

Anyway i am trying to do this with hardware (picaxe serout) since it looks that such software i am looking for may not exist.

So far i have not been able to make picaxe transmitt ASCII caracters but that has to do with the code i wrote. I need to practice it more and read the command's manual.

Again a software solution would be very helpful.

THANKYOU.

 

[Harald Kapp]

TeraTerm has a built-in macro language interpreter. You can use it to write a simple program that uses the serial port.

You could also write a small program yourself which accesses the windows COM port, e.g. using python.