Tristated output to two common Base transistor issue

[Acetronics2]

Hi, Roger

the simplest (component count ) is to use "classic" transistors and use resistors for 3.5v Hi state switching, and 1.5v Lo state switching ...

That gives "R1" = 10k and "R2" = 2.2 k ... and class "C" transistors ( gain > 400 ).
The original Logic transistors can't work this way due to the too high b>e resistor

AND do not forget the +V/2 pullup 2 resistors ...

BUT ... what do you want to realize ??? other solutions possible !!!

Alain

[RussMartin]

If my way of doing (thinking) is completely wrong, how would this kind of problem be resolved in an industrial environnement? Would they just use a 20 I/O ports PIC?

Finally, I ask myself if this can be done ... or not?

This is why engineering is an art, not a science!

Let's sum up what we know so far:

1. It is a 5 VDC circuit.

2. The LEDs require more current than a PIC pin can source or sink.

3. There are three permitted states for any pair of LEDs--one or the other is on, or they are both off. They are never both on at the same time.

4. There are 10 output channels.

Beyond this, we don't know what you are attempting to do, or why. In other words, we don't know your intended application.

And there are questions: Do you have to use such current-hungry LEDs? Why? Of 10 channels, how many LEDs are likely to be on at the same time? Do you have size and/or board space limitations?

Alain suggests using "classic" transistors (I think he means ones without the built-in resistors). This sounds like one good approach. I would go further and suggest generic switching transistors driven well into saturation. Going to a 20-port PIC would also work, with the advantage of allowing you to use transistors that are all the same.

[Jerson]

Unless it is a course requirement, I suggest you use shift registers CD4094 to latch the states of the LEDs. For 24 LEDs you need 3 x 4094 devices; and they are dirt cheap. So, why struggle?

[flotulopex]

Well,

I'm still working on a "Knight Rider" stoplight...

Because of a extremely narrow space availability inside the original stoplight, I can't add lots of components to the circuit.

The LEDs have to be powerfull enough to look like, at least, the original bulbs.

I have made a prototype with "common" LEDs and it works well BUT it is far to weak (... and I have made it with yellow LEDs witch makes it unsuable as stoplight).
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1972&stc=1&d=118910210 0">

I connected the LEDs like hereunder but this is not longer possible because of the use of transistors (btw this is just the LED connection part). The "curious" disposition of LEDs in this shematic allows me to switch-ON two consecutive LEDs and improve the fluency wipe effect - but this is another story.
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1973&stc=1&d=118910225 7">

I couldn't find SMD transistors (at DISTRELEC for those who now this supplier) without built-in resistors so I thought it is a common thing (?).

Hope this helps a little...

[Luciano]

Hi Roger,

Here are two complementary transistors from www.distrelec.ch.

Best regards,

Luciano


==============================================
Distrelec Art.No: 610375
Type BC 817-40 (NPN SOT-23, 500mA)

Datasheet:
http://www.nxp.com/acrobat_download/...7W_BC337_5.pdf

==============================================
Distrelec Art.No: 611585
Type BC 807-40 (PNP SOT-23, 500mA)

Datasheet:
http://www.nxp.com/acrobat_download/...7W_BC327_5.pdf
==============================================

[flotulopex]

Luciano,

Je dois avoir les yeux dans mes poches, c'est pas possible autrement!

Nevertheless, it looks not possible to control the LEDs the way I thought I could...

[Luciano]

Hi Roger,

Try that:

If you use standard LEDs, replace the 22 Ohm resistors with 100 Ohm resistors.

Best regards,

Luciano

EDIT: The note about the resistors is only if you use this circuit.
To drive two standard LEDs (max 20 mA) you don't need transistors.

[flotulopex]

As said, the first board I made was fitted with standard leds (picture in post #12).

But, following a good old swiss habit, I observe and think how to make things even better... and get into trouble.

Frankly, I thought this kind of question would have been common, maybe even boring.

A few days ago, I made a second prototype where I used my SMD NPN's and paired the leds. The visual effect is quite good (will make a video).

Now, if I have to make a third board with MOSFETs, this won't be so terrible (the iron is still hot).

And, please remember Leonardo Da VINCI: he invented so many things but built so few. I try to balance and do the opposite

Many many thanks to all.

NB: "Swiss precision" is not a legend and if you have a closer look to a chocolate bar, you'll notice that even the hazel nuts are lined-up! And yes, I'm a musician.

[sayzer]

Hey Roger,

I am getting a lot of good stuff via this post and all is free.

But, going back to your original issue, I was wondering why don't you consider using something like 74HC164 ? You can use three or four I/O pins and control cascaded 74HC164 ICs for 20 LEDs.

You can set all 20 LEDs with uS of times at once.

-------------------------------------

[flotulopex]

As you can see the image in my post#12, the led's PCB is quite small (narrow). It is ment to be put inside an existing stop-light case ("boîtier" in french).

I haven't enough space to put either the µc nor the power regulator on the led's PCB; this is why I made a second PCB for the "power & µc section". It is located outside the stop-light (still inside the car but not inside the case).

Finally, and this is why this all is about, I wanted to reduce the amount of wires between those two PCBs.

Voilà.

Another general info: I made my second proto with 15° leds; this viewing angle is to small, 24° must be better.