Tristated output to two common Base transistor issue

[flotulopex]

So,

Seems I have no other choice but to buy a 20 Outputs PIC.

Thank you all anyway for all your propositions and the time you spent on my problem.

BTW Luciano, I made the circuit too and yes, available current is far to low...

[RussMartin]

Luciano: After testing my circuit on a breadboard I have found out
that the circuit works, but the current available for the LEDs
is too low for your application where the forward current of
the LEDs is 50mA.

Roger: I made the circuit too and yes, available current is far to low...

Is it possible that the series base resistor (2.2K) is too large? The "brute force" method to assure that the switching transistor saturates is to give the base about ten times as much current as it needs to turn on.

Remember, the base current controls the collector current: Not enough base current means not enough collector current. Try 820 ohms, then on down (680, 560, and so on) until it works.

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

I did a little looking on line. Is this the kind of thing you are doing?

http://www.customdynamics.com/knight..._light_bar.htm

and the cool video:

http://www.customdynamics.com/videos...derz_video.htm

[Luciano]

Hi Russ,

Saturation is not possible, here are the results:




Best regards,

Luciano

[flotulopex]

I was just wondering if something like that could work.
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1986&stc=1&d=118926122 9">
But after I made some trials, it is still not doing the trick

[RussMartin]

Saturation is not possible, here are the results

Thanks for showing us an in situ analysis. Your measurements should enable you to immediately infer what is wrong:

The circuit, as configured, doesn't permit enough voltage across the transistor! (What is Vf for the LEDs?)

If you remove the 22 ohm resistor to ground, you will get different results.

[Luciano]

If you remove the 22 ohm resistor to ground, you will get different results.

This is not possible, the circuit needs the voltage divider.



Roger will have to modify his design.

Best regards,

Luciano

[RussMartin]

If you remove the 22 ohm resistor to ground, you will get different results.

This is not possible, the circuit needs the voltage divider.

. . . and it is the voltage divider that is causing the problem.

I was not referring to your original circuit, I was referring to your test circuit, suggesting you compare your results by omitting one resistor.

Maybe we should start over. Let's go back to Roger's original circuit, which can be redrawn this way:

<IMG SRC="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1987&stc=1&d=118927164 6">

This is a first cousin to a half H-bridge.

You will recall the only problem he observed was that, when the input was tri-stated to high-Z, both LEDs were on, although not fully bright. This fault was traced to the resistors built into the transistors. Alain suggested, and I agreed, that the circuit be attempted with "classic" transistors (i.e., transistors without built-in resistors).

Has anyone tried that yet?