Tristated output to two common Base transistor issue

[RussMartin]

Beg pardon, my friend; not my circuit. It is a generic redrawing of Roger's original circuit. Which transistors did you use? And did you toast them?

Thanks for the drawing! It illustrates the problem very nicely! (The red current path is through two diode junctions.)

[Luciano]

Hi Russ,

The transistors are still working.

With this circuit the LEDs are always ON.

NPN BC546
http://www.fairchildsemi.com/ds/BC/BC546.pdf

PNP BC558
http://www.fairchildsemi.com/ds/BC/BC558.pdf

Best regards,

Luciano

[Luciano]

Hi Russ,

My interest was to find a solution for the LEDs board that Roger has already made.

If there is no solution for the original LED board, then a new board with I/O port
expanders ICs + transistors is in my opinion the way to go.
(One I/O + 1 transistor for each 50mA LED). That way all the LEDs can be
ON at the same time and each LED can be individually controlled.

Best regards,

Luciano

[RussMartin]

The transistors are still working.

Well, that's good news!

If you have a few extra minutes, try the circuit this way:

<IMG SRC="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1992&stc=1&d=118928211 2">

Let us know how that comes out. You may want to tinker with the values for Rcl, maybe starting with something between 100 and 150 ohms.

Do you have some small-signal P-channel and N-channel MOSFETs laying around?

[RussMartin]

Scratch this post.

[Luciano]

Hi Russ,

These are the test results. I have used the old transistors
and also new transitors but the results are the same. The
picture below is with the new transistors.

The picture shows the two LEDs ON with about 25% luminous intensity.




Best regards,

Luciano

[Luciano]

Hi,

With the circuits we are exploring here, the two LEDs are turned OFF by setting
the I/O of the PIC as input. (Input tristated). When the I/O is set as input,
the voltage present on the I/O pin must be recognized as logic LOW or as a
logic HIGH. Undefined voltage levels are dangerous for the input buffer of the I/O.
Floating inputs on digital pins are a no-no.

Best regards,

Luciano

[RussMartin]

Thanks for an excellent result! (And the photo!) And thanks for bearing with me . . . I'm doing this on paper, not on a breadboard. I don't know what your LEDs are, so I'm having to guess at the Vf (I'm using 1.6 V) and assuming an If of 20 mA.

If you please, try increasing both Rcl's each by the same amount until the LEDs just go out, then test high and low at Rb again.

I appreciate your concern about floating inputs, but we can deal with that next. (I have something in mind.)