Sinking current from 9V into a PIC pin.

[HankMcSpank]

Hi Joe,

My point was not what you meant.
If a PNP transistor is supplied by 9V, and you drive it with +5V (less than 9V), then the transistor will not be ignited as you expect.

As easy as having it tested on real circuit.
Just drive an LED via PNP transistor by a PIC, ON and OFF.
You will see that LED will not fully turn OFF.
And for the subject, our friend should use NPN, or for the easniness, use ULN2003, ULN2803.

The LED *does* turn off ! (ie the PNP tranny is working as I intended/required - real world, not simmed or theorized!) .....ie at a PWM duty cycle of value 255 the LED is visibly off (at a duty cycle value of 250 the LED light is just detectable...the LEDS continue to get brighter all the way to a duty cycle value of 0)

Re using an NPN....how would that slot into my needs of having a PIC sink the current per LED (in otehr words the suply voltage to the LED anodes needs to be commonised - how to switch that common voltage with an NPN?).

re using the ULN2003...I have no extra space available! (that's why I want to use a single, SOT23 SMT tranny to switch the 9V to the LEDS)

[Archangel]

Re using an NPN....how would that slot into my needs of having a PIC sink the current per LED (in other words the supply voltage to the LED anodes needs to be commonized - how to switch that common voltage with an NPN?).

? ? ? Swap the emitter and collector so power flows in correct direction, then invert PWM values (or not), transistor now conducts on positive instead of NEG. Keep those digital transistors, which are avail. from Mouser in mind to save board space too, they are equipped with internal bias resistors for logic level switching, and they are still very small footprint.

[Charlie]

Any base-emitter pair can only be a diode drop apart if the device is working. So, if the emitter is at 9V then the base is at 8.4 V or so. This means that either the PIC RC5 pin would need to be above 8.4V, or tri-stated to an open circuit to turn the transistor off. At 5V or 0V, the transistor is on. So you must be changing the tri-state, not the value (0,1) of the pin.
The problem with this is that all of the pins are briefly exposed to 9V, albiet through current limiters. Maybe the device has a sense of humour about this, maybe not. If you are making a hobby toy for yourself - have at it. If you are making a product, do the support folks a favour and do a little reading about transistors.
You have not shown us your code, or which device you are using so we are guessing here, but rest assured the concerns are valid. Just because it *appears* to work, does not mean it still will next week.

[HankMcSpank]

This is the point where Hank asks for a large dose of humble pie...obviously at some stage in my frenetic breadboarding activity (moving wires around all over the shop), I've actually got 5V going to the PNP emitter (not the 9V I thought was on there).

Just to prove the point...I put 9V on the emitter...the PIC lived, but the LEDS didn't turn off when the duty cycle went to 255.

You're all vindicated! (not that I ever doubted you!)

Charlie...this is just a hobby.