Hmm,
Thanks for your replies, guys.
The power supplies I've tried are:
(a) a 9V Duracell battery, brand new
(b) a slightly older and well used 9V battery
(c) my main 10V power supply capable of supply 2A, this is the pwr supply I'm using via a 5V reg to power the PIC side of things.
(d) the regulated 5V itself from the PIC side of things.
All these supplies have been substituted for the "+5V" in the diagram above, all with the same problem. Namely, the sequential switching on of LEDs causes them as dim slightly as a group.
With reference to kinjalgp's suggestion number 2 of not using resistors to the INputs of the ULN from the PIC, I've tried that also. A direct wire connection from PIC PortB pins to the INput pins of the ULN present the same problem.
I've just tried connecting the pull-up resistors suggested by kinjalgp in his point number 3, however this has the result of turning all the LEDs on all the time regardless of PIC port logic output state, since of course the INputs of the ULN are high all the time then (V+ thru pullup to IN).
I'm really stuck here, folks!
These are just simply 20mA or so red LEDs. Surely the power supply doesn't need to be beefy to drive 8 LEDs at a uniform brightness via a ULN?
Incidentally, I've tried this also using BCX38B transistors with their emitters commoned, also a CA3083 5-transistor chip (not Darlington). Same problem results for all of these methods.
Could there be some issue with the current drive from the PIC PortB pins? That is, when one PortB pin is switched "on", the current flow from that pin going onto the base (INput pin on the ULN) and then through the emitter is also helping to drive the LED? And when more PortB pins are "on" the current is divided between them now, thus less base-emitter current per node and less brightness is the result?
Chris