Hey guys, I'm facing an interesting problem.
The attached schema is of a simple circuit that serially clocks in data to the shift register 74HC164 and individual shift register outputs drive some NPN+PNP high-side switches. The +48V in the experiment is not connected (havent tried yet), there is only +5V. LEDs D1-D8 are there for fast visual check to see if clocked-in data are correct. If I want to use the LEDs, I close the jumper JP1. Everything works fine, LEds are signalling what they should.
Now the funny situation comes when I open the jumper JP1. LEDs will continue to shine, although with a reduced intensity! As there is no other connection of their cathodes to GND, I don't see what is sinking the current. I measured the resistence between cathodes and GND with the JP1 open (without power supply) and it was at least 350k (my multimeter is broken and does not measure past 350k).
The PCB is with SMD components so theoretically there can be an unwanted copper "bridge" but I measured the pcb and fixed is very carefully.
Can you think of any reason why opening JP1 would not shut the LEDs D1-D8?
Any tip is appreciated :)
EDIT: Any other comments regarding the schematic are also welcome - if you see something that a man experienced in electronics would never do, go ahead and let me learn ;-)