I’m building a scoring display with home-made seven segment characters. There are going to be 3 or 4 5mm series leds per segment. I think 74hc595 shift registers aren’t going to be up to the job, so I ordered some tpic6b595 ICs instead, except I messed up and got tpic6c595 ICs instead. So I figured I’d try and test it by only wiring up one segment so far (ie 3 series leds) but somethings not working properly. When the leds should be fully on, that’s fine, but when they should be off they’re still half on.
This is my curcuit.
If I connect the base of the BJT to either Vcc or Gnd the leds switch on and off fine, but when it’s connected to DRAIN0 (pin3) of the shift register, I get the problem.
I saw something about the tpic6c595 having constant current outputs so may not need current limitting resistors compared to the tpic6b595, so I’ve tried various different values of resistors and no resistor at all, but nothing seems to make a difference. Do I need to change my circuit for the 6c or do I need to use 6b instead?