I'm building a project where I want to use two double-digit 7-segment-LED displays (green, common cathode). From what I've read, a shift register wouldn't be able to source enough current, so I decided upon using a SN74LS248N IC (datasheet). This IC takes Binary Coded Decimals (BCD) as input and has 7 outputs to drive the segment LEDs, minus the decimal point LED. As I understand the *248 ICs are for common cathode displays, so pins connected to active segments are HIGH.
My problem: I can't get the LED segments to light up properly using this IC. It's visible in the image below, but worse in real life. Check the photo album I've made for comparison with a circuit where the segment is driven directly. This album also includes a circuit diagram, to help illustrate the problem.
When I connect using the driver IC, the segments light up only weakly . My bench power supply indicates the circuit is using 9 mA.
Now, I've tested the display using only power leads and a 150Ω resistor. That works beautifully, using up more than three times the power: 30 mA according to my bench power supply.
Does anyone here have any clues or tips?