Problem with 7-segment display: dimly lit when off
The LED displays are being lit by current flowing through the p-channel MOSFETs (and possibly protection diodes) in the HC595s. They can only provide 0V and 5V at the outputs so when they are high (off) you have 18V - 5V = 13V across the LED display + resistor. 

You need to lower the 18V to something a bit less. I suggest an LM317 so that you can fiddle with the voltage setting, up to 16.5V or so. 







simulate this circuit &ndash; Schematic created using CircuitLab