I'm here in China trying to increase my knowledge of 7 segment leds. In attempting to build the circuit all I can get hold of is a 4511 IC and a common anode 7 segment led. These 2 are incompatible unless I use a transistor (in this case a 3904) as a current sink for the LEDs. I know that with an NPN the presence of voltage at the base allows the flow of current from collector to emitter. I was under the impression that I must always have something connected to the collector. However I have noticed that if all I want is for the transistor to act as a current sink, there seems to be no need to connect anything to the collector. I just supply current to the base, the switch closes, the circuit is complete and the appropriate LED lights up because its cathode is satisfied. Is my observation correct or am I missing something? Somehow I'm not comfortable with the collector being connected to nothing.