transistor as a current sink

[kelangfei]

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.

 

[ericgibbs]

hi,

All the transistors would be doing is inverting the voltage levels coming out of the 4511, making the 'voltage sense' compatable with common anode 7 seg LED.

Connect a 1K0 resistor from each output pin of the 4511 [total 7 resistors]
Connect the other end of the resistor to the BASE pin of the npn transistor, the EMITTER of the transistors are connected to 0V. [all 7 transistors]

The COLLECTOR of the transistors have a 330R resistor which is then connected to the 7 seg LED cathode.

 

[AllVol]

Is my observation correct or am I missing something? Somehow I'm not comfortable with the collector being connected to nothing.

A transistor can pass current from base to emitter with a proper resistor on the base leg. It is acting as a diode in this configuration.

However, to properly activate a 7-segment display, I would think you would have to go with Eric's configuration, which inverts the "1", HI signal from the 4511 to a "0" the common anode can use.

In your experiments, are you sure the segment that lights is doing so properly? There is the possibility it is lighting when it should be dark.