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How are NPN transistors used in 7 segment displays?
You can use the NPN transistors to sink current from the common cathode when you want to select an individual 7-segment display if you are multiplexing them. However, you will need PNP transistors in addition to the NPN transistors in order to source current to the anodes of the displays.
Can a 7-segment anode be switched at a time?
If you are just driving a single 7-segment digit, then the above applies directly. If you are multiplexing multiple of them, then you also want to switch the anodes so that only one digit is enabled at a time. In that case, the average on-time will be the reciprocal of the number of digits.
How many transistors are in a 7 SEG?
The datasheet for the 7 seg suggests 2.25 V drop and 20mA. With that said, the board that I am using outputs ~3.3V and about 3mA. I already have a bunch of these NPN transistors, just need some advice on how to connect it. Thanks. Well, there’s good news and bad news.
How is the current of a segx led controlled?
Here is a good way to do this: The SEGx lines are 0-5 V digital outputs from your microcontroller. All you need is a single NPN transistor and emitter resistor per segment. The LED current is controlled by the emitter resistor value, and is largely independent of the supply voltage.
Do you need NPN transistor for ULN2003 segment multiplexing?
Most examples on the net use common cathode displays, in which case NPN transistor is called for, but the ULN2003 which we are using here for each segment is a current sink, which means individual segments need to be wired to it on “minus side” – so we need a common anode type of display.
What kind of transistors are used for 7 segment multiplexing?
Most sources in the net did not specify hardware at all, and those that did were driving the segments with a 74HC595 shift register and using NPN transistors to enable one common cathode display at a time.
Are there any drawbacks to NPN transistors?
The only real drawback is the Vce (sat); it’ll be somewhere between 1.3v and 1.6v for a 20mA load. If you want a similar driver for the low side, you could look up ULN2003 or ULN2803; but probably won’t be worth it unless you have more than 3 displays to drive. Thanks SgtWookie. I knew I could do it with PNP, just trying to use what I had.