Seven Segment MULTIPLEXING

[sssuneth]

Hello there

I would like to know the the transistor's job in a MULTIPLEXING 7Segment set up

I have seen a schematic(picture attracted) and wanna select a appropriate transistor.

here is the link where i saw the schematic
**broken link removed**


1.)what is the purpose of the transistor?
2.) any recommend transistor

 

[Reloadron]

You have in that example common cathode displays. As each transistor turns on the cathodes sequentially are connected to common. The transistors are being "Strobed" and each turns on a single display when the data for that display is present. The idea is the human eye can't see this because of the strobe speed so the display looks to be always on across all the displays.

Ron

 

[crutschow]

1.) The transistor turns on (conducts current) for a particular digit while the microcontroller outputs a high signal to those segments that are being lit for that digit. In a multiplexed display each digit is sequentially lit to minimize the required number of connections from the controller. The disadvantage of this is that it reduces the apparent brightness of the digits since they are only being lit for 1/N of the time where N is the number of digits multiplexed.

2.) Most low power NPN transistors should work. The common 2N2222 (PN2222 in a plastic case) and its many variants work well, for example.

 

[sssuneth]

thanks Ron

My question is how does that Transistor acts in this circuit . I mean does it supply current towards 7seven segment?
can i use any transistor for that..?

would BC548 be fine? why?

 

[sssuneth]

thanks for your reply Carl..

1.) can i use BC548
2.) is that transistor passing current Via Collector to 7segment?

 

[KeepItSimpleStupid]

Before one can select a transistor, we do have to know the characteristics of the port. The current to the LED segment would help too.

 

[Reloadron]

thanks Ron

My question is how does that Transistor acts in this circuit . I mean does it supply current towards 7seven segment?
can i use any transistor for that..?

would BC548 be fine? why?

The transistor simply acts as a switch. It is either on or off. When we use a transistor like that it is known as driving the transistor into saturation. The BC548 you mention could be used as long as the display never draws over 100 mA which is very unlikely. That or as Carl mentioned the little 2N2222 would be a very good choice as a general purpose switching transistor, as would a 2N3904. Just about any small switching NPN transistor will work. As Kiss mentioned, it would be nice to know the port output characteristics so the correct base value resistors could be chosen. So the transistor is a switch that turns on a given display at a given point in time when the data for that specific display is present on the 7 lines (as well as dp).

Ron

 

[audioguru]

When the number "8" is displayed then all seven segments are lighted. If each segment uses 20mA then the total current in the transistor is 140mA and it might be more. The BC548 has a max allowed current of only 100mA so it cannot be used.

 

[Pommie]

Assuming the microcontroller runs at 5V then the segment current is (5-2)/330 = 9mA. Assuming all segments are lit then total current = 8*9 = 72mA. A BC548 will work fine especially as it can handle a pulsed current of 200mA. The base resistors are a little high and I'd replace them with something like 2k.

Mike.

 

[srikanthsamaga]

I totally agree with Pommie.
I have used BC548 with a 1k base resistor to drive common anode displays without any problem, I had used BC548 because I had plenty of them , or else I would have used 2N2222.

 

[sssuneth]

how much current do i need to pass to BASE of the transistor (BC548) to enable collector current??

 

[sssuneth]

could you please just tell me how did (in the original picture ) decides to use 10K resistor with each NPN transistor ?

 

[audioguru]

how much current do i need to pass to BASE of the transistor (BC548) to enable collector current??
Could you please just tell me how did (in the original picture ) decides to use 10K resistor with each NPN transistor ?

If the LEDs are 1.8V and the supply is 5V then the 330 ohm current-limiting resistors reduce the LED current to (5V - 1.8V)/330= 9.7mA.
The datasheet for the BC548 guarantees that it saturates well when its base current is 1/20th its collector current so its base current should be 9.7mA/20= 0.49mA. The port will be about 4.8V and the base will be about 0.65V so the base resistor should be (4.8V - 0.65V)/0.49mA= 8570 ohms. Use 8.2k for the base resistors.

 

[Pommie]

If the LEDs are 1.8V and the supply is 5V then the 330 ohm current-limiting resistors reduce the LED current to (5V - 1.8V)/330= 9.7mA.
The datasheet for the BC548 guarantees that it saturates well when its base current is 1/20th its collector current so its base current should be 9.7mA/20= 0.49mA. The port will be about 4.8V and the base will be about 0.65V so the base resistor should be (4.8V - 0.65V)/0.49mA= 8570 ohms. Use 8.2k for the base resistors.

That's fine if you just want to light 1 segment. To light more use 1k resistors.

Mike.

 

[Diver300]

If the LEDs are 1.8V and the supply is 5V then the 330 ohm current-limiting resistors reduce the LED current to (5V - 1.8V)/330= 9.7mA.
The datasheet for the BC548 guarantees that it saturates well when its base current is 1/20th its collector current so its base current should be 9.7mA/20= 0.49mA. The port will be about 4.8V and the base will be about 0.65V so the base resistor should be (4.8V - 0.65V)/0.49mA= 8570 ohms. Use 8.2k for the base resistors.

I think that the collector current could be up to 8 times as much if all segments including the decimal point are lit. You should allow for 80 mA in the collector, so aim for about 4 mA in the base. A 1 kΩ base resistor would do.

The saturation gain of 20 is worst case and a larger resistor might well work, but there is no reason to use anything higher than 1 kΩ.

(It turns out that I posted at the same time as Mike)

 

[aljamri]

I've used 2N2222A, 2N3904, and the following circuit used BC547, and all of them as Common Cathode application and doing fine:

**broken link removed**

the above circuit can be found in https://www.electro-tech-online.com/threads/room-person-counter-help.97193/.under the sidebar "Lift Counter".

 

[audioguru]

I've used 2N2222A, 2N3904, and the following circuit used BC547, and all of them as Common Cathode application and doing fine:

But you are overloading the PIC with a total max current of 156mA but I think its absolute max total allowed load is only 50mA.
The poor BC547 transistors also have a total max current of 156mA but their max continuous allowed current is only 100mA each.

 

[Pommie]

Yup, pic pins overloaded but 547 can handle the pulsed current (200mA pulsed). BTW, current per port = 100mA , so still overloaded.

Mike.

 

[aljamri]

But you are overloading the PIC with a total max current of 156mA but I think its absolute max total allowed load is only 50mA.
The poor BC547 transistors also have a total max current of 156mA but their max continuous allowed current is only 100mA each.

Yes, you are absloutly right, when I touched the transistor, It felts HOT, so I've connected a series resistance, (decade box) where I adjusted until I got a visible amount of brightness. If I remember the final value was 180 Ohm or below.

 

[birdman0_o]

Not to mention that poor diode!