74LS47, multiplexed 7 segment display(common Annode)

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andreikiss

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Problem with 74LS47

Hello

I'm trying to do this project as a part of a bigger project :
https://engknowledge.com/microcontroller_interfacing_multyplexing_7_segment.aspx



But I have a few problems related on switching on and OFF the 7 segment display.
I use a Atmega168 microcontroler powered at 3.3V (because it is interfaced with a ENC28J60 ethernet controller).

I cannot switch OFF the PNP transistor(2N3906) running at 3.3 V .
And I tryed a NPN transistor(BC107) but i cannot switch them ON with 3.3V.

The 7 segment display the required digit(thanks to the 74ls47) but i cannot multiplex them.

I was thinking of optocouplers ???
Do you know ... is ULN2003A of any help in this situation ?

I need 9V on the 7 segment display to make it look bright.


Any help will be apreciated,
Thank you
 

In series with the base resistor try a Zener diode: [9V – Ube - Ures – 3V] => Zener5V + 10k pullup between the base and 9V, or use a voltage divider: 9V-resistor-Base-resistor-microcontroller ..

IanP
 

So that would be using a diode in the base of a NPN ? Or PNP ?
 

Keep the same PNP transistor and put a pull up to the base (from base to 9v) and the zener from the ground (anode) to the base of the transistor (cathode). make sure the zener is reverse biased.
 

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