connect 74LS47 to seven-segment display does not work?

[jamsar13]

Hey Tom,

I think i'm having a stupid person moment, because I changed the pins that weren't connected to high, as you showed in that example, and changed the ground to digital ground and its still not working. Would it be at all possible for you to send me the file or a jpeg to see what you did?

[Tom Sedlack]

Your circuit had a command cathode LED.  Did you change it to common anode?

[jamsar13]

like i said...STUPID MOMENT...It works!....thank you so so much Tom 🙂 🙂

Sara

[Shank747]

I am having a somewhat similar problem involving the same IC 7447 and a 7 segment display common anode.

I have attached my schematic and the corresponding multisim file which i've made in multisim 11.0

I really don't understand why the dispay shows 7 when it should show 0 when all the switches are open. Can someone please help?

[Tom__Sedlack]

Take a closer look at the truth table of the priority encoder. When all inputs are high, all the outputs are high which gives you the "7".

[Shank747]

Hi Tom!

So, from what i understand, the circuit is correct, but i just need to invert this trend.

I'm not sure how to go about doing this... Do i need to use inverters along each line coming out of 74148 into 7447?

[Shank747]

OK, i figured it out! But i want to replace the 7 segment display common anode with a 7 segment display common cathode. I tried replacing the 5V line with a digital ground, but the display doesn't light up. What am i doing wrong? Thanks in advance for your help. It's much appreciated.

[Tom__Sedlack]

The 7447 drives common anode displays.  The 7448 drives common cathode displays.The two parts are the same functionally with the exception of the led output drive.

[Shank747]

Thank you for this useful information, Tom.

Using 7448 simplified my circuit considerably. In the circuit, i also replaced the common cathode output with a digital ground. But there is a problem with the display; when viewing 0 and 1 on the 7 segment display, the display is clear and stable, but the displayed number fluctuates and goes faint when displaying the numbers 2 to 7. I have attached my modified circuit. I have removed the resistor connected to the common cathode pin, but to no effect. Please advise on what i'm doing wrong.

Your help is highly appreciated, as always.

[Tom__Sedlack]

 It's never a good idea to have a single current limiting resistor for a multi-segment display as the current will change based on how many segments are being driven.  My guess is that the simulation determined that too much current was being drawn so it simulated a failure/shutdown of either the driver or the LED. That's why it was flickering.  I've attached a copy of your design with the preferred way to wire the LEDs.