7448 BCD to seven segment display

Thread Starter

Yami

Joined Jan 18, 2016
309
I've connected a 7448 to a common cathode seven segment display. Using switches for the input data. It didn't work only when I input 0V to the input of the IC does the segment lights up.
What am I doing wrong any help would be greatly appreciated
Thanks
 

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bertus

Joined Apr 5, 2008
20,664
Hello,

With the 7448 you do not need resistors for the leds.
The common cathode goes to ground, the segments go to the outputs of the 7448.
See the datasheet for more info.

Bertus
 

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Thread Starter

Yami

Joined Jan 18, 2016
309
Hello,

With the 7448 you do not need resistors for the leds.
The common cathode goes to ground, the segments go to the outputs of the 7448.
See the datasheet for more info.

Bertus
Thanks Bertus,
I've removed all the resistors going from the IC to the LEDS. The segment does not still work. When I connect any input pin of the IC to the ground it works - so the switches cannot be used to work how it should. Shouldn't it be 5V at the input of the IC to make it drive.
 

GopherT

Joined Nov 23, 2012
8,012
RBI pin turns off all output if the BCD value is zero all low (used if the segment is a leading zero for hours, you don't want 03:00, you want 3:00.

Anyhow, either make the RBI pin low, or set the input switches to any other BCD value.
 

Thread Starter

Yami

Joined Jan 18, 2016
309
RBI pin turns off all output if the BCD value is zero all low (used if the segment is a leading zero for hours, you don't want 03:00, you want 3:00.

Anyhow, either make the RBI pin low, or set the input switches to any other BCD value.
Thanks
The segment lights up when all the inputs are low (connected directly from the common to all the pins). When all the inputs are connected I get a "0" as expected and other numbers as expected too(depending on which pin is connected to the common and which pin is not connected). Then what is the use of the 5V connections through the switches?? and also how can I set the BCD values if I cannot use the switches. If I connected the switches to the common I can make it work but then I have to press the switches the opposite way to get the values. So to get "0" I have to push all the switches...
 

GopherT

Joined Nov 23, 2012
8,012
Thanks
The segment lights up when all the inputs are low (connected directly from the common to all the pins). When all the inputs are connected I get a "0" as expected and other numbers as expected too(depending on which pin is connected to the common and which pin is not connected). Then what is the use of the 5V connections through the switches?? and also how can I set the BCD values if I cannot use the switches. If I connected the switches to the common I can make it work but then I have to press the switches the opposite way to get the values. So to get "0" I have to push all the switches...
Post a photo of your circuit. The real circuit - a few photos with different angles so we can see all the wires.

Also, make sure your switches are the normally open verses normally closed type.
 

Thread Starter

Yami

Joined Jan 18, 2016
309
Post a photo of your circuit. The real circuit - a few photos with different angles so we can see all the wires.

Also, make sure your switches are the normally open verses normally closed type.
The LEDS are on when the LT is connected to common so that works.
 

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Thread Starter

Yami

Joined Jan 18, 2016
309
1kΩ pull-down resistors are too high to sink the input currents of 7400 TTL gates.
Try 100Ω instead.
Great it has done the job!! its working perfectly. I still can't get my head around the pins LT, BI/RBO and RBI, and also how would I make a counter using J-K flip flops.
Thanks so much everyone for the help :)
 

MrChips

Joined Oct 2, 2009
21,679
/LT = Lamp Test is used to check that all the LED segments are working. Connect /LT to GND and the display should show 8.

BI/RBO and RBI are used when you have multiple 7-segment displays and you want to blank leading zeros.

BTW - connect with our good member @R!f@@ and he might be able to give you personal assistance.
 
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