My Simple Scoreboard - Need help.

[mihailok]

Hello, I am working on building my own scoreboard, but require some help. This is basically how I want the end product to function: . I'd like to make the 7 segments about 6 inch's high, the power to run off the mains and was thinking of using buttons as used on a pinball machine.

I have been browsing threads here and found 'this' schematic by hdc090360 that I think should suit the purpose, but it just needs to be tweaked to the specifications.

So far, I have altered and got a good understanding of how the scematic works and have come up with so far in multisim. I haven't added the second group of lights as I figure its just a matter of attaching the CLR's (on the 74LS192N's) to the reset switch. As you can see, I have left out the power supplies and the values of the resistors. Because, simply I don't know how and was hoping you can please help with that?

Also to note, I found 'this' design for the LED's I was hoping to achieve. The one I have drawn is just resembling what I wanted to achieve. I was thinking maybe it could be similarly linked up like **broken link removed**?

Thank you for your time and consideration,

Michael

 

[alec_t]

R1,R2,R18 only provide a 1ms time constant when used with 1uF. They should have a higher value (say 30-50k) for better contact debounce.
74LS logic outputs can only sink 8mA. Will that be enough for your LEDs?

 

[mihailok]

Hi alec_t, thanks for the reply. Regarding the resistors, I was just using the suggestion made in the previous threads. So thanks on correcting that. I shall be obtaining the LED's over the internet in the UK, so if you have a suggestion on which to use, that would be great. Obviously the brighter the better. But, do you believe there are better options than using the 74LS logic for this project?

Thanks again!

Michael

p.s. for the design of the lights I sent earlier, they used 14 lights per segment, so one number holds 98 bright LED's of 5mm.

 

[alec_t]

Further research indicates that although 74LS in general has 8mA outputs, the 74LS47 is an exception and can sink 24mA. Sorry if I misled you. I can't recommend any particular LEDs. Red LEDs in general have a Vf of ~1.8V-2.0V (other colours have a higher Vf). The 74LS47 output transistors can withstand up to 15V apparently, so if the LED supply voltage were 15V the maximum number of LEDs in series that each ouput could drive would be 15/2 = 7 (but that would leave little headroom for current-limiting). So you would need 2 strings of LEDs per segment, which would restrict current to <12mA per LED.

 

[mihailok]

Doesn't sound like good news bro. But thanks all the same. What are my other options? I am not limited to using those chips. Thanks again.

 

[dougy83]

You can buffer each output of the 74ls47 ICs with a PNP transistor in emitter-follower configuration. You'll not have any issues with output current then

 

[alec_t]

I agree with dougy83. You would still have to be careful not to exceed the 74LS47 output transistor voltage rating.

 

[mihailok]

You can buffer each output of the 74ls47 ICs with a PNP transistor in emitter-follower configuration. You'll not have any issues with output current then

So Dougy83, the buffer given will allow for a stronger output to run the LED's? Wouldn't that require a transistor for A,B,C,D,E,F,G on each of the 74LS47N's?

p.s. im sorry for my lack of terminology and knowledge, im trying

 

[dougy83]

Yes, it requires a transistor for each of the outputs of the 74LS47s

 

[mihailok]

Something like this?

 

[dougy83]

It's not what I was referring to, but yes, something like that will work.

 

[mihailok]

Hang on a moment, wouldn't that require being an NPN formation for how I have drawn it? being a current from the 74LS47 signalling the 12V current to pass to the LED's?

 

[dougy83]

Hang on a moment, wouldn't that require being an NPN formation for how I have drawn it? being a current from the 74LS47 signalling the 12V current to pass to the LED's?

The output of the 74LS47 is an open-collector transistor. When it is on (LOW), it pulls the output towards 0V, which will turn on your PNP as you have drawn it.

 

[mihailok]

Thanks dougy83, I shall get drafting right away with the info you guys have given and see what I can come up with.

 

[mihailok]

Ok, so I have simplified it a lot. But the power source to the 74's can be provided by 5V now?

 

[alec_t]

You're nearly there. Each LED string should have its own current-limiting resistor. Without that, because of LED manufacturing tolerances, you might find that different strings have quite different brightness. And yes, the 74LS logic needs a 5V supply.

 

[mihailok]

You're nearly there. Each LED string should have its own current-limiting resistor. Without that, because of LED manufacturing tolerances, you might find that different strings have quite different brightness. And yes, the 74LS logic needs a 5V supply.

Thanks alec_t, that makes sense. What I shall do now is finish drawing it all up and come up with a specifications list. Then, I will need some help with working out the values for the resistors and other components. But, I am really happy to see it all progressing well and that I am learning so much. Very big thanks.

 

[mihailok]

You're nearly there. Each LED string should have its own current-limiting resistor. Without that, because of LED manufacturing tolerances, you might find that different strings have quite different brightness. And yes, the 74LS logic needs a 5V supply.

Hoping I am on the right path here.

 

[alec_t]

Well each string now has 7 LEDs, so the 12V supply isn't high enough

 

[mihailok]

Alec - you just had to go and pop my bubble, hehe. Wouldn't I need to base the voltage anyway on the type of LED's I shall be using? Like if I chose one of a higher MCD? Thanks.