# Building a digital scoreboard

Discussion in 'General Electronics Chat' started by ohmer fudd, Mar 25, 2008.

1. ### ohmer fudd Thread Starter New Member

Mar 25, 2008
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I have been considering building a seven segment led display for keeping score when playing horseshoes. I would like to build the display using 5 leds per segment which would be 35 in total when all the leds were in use. This would draw about 700ma. I have a 7805 voltage regulator that could be used for the power source. I have been reading about circuits & components that are used for a regular seven segment led display, but I have a feeling that they will not work for the setup I am talking about. Any information about schematics, components that I should use for this setup would be greatly appreciated. I am a real newbie. Thanks in advance for any help I may receive.

2. ### aurelien.pean New Member

Mar 25, 2008
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You can read some datasheet on radiospare site or others. A 7805 is a good solution it seems. Hoping you will manage, but it is not really difficult.

3. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
What voltage would be 7805 be working from? There is a gotcha waiting for you, in that each LED takes at least 1.5 volts to turn on. 5 in series will need more than 7.5 volts to operate. Moreover, they have no resistance to current once they start to conduct, so some control of current is going to be necessary. A resistor may do fine for that purpose.

4. ### SgtWookie Expert

Jul 17, 2007
22,201
1,807
Well, for each segment, instead of connecting 5 LED's in parallel, you could connect them in series. If you were using super-bright red LED's rated 20mA @ 1.9Vf (forward voltage), you would have a Vf(total) of 9.5. You would then be better off with a 12V supply, and with all segments on you would need 100mA of current.

In order to ensure a 20mA supply for your LEDs, you calculate (for each string of 5 LED's):
Rlimit = (VoltageSource - Vf(total) ) / CurrentRequired
Rlimit = (12 - 9.5 ) / 20mA
Rlimit = 2.5 / 0.02
Rlimit = 125 Ohms.

For the rest of your circuit, take a look at these CMOS IC's:
4553 - CMOS 3-Digit BCD Counter
http://www.alldatasheet.com/view.jsp?Searchword=MC14553B
4543 - CMOS BCD-TO-SEVEN-SEGMENT LATCH-DECODER-DRIVER FOR LCDS
http://www.alldatasheet.com/view.jsp?Searchword=MC14543B
The 4543 is capable of driving both LCDs and LEDs directly.
These CMOS ICs are made by various manufacturers. Using these two together will greatly simplify your circuit - and you can run them from 12v.

5. ### ohmer fudd Thread Starter New Member

Mar 25, 2008
3
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Thanks beenthere for the quick response. The output voltage of the 7805 would be 5v. I would like to have the 5 leds be in parallel which should draw about 20ma per led, with the resistors that I am using. My problem is which ICs should I use. Some sort of schematic would be great if I could find one. As you probably know by now I am very new to this, with just enough knowledge to get into trouble.

6. ### ohmer fudd Thread Starter New Member

Mar 25, 2008
3
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Thanks SgtWookie;
Good stuff & thanks for the info. I like your idea of using 12 volts better as this would eliminate running power to the horseshoe pit. Since I would only be using 2 sets of 2 digits for the scoreboard I guess a cmos 2 Digit BCD counter would do. You woudn't have access to a schematic that I could use just to keep me on the right track.

7. ### SgtWookie Expert

Jul 17, 2007
22,201
1,807
Well, you could just use the 4553/4543 and not connect a 3rd display.

You could also use a CD4518B dual BCD up-counter, with a couple of CD4055 BCD to 7 Segment Display drivers.

There are schematics for constructing such circuits right in the datasheets.