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7 segment LED counter

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ttebeau

New Member
I have purchased and bread boardedd a circuit which is made up of 74ls90, BCD counter, 74ls47 seven segment display driver, and some 470 ohm resistors, and 7 segment common cathode displays made by Kingbright. I found this circuit on Aaroncake.net /circuits/counter.asp. I already have over $140 invested in this. I get the numbers to come up but they are random, and sometimes I get random letters. This circuit is to replace a shuffle board game that had 2 PC boards wiped out by 110 volts.
I need some help in getting this thing to work properly, so I can press on with getting the shuffle board back on line.

I hoped this is the way to get this problem out there.

Thanks
Tom
 

audioguru

Well-Known Member
Most Helpful Member
The 74LS series of ICs were used in the seventies. 32 years ago!
Like any counter, they count every bounce of the contacts of a switch, pushbutton or relay. The contacts bounce many times per push.

Look in Google for a Debounce circuit.

None of Aaron's circuits have a very important supply bypass capacitor. Most old 74LS ICs need a 0.1uF ceramic disc supply bypass capacitor each.
 

k7elp60

Active Member
The first error I see is that you stated the displays are common cathode. The74LS47 is to drive common anode displalys. The circuit shown will come on with a random number showing because the 74LS90 is not reset to 0 on the application of power.
What circuit are you using to trigger the counter? It must be bounce free and TTL compatible.
 

Torben

Well-Known Member
Audioguru and k7elp60 have already answered your question. I'm just going to expand on that a bit:

That circuit as posted is just a basic textbook-style outline--it doesn't contain everything you need to actually make it work properly in the real world. If you search this site you will find a lot of other people have had trouble with it for just that reason.

o You need a bypass capacitor on the power pins to get rid of power-supply spikes caused when the counter counts. Otherwise the thing can get false input triggers. Check page 554 in the Art of Electronics for more information.

o You really do need a debounce circuit on the input--even a good switch will make and break contact several times over the first millisecond or so after being switched before it settles down. A debouncer just turns this into one nice transition from On to Off (or vice versa). Read this thread for more information and instructions on how to do it.

o When you turn the counter chip on, its state at the moment after power is applied is undefined unless you give it a reason to reset to 0. Check out this thread for more information. In particular, the second-to-last post in the thread (by Sebi) shows how to wire it a power-on reset circuit to the 74LS90.

Hope at least the links help. I found them (and this board) very helpful when working on my foosball table last year. I keep meaning to clean up my finished schematic and post it with some pictures. :)


Good luck!

Torben
 
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