# LED counting circuit

#### Sparky49

Joined Jul 16, 2011
833
Hi all,

after a wee break, I'm back with more questions for you!

I'm try to design a simple money box where a push of a button adds ten pence to a total amount.

To do this, I'm planning to using a monostable circuit (to avoid 'bounce') which provides an input to a clock on a counter. This then counts up to nine, until the next 'input' when it resets itself to zero, and counts 'up one' on the next counter.

I'm trying to avoid micro controllers, as I'm not experienced enough for them yet.

The question is, would the circuit I've drawn work?

Could anyone recommend a good counting IC?

Sparky

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#### RiJoRI

Joined Aug 15, 2007
536
CR = Chip Right
CM = Chip Middle
CL = Chip Left

(1) CR will count from 0 to 8. Upon the next pulse, the AND gate output will go HIGH, resetting CR.

(2) What happens when CM's AND gate goes HIGH? Do you see the signal going not only to the reset pins, but also to the clock input? You may have a race condition here.

(3) It's been several millenia since I worked on this type of stuff, but try looking for a decade counter with a carry output. Feed the carry signal to the clock input of the next chip.

HTH,
--Rich

#### SgtWookie

Joined Jul 17, 2007
22,230
The 4040 outputs binary counts; you really need decimal.

If your output will be 7-segment LED's, consider using 4543's and 4553's. The 4553 is a 3-digit decimal counter, and the 4543 is a decimal to 7-segment display driver.

Here is a link to OnSemi's datasheet for the MC14543B:
Here is a link to OnSemi's datasheet for the MC14553B:
On page 6, figure 5, there is a schematic for a 6-digit counter display using two of each of these ICs, plus some transistors and resistors.
You can simply use 1/2 of the schematic for a 3-digit display. Connect the strobe (LE) to ground. Keep RESET (MR) low unless you wish to reset the counters to zero.
This is a decimal up-counter only. You'll be hard-pressed to find another two-IC combo that will display up to 3 digits.

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#### Sparky49

Joined Jul 16, 2011
833

Sgt Wookie, may I ask, how doyou find suitable ICs? Is it just a wide knowledge of different ICs, or is there some sort of search engine for ICs?

Thanks.

#### SgtWookie

Joined Jul 17, 2007
22,230
Well, let's just say that I've been doing this for a few weeks longer than you have.

However, there are various parametric search engines where you can just put in the function that you want, and various options will be shown.

Texas Instruments has (or used to have) downloadable Excel spreadsheets for various logic IC lines. They are pretty handy, as you can use the Search function to find the features you want, and the spreadsheets have embedded links to datasheets in them.
Here's a link to their Search page; I just started off by entering CD4 to get their CD4000 parts.
http://focus-webapps.ti.com/general...eratureRange=-1&noOfResults=100&Search=Search
You could just as well plug in 74HC to get high-speed CMOS TTL IC's, or whatever combination you wanted.
Digikey has a pretty good parametric search engine. Fairchild Semiconductor does as well.

#### nDever

Joined Jan 13, 2011
153