# digital clock circuit

#### nikon

Joined May 3, 2007
17
Hi you all,
I learn Electronics at school and I found it very intersting.
I decided to improve my skills so I took on myself a nice project.

Building a digital clock.
How ever , I dont want to use ready chips.
Only the basic compontes: Flip Flops,gates and such.

As a start I tought how to manage the first numeral seconds:
(I mean:If I have the "53" seconds, I wish to show only the 3 )

4 T FF
T will get +Vcc.
Decoder BCD to 7SEG

So please take a look at the picture that attached to the post
and answer on the questions that I wrote.

Thanks.

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#### recca02

Joined Apr 2, 2007
1,214
the image isnt clear and my ideas abt pinout diagrams aint clear either
so do you wish
to use mod 6 counter for ten's position?
you can use the 10th count of the decade counter to drive the mod 6 counter.
which in turn will drive similar type of ckt for minute for its 6th count.
similarly it minute will drive the hour's ff circuitry (mod 3 and decade counter) is this what you plan on doing?
do you plan on using six 7 seg displays?
u may also need to have a presetting circuit to set the clock to a particular time.

#### nikon

Joined May 3, 2007
17
I saw its too big on me so I decided to do only the units of the seconds.
That means:
1 x7SEG
4 xT FF

I added an "OR" gate in order to prevent "printing" "10","11","12","13","14","15".
So when the counting getting to 10 its reset immediedtly.

It likes:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 .....

If something is missclear please tell :]

#### recca02

Joined Apr 2, 2007
1,214
there shud be an input from a clock pulse generated by a timer ckt say
555 timer to the first ff. (an astable multivibrator with freq of i hz shud do the trick if u want
actual seconds counting at the display).
for the supply connections see the pinout dia for ground and common etc,
for the bcd to seven seg decoder.

some of the pins in the decoder like ripple blank output (i think thats what it stands for not sure though) might not be needed for this case.

#### lightingman

Joined Apr 19, 2007
374
Yes that is rite..You will need a clock input of 1 Hz to the first stage of the set of FF's.As for the decoder, it depends what one you are using.Generaly if there is an LT pin, that is lamp test and will display all segments, if there is a line over the LT it means that the input is "active low" I.E. it will light up all segments when it is pulled to logic "0".Yes the RBI pin is the riple blanking input, and should be tied to logic "1" if it has the line over it, or "0" if it does not.If the decoder has an LE pin on it, that is the latch enable pin.Hope this helps.Daniel.

#### nikon

Joined May 3, 2007
17
could you add a picture of the circut?
And how can I put a 1HZ input to the first FF?

#### Salgat

Joined Dec 23, 2006
221
Although you can use a 555 timer for the 1Hz frequency we want some actual accuracy coming out of this thing. Go for a Crystal Oscillator and use voltage dividers to get it down to 1Hz(decade counters are good for dividing the frequency of a pulse by 10). For my clock I use 74HC390 decade counters and just tied the frequency into the input, and tied Q8 bit to the the next input(since the 8s bit will only pulse once for every 10 pulses inputted, from low to high when it hits 8 and from high to low when it resets to 0). I've attached a crystal oscillator thats using a 1MHz signal. You can use 22pF capacitors or replace one with a trimmer cap to fine tune the frequency. Be sure your OR NOT gates are CMOS, I believe it is the 4001.

EDIT: Also use a Decade counter to avoid having to add extra gates, a 7447(decoder/driver) and a 7490(decade counter) for counting your digits. I attached one of my old schematics I made, you'll have to fix how the two digits connect, how the NANDS connect is wrong.

EDIT #2: I just fixed it, it should be all correct.

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#### nikon

Joined May 3, 2007
17
This was what Im talking about, however instead of "74LS47" and ""74LS90"
I nees to use FF and logic gates and a "pulse giver" that I will connect to a
clk input of the first FF.

*By the way, with which prog. you paint this circuit?

#### nikon

Joined May 3, 2007
17
Yes but can yo show me a circut that do the exacly same thing just with
T FlipFlops and Pulse generators (of course that I can use also a BCD TO 7SEG DECODER?

I know that I need to use an OR gate ,at the seconds units(0), that will check if the sum of the
"Q"'s wil be 1010 (means 10) and if it is:
reset and add a pulse to the seconds tens (10).

I will thank you very much if you could help me before tommorrow night.

#### nikon

Joined May 3, 2007
17
very Sorry on edit but after 3 edits it started to stuck to me
my only request: Is my circuit (That I attached) is ok and aspose to work?
and now how much Flip Flops I need to use to the Minutes Units and to the Minutes Tens?
It has to be less then 4 I think, but how much exacly....?
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#### Retiredguy

Joined Feb 24, 2007
28
Looking at your circuit, it seems to me as though the seconds part of your circuit should work, however you do not need the FF (U6A) for the ten's since it will never be used. This only counts to 6 or 0110 binary before it resets. The minutes part of your circuit should be the same as the seconds, it will only count to 59 before it resets, like your seconds. take the reset from your 10's seconds and use it as the clock for the minutes part of yout circuit. So you will need another 7 FF's For the minutes section. When you get the minutes section built, use the reset from the 10's as the clock for the hours part and for this you will only need 5 FF's ( 4 to count to 10 and one more for the ten's if you are making a 12 hour clock)

#### nikon

Joined May 3, 2007
17
I have a little prob with the hours.
You was right that is not neccesery to use 4FF in the Minutes and Seconds Tens.(only 0-6, so the max is 111 = 3FF).

Well in the hours i need to do a 24 clock.
01 11 21
02 12 22
03 13 23
04 14
05 15
06 16
07 17
08 18
09 19
10 20

As you can see the Units of the HOURS is used in different times.
for example: 01,11,21 - they have the same Hours Unit
so how can I know for sure when its 01 and when its 11 and when 21?

Its seems to be very hard.

#### recca02

Joined Apr 2, 2007
1,214
why do u need to know the hours units,
i mean from what i understand you wud again be having a unit's counter separately and tens separate along with separate displays.
u only will need units counter to drive the ten's which wud only have to count 3 states. (i havent analysed your ckt so i may be wrong here)

( btw this is my advice to every one that they keep refreshing the page every once in a while, i ve seen ppl having been given reply
but they didnt even know unless they logged in again after hrs or even days )

#### Retiredguy

Joined Feb 24, 2007
28
OK, To make a 24 hour clock, for your hour's section, you would need to design your circuit with one more FF, 4 for the single hour which will count 0 through 9 and two more to count 0 thru 2 for the tens of hours similar to the minutes section. You could then use logic gates to reset your whole clock when your clock strikes 24:00

#### nikon

Joined May 3, 2007
17
At the ends I decided to do a 12 AM\PM clock.

I added a STOP option and CONTINUE.
a RESET option seems to be problemastic.

#### nikon

Joined May 3, 2007
17
*BUMP*

Can you may give me a little clue about what should I connect to the
first FF's CLK instead of the currenct wire?
Please dont ask why I need a replacement and only give me a clue.

I repeat, only a little clue,(I want to think by my own) because I tried bazilion options but I dont have

#### recca02

Joined Apr 2, 2007
1,214

and first ff wud be?

is it the first ff for hour's counting? again is it for units or tens?

#### nikon

Joined May 3, 2007
17
The Seconds Units first FF CLK connected to a PulseGenerator.

But what should I connect to the Tens Seconds first FF clk?
(and not "connecting the wire that checks if the Units Secs = 10",
im looking for another option because Ive been told that its not allways works)

If you have another solution please just give me a clue.

#### recca02

Joined Apr 2, 2007
1,214
i thought it shud work(have u tried it?),
right now i cant think of another way,
perhaps somone else can,
good luck.