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2 minute countdown timer using 74190N

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Mark Leghrand

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Hi! Can anyone here help me about my problem? I already had my circuit diagram which displays 2:00 at start up. The problem is, the digits will count
(for2)
2 1 0 9 8 7 6 5 4 3 2 1 0 9 8.....and so on. When it reaches 0, the next digit beside the 2 becomes 9.

you know, I want for my circuit is to be a real count down timer. Please help me. Thanks.

Here's my schem...
 

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I don't understand. I think it works like this:
2:00, 1:00, 0:00, 9:90, 8:90, 7:90, 6:90,
I think you want it to go:
2:00, 1:59, 1:58, 1:57, 1:56......
Your 'clock' goes into the minute counter while it should go into the second counter.
 
hi Mark,
Any reason why you have chosen the Ripple clock rather than the Min/Max for clocking.?

From your description you appear to have a double clock pulse at each cascade clock down.
E.
 
I don't understand. I think it works like this:
2:00, 1:00, 0:00, 9:90, 8:90, 7:90, 6:90,
I think you want it to go:
2:00, 1:59, 1:58, 1:57, 1:56......
Your 'clock' goes into the minute counter while it should go into the second counter.
indeed! Unfortunately the circuit worked like that. 2:00, 1:00, 0:00, 9:90, 8:90, 7:90, 6:90,.....
Yes I will immediately modify the circuit (clock). but How will I be able to command the the tens digit to stay at 0 when it already passed through
0-5-4-3-2-1-0. For instance, the timer is already at this state, 0:09 to 0:00.
 
hi Mark,
Any reason why you have chosen the Ripple clock rather than the Min/Max for clocking.?

From your description you appear to have a double clock pulse at each cascade clock down.
E.


May I ask what is the Min/Max clock for? I actually can't understand the data sheet
 
hi Mark,
It 'pulses' High at the 9 to 0 count and also on the 0 to 9 count.
Is easy for you to swap the RCO connection over to the Min/Max pin.?
E

EDIT:
You appear to be clocking the MSD [ the 2's 74190], you should be clocking the LSD [seconds units] for a Down counter.
 
hi Mark,
It 'pulses' High at the 9 to 0 count and also on the 0 to 9 count.
Is easy for you to swap the RCO connection over to the Min/Max pin.?
E

EDIT:
You appear to be clocking the MSD [ the 2's 74190], you should be clocking the LSD [seconds units] for a Down counter.


What I did is that I connected the the Max/ Min to the clock of the next counter.

BTW what is LSD?
 

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LSD , is Lowest Significant Digit example: 2.00 [ the Red is LSD , Seconds units]

The Clock line should be input to the LSD 74190 Clock pin.
The output min/max pin from the LSD should goto the Clock input on the seconds Tens, the min/max of the Sec Tens should goto the Minutes clock input .

EDIT, so like this
Minutes << Ten Secs << Unit Sec << Clock source input
 
Connect this way, sorry about poor quality drawing mods.
The clock needs to be 1Hz.
 

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LSD , is Lowest Significant Digit example: 2.00 [ the Red is LSD , Seconds units]

The Clock line should be input to the LSD 74190 Clock pin.
The output min/max pin from the LSD should goto the Clock input on the seconds Tens, the min/max of the Sec Tens should goto the Minutes clock input .

EDIT, so like this
Minutes << Ten Secs << Unit Sec << Clock source input


Alright sir I now get it. The second issue now is the seconds tens. Originally, 0 is displayed. I want it to be 5 when the countdown starts. Do I need to add something?
 
him
Do it the same way as you did the '2' on the Minutes digit.

Hold pins B and D High on the Secs Tens 74190 [ like pin B on the Minutes]
 

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Isn't it 1010? like what I did in the 2 minutes sir, i put B on high so 0100.

Woops! Sorry Mark.:confused:

I dont usually put the parallel pins in ABCD I use DCBA... you are correct

EDIT.
Its conventional to write decimal '5' as 0101 , so I draw as D,C, B, A, where 'A' is the LSD
 
Hmm sir I did what has to be done. Yes setting A and B high gives me 5 at start-up but I want a zero then it will go 5 on start of the coutdown.
it should be 2:00 then 1:59
 

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that's very fine sir Eric. :)


sir I just read this.
"Doing the Impossible

Not impossible, just difficult. Difficult we do right away. Impossible just takes a little longer.

You could use a Quad 2-Input Mux such as the 74LS157 to set the counter values for the divide by 6 counters. The Mux SELECT input is controlled by the Load switch. This allows selection of either the load switch values or the divide by 6 values for the Counter Preset. (The Mux is not needed for the counters that go to 0 and rollover to 9.)

The Mux Select input goes to J7. The Mux I0 DATA inputs go to the Counter Preset Switches and the Mux I1 data inputs go to the desired count preset value for a divide by 6. The Mux outputs go to the counter DATA inputs. In the J7 load position (Mux Select Low), the counter presets are thus connected to the switches. In the J7 run position (Select High) the presets are set to the hardwired values to cause the proper divide ratio. For example the 10's minute digit would be loaded to binary 5 on the next count after it reaches zero.

Since the counter load control is asynchronous you will need a D flip-flop to delay the counter load signal until the count following 00 for the counters you want to divide by 6. Otherwise it would instantly load 5 when it reached 00. Connect the RCO signal from the counter to the D FF input and the FF Q output to the counter LOAD input. The FF clock should be 2Hz for proper sync of the LOAD signal.

You will also need to add 2-input AND gates at the divide by 6 counter LOAD inputs. The AND inputs are the FF output and J7 so either can control the LOAD function for the divide by 6 counters.

Hope you can follow all that. I could draw if for you but what's the fun in that. As they say in the textbooks, that's left as an exercise for the reader. It's not a trivial addition to the circuit but, I think, doable."


can anyone guide me how to do this? Whats wrong with my circuit? The tens digit "0" isnt now working anymore.

here's my work
 

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hi Mark,
I have been working on the problem for a while since we chatted and I can see no easy solution, it gets very messy.

E
 
BTW what is the difference between the ansynchronous and syncronous coounter?
All to do with the clock... Asynchronous has cascading clocks, whilst synchronous are clocked all at the same time.
 
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