I've searched the forums and there is a thread with this exact title and a very similar question already posted, however the person only reported they figured it out and did not share his method.

I'm trying to build a countdown clock, I'm at step one which is simply getting one of the circuits to run. I can't get the counter to work at all. I'm using multisim 9 educational, I've attached the diagram of what I have so far. I've pulled the count enable line to high and low with no result.

Any advice is greatly appreciated.

Attached Images

clock3.jpg (180.5 KB, 56 views)

 

Pin 4 enable needs to be low. The load terminal needs to held high and brought low for an instant.
I think you need to put the 220 ohm resistors to Vcc and let the dip switch pull the appropiate load terminal low.

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Thanks for the reply!

So is what I have for the most part correct? I'm attaching my latest revision.

A couple questions:

(pin 11) How would I go about setting the load to low and then holding it high? I'm not sure how to do that.

(pin 12 MIN/MAX) Should I do anything with this? I'm not quite sure what it is for.

Attached Images

clock4.jpg (198.9 KB, 31 views)

 

So this circuit now works from:

99:99:99

But I can't get it to work for:
23:59:59

Again any help anyone can offer is greatly appreciated.

I modified it further an can now get it to set a time to count down. I need to figure out a way to have the div counts start back from what they are set. Currently it counts 5,4,3,2,1,0,9,8,7,6,5,4,3,2,1,0,9... (where it starts at five)

Attached Images

clock6.jpg (185.7 KB, 22 views)

 

When counters are cascaded like this one of the outputs can be used to load the counters. I am not sure how to do it, perhaps another forum member can be of help. You do need some despiking capacitors of 100nF on a working circuit.

__________________
The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.

 

You are using all digits to divide by 10 when some should be dividing by 6. Think about it.

 

I've got it so it sets the time now, so I can set it to whatever I want. But you've hit the nail on the head, everything I've tried to do to do a div6, div5, and div3 just mess up the circuit. I tried putting in a D Flip flop, and I've tried logic gates but I can't get any of it to work. If anyone can help lead me down the right path I'd greatly appreciate it.

I think that for the first div6 I need the logic to do the following:
The 74LS190 U4 chip -- when MIN/MAX (pin 12) goes high it means that it has reached zero, so I need to pull the LOAD (pin 11) to Low without pulling the whole line that is connected to the J7 switch. The J7 switch is being used to set the number of hours, minutes and seconds. When it pulls the line to ground the numbers from the bottom row of switches are loaded. Then when I put it back to high the circuit counts down. (with the exception of the div by # not working.

I just want to figure out what I'm doing wrong...

I've tried the following with a D flip-flop, but it messes the circuit up. It doesn't count correctly and I have to completely disconnect the set line (Switch J7).

Attached Images

DflipFlop.gif (4.0 KB, 17 views)

 

I think I may have used the wrong chips for this. I'm not quite sure how to do a div6 while still being able to preset the numbers. I'm beginning to think it is 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 J7 Preset/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.

 

Thanks, I'll give this a try this weekend.

 

I tried following this, but I get lost when trying to connect all the inputs. I hate to ask but is there any way someone can illustrate this? I've changed the clock to what I believe is synchronous, so now what do I do instead of the D FF?

Attached Images

clock15.jpg (188.6 KB, 10 views)

 

I thought I had this figured out, but now it counts like this: 15-14-13-12-11-10-9-58-57-56...

I have no idea why.

Attached Images

clock16.jpg (195.4 KB, 3 views)

 

I modified this back to the origional circuit you looked at, I've tried everything I can think off with the clock input of the D FF, I put in a second clock at 2Hz, I've connected it to everything I can think of and it doesn't count quite correctly. It will either not count down the tens of seconds or it will miss a time, it counts the 5 twice when it flips. 00-59-58-57-56-54-53-52-51-50-59-58-57-56-54-53-52-51-50-49-48...

I've attached what I've managed to get done, but again I'm not quite sure what to do with the clock input line. There is one other issue here as well, when you pull J7 high, it overides the time you input into the switches, just wipes it right out and pushes the time to a 5.

Attached Images

clock17.jpg (204.5 KB, 7 views)

 

The 2 Hz clock should go to the FF CLK, but that's assuming the main clock is 1Hz. You're showing a 200kHz clock. Is that just for test purposes? The FF clock should be twice the input count clock so for a 200kHz input clock, the FF clock should be 400kHz. Also the two clock edges must be simultaneous i.e. the 1Hz clock goes high with the 2Hz clock edge.

 

I have the clock ramped up for testing.

I thought that doubling the clock may be what you meant, so I tried it with 400kHz and it doesn't work.

I'll attach a diagram of the circuit, basically it counts 5 for the tens of seconds twice and does some really strange stuff with carrying through to the minutes place. It's kind of hard to explain. I'll attach my mulitisim file for anyone who may want to have a go at this. I'm about to throw my hands in the air, I've spent an awful lot of time on this and it's just for my own purposes. I recently got my hands on a copy of multisim so I thought I'd see what I could remember from college physics, which is apparently very little.

Attached Images

clock21.jpg (197.4 KB, 8 views)

Attached Files

countDownClock.zip (313.2 KB, 15 views)