I have a project to create a stopwatch using some cmos ic. i use cd4060 and cd4027 as my 1 hertz clock and using 4 cd4026 as my counter and decoder to my 7 segment display. My circuit supposed to have 99 minute count maximum. But in my case now after it reaching 9 minute 59 second, they all back to zero. Can someone tell me what is wrong with my circuit?

 

hi,
Do you have an oscilloscope, if yes, check the following.
When SM LED reaches count of '5' , U5 pin #5 should go Low, ie; 0V
Keep clocking and when SM passes thru the '9' to '0' roll over, U5 pin#5, should go High ~+5v,
U6 pin#1 should follow U5 pin#5.

Let us know what you measure.

Does the PM LED always display '0' ?

BTW: I see no capacitive power rail decoupling on the circuit diagram.?

E

 

I have a project to create a stopwatch using some cmos ic. i use cd4060 and cd4027 as my 1 hertz clock and using 4 cd4026 as my counter and decoder to my 7 segment display. My circuit supposed to have 99 minute count maximum. But in my case now after it reaching 9 minute 59 second, they all back to zero. Can someone tell me what is wrong with my circuit?

That sounds like the Carry Out of U5 goes somewhere other than the schematic shows. Either Reset or maybe power.
Is the Master Reset line grounded? Pushing the Reset button does what?

 

hi,
Do you have an oscilloscope, if yes, check the following.
When SM LED reaches count of '5' , U5 pin #5 should go Low, ie; 0V
Keep clocking and when SM passes thru the '9' to '0' roll over, U5 pin#5, should go High ~+5v,
U6 pin#1 should follow U5 pin#5.

Let us know what you measure.

Does the PM LED always display '0' ?

BTW: I see no capacitive power rail decoupling on the circuit diagram.?

E

Okay i did what you instructed eric but i use my volt meter because i don't have oscilloscope. The carryout from U5 apparently always high even when the minute displaying 5-9. The input clock from U6 has the same condition, always high.

Then i notice that in my circuit PCB, the conductor track for pin 3 (DC Input) of U6 was adjacent with conductor track for carryout of U5 that somehow making them short circuit. So after i clear some conductor track between them my circuit is working now. Thanks for pointing out how the CD4026 carry out work.

And yeah my circuit doesn't have capacitive decoupling, is that necessary for my circuit? And what exactly it does?

That sounds like the Carry Out of U5 goes somewhere other than the schematic shows. Either Reset or maybe power.

the carry out is going to the input of U6 only, but as i say before that it was adjacent with the DC input track in my PCB and making them short circuit

 

something likely rests the whole thing. the biggest suspect is U5's CO pin.

U4's CO pin is not reliably connected.

 

something likely rests the whole thing. the biggest suspect is U5's CO pin.

U4's CO pin is not reliably connected.

yeah the U5 co pin is the main problem here. BTW why is U4 CO not reliably connected, is it because i put a 10k Resistor there?

 

Then i notice that in my circuit PCB, the conductor track for pin 3 (DC Input) of U6 was adjacent with conductor track for carryout of U5 that somehow making them short circuit. So after i clear some conductor track between them my circuit is working now. Thanks for pointing out how the CD4026 carry out work.

And yeah my circuit doesn't have capacitive decoupling, is that necessary for my circuit? And what exactly it does?

hi denny,
Pleased you have found the problem.
Decoupling of the power rails is required in order to suppress electrical noise on the power rail, generated when adjacent devices switch On/Off, entering another device via its power rail connection.
Noise can cause random problems which may make the circuit malfunction under certain conditions,,, always fit decoupling.
E

 

hi denny,
Pleased you have found the problem.
Decoupling of the power rails is required in order to suppress electrical noise on the power rail, generated when adjacent devices switch On/Off, entering another device via its power rail connection.
Noise can cause random problems which may make the circuit malfunction under certain conditions,,, always fit decoupling.
E

Hmm i never know that. Where do you put the capacitor then, on the vcc? And how many farad do i need?

 

hi,
It is usual to have say a 100uF and a 100nF near to the power supply regulator.
The power rails should be decoupled near the actual IC's, a 100nF cap, say every 2 or 3 inches along the rails is typical.

On some fast switching circuits it is wise to have a decoupling cap close to every IC, typical values are 47nF

E

 

hi,
It is usual to have say a 100uF and a 100nF near to the power supply regulator.
The power rails should be decoupled near the actual IC's, a 100nF cap, say every 2 or 3 inches along the rails is typical.

On some fast switching circuits it is wise to have a decoupling cap close to every IC, typical values are 47nF

E

So i need to put 100uF and 100nF from the power supply. Then 100nF on every ic i have in my circuit right? (in my case 6 capacitor for 6 ic)

 

So i need to put 100uF and 100nF from the power supply. Then 100nF on every ic i have in my circuit right? (in my case 6 capacitor for 6 ic)

hi,
I would do that, the use of 47nF [at least 6V working voltage] would be acceptable, usually they are physically smaller.
E

 

Hmm i never know that. Where do you put the capacitor then, on the vcc? And how many farad do i need?

The caps are a good idea, but, as you can see, it isn't always necessary. CMOS is pretty tolerant of noise on the power lines. TTL stuff was harsh on power lines and the caps were a necessary habit. If you need them, use them. If not, don't. From what you have said you don't have a problem you can attribute to no caps.

 

hi denny,
This link is to cap decoupling document, CMOS as well as TTL require decoupling.
http://www.brighthubengineering.com/diy-electronics-devices/56501-how-to-decouple-a-logic-circuit/

For other information Google: digital CMOS ic decoupling

E

BTW.
As your circuit is driving 7 segment LED's, which require a 5mA to 15mA segment operating current, it is more important that you have decoupling.

 

hi denny,
This link is to cap decoupling document, CMOS as well as TTL require decoupling.
http://www.brighthubengineering.com/diy-electronics-devices/56501-how-to-decouple-a-logic-circuit/

For other information Google: digital CMOS ic decoupling

E

BTW.
As your circuit is driving 7 segment LED's, which require a 5mA to 15mA segment operating current, it is more important that you have decoupling.

Technically good advice, of course, but his circuit appears to work fine such as it is. I agree it wouldn't hurt, but it might need him to make a new board.

 

hi hp,
This is one of the pitfalls of not decoupling, it will probably work OK most of the time without it, but it has the potential [ pardon the pun] to malfunction at a critical moment.
E

 

The master reset signal is tied to GND, probably screwing up the conditional resetting around U4.
Any of the 8 unnecessary connection dots might actually be non-connections, causing other problems.

ak

 

hi hp,
This is one of the pitfalls of not decoupling, it will probably work OK most of the time without it, but it has the potential [ pardon the pun] to malfunction at a critical moment.
E

In theory you are correct. In reality his circuit does work.

 

In theory you are correct. In reality his circuit does work.

Yes, when operating in the environment existing at this time.
As the Components, Temperature and the Environment change over time, it may not always be the case, so fit decoupling caps.

E