Author Topic: digital clock with an alarm  (Read 1084 times)

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Offline Rajan ShahTopic starter

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digital clock with an alarm
« on: July 20, 2019, 10:00:16 am »
I'm building a digital clock as my side project to know how counters and other stuffs work. I've implemented it in pcb for seconds and minutes portion till now. As soon as I give Vcc and ground, the minutes portion starts from 20 instead of 00 and then resets at 40 instead of 60. I've checked the circuit. Everything's okay. I've used 555 timer, 7490 ic and 7447 ic. For the Vcc and ground, I've given parallel connection to all PCBs. Is this problem because of Vcc and ground or is it any other factor?
 

Offline Chriss

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Re: digital clock with an alarm
« Reply #1 on: July 20, 2019, 10:08:16 am »
Hi
Can you please post a schematic about your circuit?

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Offline Rajan ShahTopic starter

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Re: digital clock with an alarm
« Reply #2 on: July 20, 2019, 10:15:16 am »
This is the circuit up to minutes.
 

Offline Chriss

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Re: digital clock with an alarm
« Reply #3 on: July 20, 2019, 10:30:17 am »
Is it every time starting from 20 when you give power to the circuit?

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Offline Rajan ShahTopic starter

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Re: digital clock with an alarm
« Reply #4 on: July 20, 2019, 10:36:09 am »
yes. And sometimes, in second it starts randomly from 98, 99 and so on. I don't understand what's wrong.
 

Offline Chriss

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Re: digital clock with an alarm
« Reply #5 on: July 20, 2019, 10:40:31 am »
I'm not sure but it smells to me like a spiky bouncing switch.
If you have a scope try to measure the power line and see if there is some noise in the power rail.

It is like you turn on/off rapidly the power for the device...

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Offline phlegeton

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Re: digital clock with an alarm
« Reply #6 on: July 20, 2019, 10:52:28 am »
On the schematic it's hard to see.. but are you using decoupling caps on the IC's ?
 

Offline tooki

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Re: digital clock with an alarm
« Reply #7 on: July 20, 2019, 05:13:41 pm »
Lemme guess, in reality you’re in the same class as this guy who just posted, too? https://www.eevblog.com/forum/projects/12-hr-digital-clock-with-an-alarm/
 

Offline Rajan ShahTopic starter

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Re: digital clock with an alarm
« Reply #8 on: July 20, 2019, 05:40:44 pm »
No, I don't know him. By the way, could you give me any ideas?
 

Offline phlegeton

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Re: digital clock with an alarm
« Reply #9 on: July 21, 2019, 09:26:03 am »
There are already a few suggestions mentioned in this topic. Did you already took a look at them ?

A couple of things I can think of:

- Did you measure voltages ?
- If the counting always starts at 20, and resets at 40 , it looks like you swapped some bits.
- Are the 7490 configured for Bi-Quinary ar BCD count ?
- As mentioned earlier in this post: check for spike on the voltage rail, make sure you use decoupling caps.
- If you have a scope (and when playing with digital circuits, you should have one) probe around. Measure stuff.

To troubleshoot, go back to the basics. Which means, check each and every portion of the circuit. So disconnect reset and alarm portion of the circuit.
Next test each counter and display driver individually, check if each counter counts from 00-99. (and as a bonus, since you use the 7447 ,use the test lamp feature to test each segment of your displays)
Next, connect the reset part of the circuit. Test if each counter reset at the right count. And that each counter in the chain receives a correct pulse.
Then connect up the alarm part of the circuit and test it.

But this is basic troubleshooting stuff..
 


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