digital clock with an alarm - Page 1
Forum section: electronics / projects

author: Rajan Shah
author_rank: Newbie
timestamp: July 20, 2019, 10:00:16 am
content: 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?

author: Chriss
author_rank: Frequent Contributor
timestamp: July 20, 2019, 10:08:16 am
content: HiCan you please post a schematic about your circuit?Sent from my SM-J500F using Tapatalk

author: Rajan Shah
author_rank: Newbie
timestamp: July 20, 2019, 10:15:16 am
content: This is the circuit up to minutes.

author: Chriss
author_rank: Frequent Contributor
timestamp: July 20, 2019, 10:30:17 am
content: Is it every time starting from 20 when you give power to the circuit?Sent from my SM-J500F using Tapatalk

author: Rajan Shah
author_rank: Newbie
timestamp: July 20, 2019, 10:36:09 am
content: yes. And sometimes, in second it starts randomly from 98, 99 and so on. I don't understand what's wrong.

author: Chriss
author_rank: Frequent Contributor
timestamp: July 20, 2019, 10:40:31 am
content: 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...Sent from my SM-J500F using Tapatalk

author: phlegeton
author_rank: Contributor
timestamp: July 20, 2019, 10:52:28 am
content: On the schematic it's hard to see.. but are you using decoupling caps on the IC's ?

author: tooki
author_rank: Super Contributor
timestamp: July 20, 2019, 05:13:41 pm
content: 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/

author: Rajan Shah
author_rank: Newbie
timestamp: July 20, 2019, 05:40:44 pm
content: No, I don't know him. By the way, could you give me any ideas?

author: phlegeton
author_rank: Contributor
timestamp: July 21, 2019, 09:26:03 am
content: 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 countingalwaysstarts 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..