Digital Clock with an alarm

Thread Starter

Bruce Wayne 007

Joined Jan 24, 2019
4
Hello everyone.
I'm working on a digital clock circuit for my hobby project. I completed the circuit but I couldn't figure out how to add an alarm to the circuit. A little help would be appreciated. Thanks.
(I've made the digital clock circuit using 7490 ic, 7447 ic and logic circuits). The simulation pic is attached below:
 

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JohnInTX

Joined Jun 26, 2012
4,499
You could use a couple of 'magnitude comparators' or 'identity comparators'. These are chips that accept two sets of inputs, in this case the alarm setting and current time, and indicate when they are equal. Some possible choices can be seen by searching ti.com for one of those terms.
http://www.ti.com/sitesearch/docs/universalsearch.tsp?searchTerm=magnitude comparators#q=magnitude comparators&t=product&linkId=2

The easiest way to set the alarm I think would be to use encoding switches - the little dial switches that generate a direct BCD code that corresponds to their setting. The switch outputs would be connected to one side of the digital comparator and the counter outputs to the other side. When equal, set a flip flop that enables a beeper.
https://www.e-switch.com/product-catalog/rotary-dip#.XKo5KNh7k-U
Search amazon.com for bcd thumbwheel switch for some others.

You could try to emulate the generally horrible (IMO) way of setting the clock by replicating the counter chain so that you can count through the alarm time by holding button(s) and use multiplexers (ref: 74xx157) to select the display decoder input between the alarm count and the time. Feed the alarm count into the digital comparators. Lots more chips there.

But if you're going to use 7400 series logic, something along those lines would do it.

A couple of observations.
7490 is a ripple counter which means that increments 'ripple' through the chain. While this is happening the time output can be invalid. It's a fast ripple but the comparators are plenty fast to pick up the invalid data. Be prepared to qualify the 'equal' output by an RC filter or a gate that blanks the output on the counting edge of the input clock.

The 555 likely won't have the long term accuracy for a practical clock. Consider a crystal oscillator or if you are line powered, tap the secondary of the power transformer with suitable level shifting and divide down for a timing signal.

Have fun!
 
Last edited:

Thread Starter

Bruce Wayne 007

Joined Jan 24, 2019
4
You could use a couple of 'magnitude comparators' or 'identity comparators'. These are chips that accept two sets of inputs, in this case the alarm setting and current time, and indicate when they are equal. Some possible choices can be seen by searching ti.com for one of those terms.
http://www.ti.com/sitesearch/docs/universalsearch.tsp?searchTerm=magnitude comparators#q=magnitude comparators&t=product&linkId=2

The easiest way to set the alarm I think would be to use encoding switches - the little dial switches that generate a direct BCD code that corresponds to their setting. The switch outputs would be connected to one side of the digital comparator and the counter outputs to the other side. When equal, set a flip flop that enables a beeper.
https://www.e-switch.com/product-catalog/rotary-dip#.XKo5KNh7k-U
Search amazon.com for bcd thumbwheel switch for some others.

You could try to emulate the generally horrible (IMO) way of setting the clock by replicating the counter chain so that you can count through the alarm time by holding button(s) and use multiplexers (ref: 74xx157) to select the display decoder input between the alarm count and the time. Feed the alarm count into the digital comparators. Lots more chips there.

But if you're going to use 7400 series logic, something along those lines would do it.

A couple of observations.
7490 is a ripple counter which means that increments 'ripple' through the chain. While this is happening the time output can be invalid. It's a fast ripple but the comparators are plenty fast to pick up the invalid data. Be prepared to qualify the 'equal' output by an RC filter or a gate that blanks the output on the counting edge of the input clock.

The 555 likely won't have the long term accuracy for a practical clock. Consider a crystal oscillator or if you are line powered, tap the secondary of the power transformer with suitable level shifting and divide down for a timing signal.

Have fun!
 
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