In this project, we will help you design and build a digital clock with 24 hour count time which can be adjusted using a mobile app. The clock runs from 00:00 to 23:00 and then back to 00:00. Our display will have eight digits, two digits for the minute’s section and two for the hour section. The specialty of this clock is that it has very low power consumption and condensed layout.

We have used a simple 0–9 counter for the ones digit of minutes display and a 0–5 counter for the tens digit display of minutes. For the unit’s digit of hour, a resettable 0–9 counter has been used, and for the ten’s digit of hour a resettable JK-flip-flop has been used. In addition to these, a few NAND gates and OR gates have been used to ensure proper functioning of the clock.

There is only one input for the entire system: the clock input into the 0–9 counter used for the ones digit of minute display. The clock inputs for the other blocks are derived from the output of the previous blocks.

We have also designed a four 7-segment decoder which can be used for the actual display of digits as we see on a real digital clock. The outputs of the decoders would be connected to a seven segment common anode display.


In this part, you will implement your software/theoretical design on a breadboard for prototyping before finally implementing it on a strip board (electronics prototyping board). In designing and simulating the clock, the following sections will be designed separately:

  1. Power Supply
  2. Pulse Generator
  3. Clock Circuit
  4. Display Circuit
  5. Integration


Our electronics design will need a 5–7V supply. We will design a power supply that provides +5V output. The transformer steps down 220V to 12V and a bridge network of 4 diodes rectifies it. The 7805 regulator serves the purpose of regulating the power supply to produce a steady 5V DC.

Required Instruments:

A transformer (110/220V 50/60 AC 12 1A)

Capacitors: 470uF and 100uF

A 7805 regulator

4 diodes: (1N 4007)

Breadboards (approximately 5)

Connecting wires

Connect your circuit as follows:


The pulse generator is the heart of the clock — the time base. We will be using NE555N timer which in a stable mode configuration continuously produces pulses at its pin 3(output) .The pulses are square wave and using a fixed resistor will give you an exact frequency of 1Hz. To vary the frequency you will have to replace the resistor with a variable resistor.

For this, you may need: a 555-timer, a well regulated power supply, the power supply designed in the previous section and some passive components which are on the diagram.

We have already simulated the designed circuit and made adjustments where necessary.

Connect your circuit as follows:

This circuit should produce a frequency of 1Hz (equivalent to a period of 1 second). This will be used as the time base for our clock.


This will be the circuit for extracting time information from the time base. Our time will be extracted in the HH:MM:SS format.

Use the following schematic to design the clock, display and alarm circuit:

We have done the circuit design and calculations, I have also simulated the circuit and nearly made all the necessary changes and appropriate adjustments. During the design implementation, following IC data sheets will be very important. This is simple, all you need to know is to the number on the components and a good internet connection.

You may need: 4 counters (ICs)-74LS90, logic AND gates (ICs) 74LS08, Drivers IC 74LS47D and your power supply. You will also need 6 segment display (Common anode) and a resister (220 Ohms) before each of the displays.

For display circuit you will require the following components: display drivers and seven segment displays (make appropriate choice between common anode and common cathode), your power supply and some resistors.

On the last part is the alarm clock section, the kind of alarm clock designed here will be the type that goes off after every 4 hours. Here you will need comparators, a buzzer, a tone generator (most likely a 555 astable circuit), your power supply and some logic gates (ICs).


For manual time setting/resetting we need a manual switch .We will need two debouncing circuits to help us input a manual switch signal into a digital circuit; one for the hours and another for the minutes. With debouncing circuits, single press doesn’t appear like multiple presses.



2-Schmitt trigger-74HC14

2- 4.7K Resistors & 2–100K Resistor

2 diodes

2–10 uF capacitor

Here’s the circuit:

The basic idea is to use a capacitor to filter out any quick changes in the switch signal.

The first thing you’ll notice is there’s a diode in the circuit. That helps the capacitor to charge up quickly when the switch is opened after being closed.

The second thing you’ll notice is R2. This resistor prevents the capacitor from dumping its charge into the switch contacts and burning or pitting them. The two extra components won’t significantly affect the timing of the circuit when it’s in use, so you don’t have to recalculate all your values.

We have successfully designed the 24hr digital clock, in the next blog, we are going to enter into the world of IoT (Internet of Things) and control our clock using our mobile phones. Keep checking this blog to find more updates.

Originally published at On Monday, 4 May 2015

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