K105 Digital clock with digital gates

A clock plays a central role in our life, we use the time to plan everything that affects our lives, from work to entertainment. In this project we are building another kind of clock, with simple integrated circuits, which will be very useful hanging on a wall of our house or our office.

The clock that we will build here is based on logical gates – digital circuits. The main components are the integrated CD4520 4bit binary counters, which together with the 7-segment decoders, the CD4060 generator, the CD4027 and the AND (CD4081), OR (CD4071), NOT (CD40106) and the other passive elements complete the construction.

Circuit description

Circuit operation is described as follows: The square pulse generator IC17 (CD4060) containing a crystalline oscillator produces square 2Hz pulse frequencies, which after splitting from the flip-flop IC18B (CD4027) obtains the 1 Hz frequency reference for our clock.

The reference frequency 1Hz feeds the IC10B and IC10A counters which together with the AND (IC11C and IC11D) logic gates list the secondary 00 to 59, which are displayed on the 7 segment display via the IC6 (CD4511) and IC5 (CD4511) decoders. The next pulse of the clock, after 59, has zero seconds and a pulse is sent to the minute counter.

The IC8B and IC8A counters, along with the AND gates (IC9C and IC9D), count the minutes from 00 to 59 that display the minutes on the display with the help of the IC4 (CD4511) and IC3 (CD4511) decoders. In the next pulse of minutes, after 59, the numbers of the minutes are zero and a pulse is sent to the hour’s counters.

The IC7B and IC7A counters together with the gates IC9A, IC12F, IC12E, IC12B, IC12C, IC13A, IC13B, IC13D, IC13A, IC14A, IC13A, IC14A, IC15A, count the hours from 00 to 23, along with the help of decoders IC2(CD4511) and IC1(CD4511) appear on the display. The layout of the gates is such that the counter of the hours is reset after 23.

To set the time, buttons S1, S2, and S3 are used. When S1 is pressed it activates the S2 and S3 buttons to set the hours and minutes respectively. Passive components around the buttons are used to eliminate contact noise. With the help of the NOT gates, which are smith trigger type, the corresponding square pulses are produced, and with the help of the IC15C, IC14B, IC25B, IC14D and IC15B logic gates integrate the minutes and hours counters.

The clock is powered by a 12Volts external power pack. In order to continue to operate at a power failure without losing time, it is necessary to connect a 9V battery to the corresponding sockets. When the power is off, the clock is powered by the battery and continues to operate although the decoder input B1 is set to LOW resulting the display being black. This is achieved by the appropriate design of the circuit using the D1, D2, D3 and D4 diodes and the surrounding passive components. When the power is restarted, the clock continues to count the time and continues to display the time.


To manufacture the clock, we use double-sided PCB that can be built on your own or can be ordered to manufacture it in high quality with punched metallized holes.

The placement of the construction should start from the bases of the integrated circuits. In the case of the PCB you make it yourself and it does not have metal holes, you should use gold-plated bases and use a small nose soldering iron. Then solder, the resistors, the diodes, the 7 segment displays, the capacitors, the crystal, the buttons (in the correct orientation) and the connectors.
When you have finished fitting all the materials, clean the board both sides with absorbent cleaning spray or acetone to remove the flow that we used during welding as it will present problems with the operation of the structure if it remains.

Then place the IC components into the corresponding bases, taking care to properly orientate them. Our power supply is powered by a 12V power adapter and the clock should work immediately. Then you must manually set the right time. Press the SET button, set the hours and minutes, press the HR and MIN keys respectively.