24 Hour Digital Clock (non Microcontroller) Version II

This is an updated version of my 24 Hour Digital Clock circuit, and I laid out the PCB and had it manufactured. This can be powered from 5, 9, or 12 volts DC, and has a battery backup (batteries do not charge in this, just regular AA batteries) with indicator lights to let you know the battery status and when they should be replaced.

This is a 24 hour clock, using CMOS ICs for the counting. All parts are common and easily sourced. On this clock I added a red acrylic panel over the 7 segment LEDs to make reading the time easier, but you can easily omit this if you like.

For this build, you will need the PCB, schematics, and parts. On the PCB I use component reference numbers, not values, so the schematics are needed so you know the where parts go.

You need knowledge of soldering and basic hand tools. Use of a drill is needed to drill the holes for mounting the battery holder.

Full credit goes to Dan, danyk.cz for the clock design, as that is brilliant, I have added the power and battery backup circuits. 

Parts Required:

5162AS common cathode 7-segment LED display, .56”, x6

CD4026BE decade counter, x7

CD4060BE binary counter, x1

TK072 op-amp, x1

LM7805 voltage regulator, x1 plus heatsink, screw, washer, and thermal pad

1N4007 diode, x2

1N4148 diode, x9

1N5819 Schottky diode, x3

100K trim pot, 3296W (symbol 104), x1, (set to mid range)

10M resistor, x1, (all resistors are 1/4 watt 5% is fine, you will see a few 1% in my build as that is what I had on hand)

100K resistor, x5

4.7K resistor, x2

470K resistor, x2

270K resistor, x1

10K resistor, x2

1K resistor, x1

22 pF monolithic disc capacitor (symbol 220), x2

100 nF (.1 uF) monolithic disc capacitor (symbol 104), x12

1 nF monolithic disc capacitor (symbol 102), x1

32.768 kHz oscillator, x1

momentary NO tactile switch, 2 pin, x2

mini slide switch, SPDT, 3 pin, x2

8 pin socket, x1 (I use machined pin socket)

16 pin socket, x8 (I use machined pin sockets)

header pin, x2, (I use a red one and a black one)

5, 9, or 12 volt DC power supply, x1

battery holder for three AA batteries, x1

nylon standoffs and screws, as needed

red transparent acrylic sheet, as needed (I used 1mm thick)

Tools Required:

Soldering iron, solder, solder wick & liquid flux if mistakes are made

Flush cutters

Drill and a 3.5mm precision drill bit (I use a Star-M wood bit for this)

Small screwdriver, for attaching heatsink to the voltage regulator

Acrylic cutter or utility knife

Additional Item:

Frame of your choice, I make my own frames and use clear acrylic to slide over the clock.

You can order the PCB through PCBWay, (affiliated), the schematics and parts list is also available there for download.

After ordering the PCBs, go ahead and gather up the parts you will need then just sit back wait for the boards to arrive.

Once you have the boards, install some short standoffs on the underside of the board, I use 6mm with 3mm thread, this will keep the board off your worksurface.

Start installing the lowest profile parts first, which is the resistors and diodes. I use a piece of electrical tape to hold a part in place to solder it. The photo shows all the resistors and diodes in place.

Install a part, secure in place with tape, solder, remove tape, trim leads. This is the basic steps for most parts.

Keep in mind the markings on the diodes as they are polarity specific and go in a specific direction, the band indicated on the PCB is for the white or black circle on the diodes.

Comments on the photo will explain some of the circuitry.

Before we get too far along, place the battery holder on the board, use a pencil and mark the two mounting holes. Use a drill and a precision 3.5mm self centering bit and drill the mounting holes, remove any dust, set the battery holder aside for installation in a later step.

Next lowest components are the monolithic capacitors. Most are decoupling caps for the ICs and some are filter caps for power, and one is for switch debounce.

Install the header pins, I do use separate pins for positive (red pin) and negative (black pin). Easiest way to install header pins is install them in a small breadboard, turn both over, and solder in place, remove the breadboard, and the pins are installed.

On the PCB, I did not indicate + or - on the header pins or or the battery connections, I do apologize for this. I will make it clear on the photos, where + and - connect.

The sockets are straightforward, put the notch on the socket where the notch is indicated on the PCB.

Switches are straightforward, just install them and solder.

SW2 when in the 9-12V position, directs power through the 5 volt regulator, when the switch is in the 5V position, D12 (Schottky diode) prevents power from going back through the regulator.

SW1 (Hours) and SW3 (Minutes) simply increase the frequency coming out of U1 to increase the count speed.

At this point, it is ok to remove the standoffs from the bottom of the PCB and install taller ones on the top of the PCB, so when your turn the board over, nothing is making contact with the workbench, except the voltage regulator but that goes in last.

The 7 segment LEDs, the red and green LEDs, trim pot, electrolytic capacitors, oscillator, battery holder, and voltage regulator. Pay attention to polarity when installing the single LEDs and electrolytic capacitors.

The oscillator footprint on the PCB are very small holes that are very close to each other. When soldering these connections, hold the soldering iron from the outside of a pad, not between then you may bridge the pads.

The battery holder can provide power to the clock when there is a loss of power to the clock, this is controlled with Schottky diodes. When power comes back on to the clock, the Schottky diodes disconnect the battery. This is done so you do not have to set the time every time there is a power drop (I live in Thailand, power drops often).

The batteries used are just common AA batteries, NOT rechargeable as this circuit is NOT charging the batteries.

D15 will be lit green when you have good batteries installed AND the clock connected to power and turned ON.

D16 will be lit red when battery power is low, like one or all the batteries are near dead AND the clock is connected to power and turned ON. Or when operating the clock without the batteries.

The voltage regulator is only working when switch SW2 is in the 9-12V position, when the switch is in the 5V position a Schottky diode, D12, prevents 5 volts from feeding back through the regulator.

The reason I use header pins for power is that will mount this clock in a wood frame with clear acrylic front. The frame will have a power jack and I use wires soldered to the jack with Dupont connectors to connect to the header pins.

For testing, I am just using a USB cable to a modified power jack with Dupont connectors on the wires.

Before turning this on, set SW2 to the correct voltage of your power supply. Make connections to the header pins, and turn the clock on.

Turn the clock on, feel free to press, one at a time, minutes or hours, to see how the clock advances.

Note: Occasionally, when you turn on the clock, the digits will show random segments in the LEDs, just press the Hour setting button to cycle through the digits to clear everything to zeros and numbers, then you can set the time after that.

The use of the red filter (transparent red acrylic) is from this project, CMOS Counter Clock, with full credit to Lonesoulsurfer.

Why use a red filter? It makes the LED segments stand out, making the clock easily readable, even in full light (my workbench light was on for this photo, and the previous photos showing the clock operational without the filter).

I used a piece of 1mm thick red transparent acrylic and measured the length of the PCB, 11.5cm, and the width I needed to cover the 7 segment displays equally on the top and bottom of them, that was 2.3cm. I clamped a steel rule on top of the acrylic to my work surface, used a acrylic cutter (a utility knife will work equally as well) and scored the acrylic and snapped it apart.

Then I positioned the acrylic under a blank PCB and marked the holes for the standoffs. Drill those holes with a 4mm self centering bit (so you can adjust the acrylic to ensure it is even with the board).

Install the filter standoffs, with screws from the underside of the board, that are just slightly higher than the 7 segment displays, I used 10mm standoffs. Place the filter on top of the standoffs and install the screws on the top through the filter.

You can see from the photo, it makes a world of difference.

Now you have an operational clock.

I will add a photo of the clock mounted in a frame after the frame is fabricated.