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Construction of another digital clock

Posts: 159
Joined: Fri Mar 09, 2012 6:16 pm
Location: Mayabeque, Cuba

Construction of another digital clock

Post by Miguel López » Tue Jun 14, 2016 9:45 pm

Hi everybody

I have begun the construction of my next digital clock. As I always want to make something new, for fun and learning, this time I will use three twin multiplexed seven segment lamps, with commom-cathode LEDs. I have never built a multiplexed display before. I will use SN74XX TTL chips and some CMOS.

There are several block which need to be designed:
- Lamps circuit, with segment handling, and digit selection inputs
- Digit selection and data selection circuits
- The clock (I will use the same circuit that I used for my Nixie Clock)
- Power supply and UPS system (same as previous one)

So, I will focus on the first two blocks. I have already designed the PCBs and mounted the components. I'm using about 1 kHz for display sweeping, so digits appear very steady.

The seven segment decoder will be the old good 7447. As this lamps are commom-cathode type, and the 7447 is designed for common-anode operation, each segment will be powered by a PNP transistor driven by the 7447 outputs. This way the transistors act as inverters, and so the common-cathode operation is achieved.

Then, each digit (7S lamp) will be enabled using an NPN transistor which will be driven by the selection circuit. Sequence is:
1.) All lamps OFF
2.) First BCD number is selected (no lamp is ON yet)
3.) Lamp corresponding to that BCD number is enabled (ON)
4.) This lamps is turned OFF (all lamps OFF again)
5.) Next BCD number is selected.
6.) Lamp corresponding to that BCD number is enabled (ON)
7.) Repeat for the rest of the lamps, and then repeat the entire sequence continuosly.

The data selection will be using 7403 open collector gates. The BCD output of the same order of every digit will be shorted and connected with a 1k resistor to VCC. When a logic "1" is applied to the other input of the NAND gate, the output is then dependable of the other input and the it will appear at the output inverted, so the four outputs of the 7403s must be inverted before be applied to the 7447.

Here some pictures to show at what stage I am.
picture shows all of the lamps ON, althought indeed there is only one ON at the same time, but as the refresh time is fast, they seem to be all ON.
this picture is the data selection and lamp enabling circuit. Uses a Soviet K133LB1 for oscillation, a 4518 for frequency division, a 7442 for decimal counting, and two 7400, and two 7404 for combinational logic.
picture is the back side of the display, which will contain the data selection circuit and inverter ( five 7403, one 7400, one 7447 (already mounted), and some transistors)

Posts: 721
Joined: Fri Jul 29, 2011 10:16 pm
Location: Coningsby, Lincolnshire

Re: Construction of another digital clock

Post by Lloyd » Tue Jun 14, 2016 11:29 pm

Hi Miguel,

I will be following this thread with interest, as I like building digital clocks too! So far I've half built one using CD4### logic, CD4026 for the display drivers, some rather bright 1.5" LED displays, and 2 smaller ones for the seconds. This is the circuit I have trimmed down for mine, I haven't added any of the bits for driving the slave clocks, or the radio control parts, although I might do at a later date.

Mine doesn't use multiplexing for the display, so it will be interesting seeing yours and how it works.

Best of luck!


Posts: 159
Joined: Fri Mar 09, 2012 6:16 pm
Location: Mayabeque, Cuba

Re: Construction of another digital clock

Post by Miguel López » Wed Jun 15, 2016 4:32 pm

I couldn't open the link. Anyway, the CD4026 is a very good choice. I don't have those, but I have several CD4518 which I have used on other clock projects with very good results.

This is the circuit that I'm using for sweeping. The outputs of the 7404s select the right number, while the output of the 7400s (once inverted) provide enable pulse for the right lamp. As it is said in the picture, I replaced the 7490s by a single 4518.
Sweep schematic 1.jpg

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