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.