This project uses ten pushbuttons to display the digits from 0 to 9 on a single common-cathode 7-segment LED display. The circuit is built entirely from discrete components, using 19 transistors and 10 diodes to implement the digit-selection logic without a microcontroller or integrated circuit.
Each pushbutton corresponds to a digit. Pressing a button activates the required segments to display that number.
How does it work ?
The circuit consists of ten pushbuttons, a transistor-and-diode network and a common-cathode 7-segment display.
A 7-segment display contains seven individual LED segments, labelled a, b, c, d, e, f and g. By illuminating different combinations of these segments, it is possible to display the digits from 0 to 9.
Each of the ten pushbuttons selects one digit. When a button is pressed, it activates a corresponding combination of transistors, allowing current to flow through the segments required to display that number.
The circuit uses 19 transistors to control the display. The ten diodes play an important role in reducing the number of transistors required. Instead of building ten completely independent circuits, the design reuses parts of the logic shared by different digits.
For example:
Digit 7 is built from digit 1.
Digit 0 is built from digit 7.
Digit 3 is built from digit 1.
Digit 9 is built from digit 3.
Digit 4 is built from digit 1.
Digit 8 is built from digit 2.
Digit 6 is built from digit 5.
Another important feature is the shared node formed by segments a, g and d. This combination appears in the digits 2, 3, 5, 6, 8 and 9. Reusing this common logic helps reduce the number of separate transistor stages required.
The diodes provide paths for combining these shared signals while helping prevent unwanted current paths between different parts of the circuit. This diode-based logic makes it possible to implement the ten digit combinations with only 19 transistors and 10 diodes.
Electronics component Fun Kit (with breadboard & power supply module)
General purpose transistors - BC547 (NPN) & BC557 (PNP) pack
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