15 x 7 LED display with PIC16F628

This project is provided as a `prototyping project` as no PC board has been produced. The photographs show the project built on a matrix board using surface-mount components and very fine wire. The final design will require a double-sided board with plate-through holes, especially for the matrix of LEDs, but for the moment you can refer to the photographs and construct the circuit yourself. Don't be put off by the appearance of the circuit. It is really just a duplication of a very simple circuit driving a LED that can be built on a matrix board with 0.1` spacing. Most of the surface-mount components can be positioned so the legs will reach a solder-land and you won't need a special surface-mount board.
15 x 7 LED display with PIC16F628 - schematic

The voltage on the PIC chip is critical and the author had some random faults develop on the screen when the voltage dropped below 4.5v. As soon as any unusual fault develops, the first thing to do is check (replace) the batteries. The author also had a problem with the PIC chip when the supply was above 5.6v (in another project). The circuit looks very complex but it consists of just two chips, 105 LEDs, a set of driver (buffer) transistors and 6 tactile switches. All the transistors and resistors seem to disappear when surface-mount components are used and the whole project can be constructed on matrix board or on a double-sided plate-through hole PC board. The 4017 is a "decade counter" and this means it has 10 outputs, starting with output pin 3, only one output is HIGH at a time. The 10th output is pin 11 and then the counter starts at pin 3. The program starts by clocking the 4017 until pin 9 is HIGH. This pin is connected to the input of the PIC chip and the program now knows the state of the 4017. This is called SYNCHRONISING. See more on this below. The '628 has two ports, Port A and Port B. Port A has 7 output lines (plus one input-only line) and Port B has 8 output lines. This creates our 15 lines for the screen. The screen is laid out as 15 LEDs "across" and 7 LEDs "down." The data (or information) is fed in at the top of the screen from the PIC chip to the 15 LEDs and the LEDs are connected to the 0v...

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