Join GitHub today
GitHub is home to over 28 million developers working together to host and review code, manage projects, and build software together.Sign up
An 8-digit seven segment display with driver circuit from easily obtainable components.
Fetching latest commit…
Cannot retrieve the latest commit at this time.
|Failed to load latest commit information.|
===================================================================== = ICMI Seven-Segment Board = ===================================================================== * Author : Hendrik Busch * Current version : 1.1 * License : Creative Commons Attribution-NonCommercial- ShareAlike 3.0 Germany License. The Seven-Segment Board offers up to 8 seven-segment digits with a control circuit made up of several cheap and easy to obtain components. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ WARNING: The current board still has a design flaw. Since the ULN2803 and the 2N7002 each incur a voltage drop when active, 5 volts is not really enough to light the display properly when driving high refresh rates for the POV effect. The first counter measure given the current design is to insert lower current limiting resistors (say 10 ohms instead of 150). The other measure would be to increase the voltage, but with the current design voltage can only be increased to 7 volts as this is the maximum voltage for the shift register and the decade counter. So an increase from 5 to 6 volts currently seems the most reasonable. The next design iteration will feature seperated supply lines for the LED segments and the TTL logic, just to make sure. +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ Seven-segment LED circuit ------------------------- The board uses dual common anode seven-segment displays, meaning that each component feature two digits with seven segments each. Each digit has its own anode that is common to all segments in that digit (thus common anode) and each segment has its own cathode, resulting in two anodes and 14 cathodes per component. Each segment cathode of a digit is connected to all the other cathodes for this segment on the other digits. The LED circuit therefore features 8 anodes (one for each digit) and 7 cathodes (one for each segment). Switching circuit ----------------- The switching circuit uses a minimum of I/O 5 pins (excluding power). Two are needed for a 74HC4017 decade counter that is responsible for sourcing power to the digits, one at a time. Using a software timer, the decade counter switches through each digit so fast, that the human eye cannot distinguish between the single digits being turned on and off but sees a steady display with all eight digits lit (this is called the POV (persistence-of-vision) effect. Exploiting this effect, the software and circuit only have to worry about displaying a single digit at a time and switching between digits. In order to control what each digit is actually displaying, a shift register (74HC595) is used to set the state of each segment. As the shift register is not made to sink current, it is connected to an ULN2803A darlington array. If one of the shift register pins is HIGH, the corresponding switch in the darlington array sinks the incoming LED current on that pin, effectively lighting the corresponding segment. The software must ensure that the digit switching and the segment configuration are in sync, otherwise the display content may show as distorted of shifted. Version History --------------- - v1.1 - The CLEAR pin of the shift register is now connected to VCC and not to GND as before. Pulling it to GND in v1.0 resulted in a continous purge of any data loaded into the register and thus rendering the display unusable. - v1.0 - Initial revision