(Note: The image shows three kits connected together - each kit contains one digit)
This board is a relatively simple design. Each of the 7 segments of the LED is controlled by one bit of a shift register. The decimal point is also controlled by the shift register. This means each board is controlled via an 8 bit binary number. You will need a 12V supply to drive the LED units, along with a 5V supply (which can be provided from an Arduino or microcontroller supply).
Data is moved into the shift register serially (check the example code for more details). The LATCH pin is taken LOW. Serial data is then moved into the shift register by first setting the DATA line to 0 or 1, then pulsing the CLOCK line. When 8 bits have been shifted in then the LATCH pin is taken high and the output displays whatever values are in the register.
The output from the shift register is just 5V, which is not enough to drive the large 7 segment LED display. This requires 12V and curent limiting resistors (330 ohm for 12V). A UN2003 7 transistor array is used to control each of the 7 segments. These transistors control the low side of the device. The 7 segment display must be a common-anode type.
The decimal point is a special case as it only contains one LED, hence must have a different current limit. An additioanl NPN transistor is used along with a 1k current limiting resistor to control the decimal point.
If you have more than one board in series (they are designed to fit together to make large numerical displays), then you need to clock in all the data to control every part of the display. For example - if we have three digits then we must clock in three 8-bit digital numbers and then set the LATCH high. This will then control all three digits. Check out the instructions and the exmple code for more information. The device is wired with the following connectors (P1 (input) and P2 (output) are as follows):
We have been asked to provide large visible displays, mainly to show power and energy. These are generally run form 12V DC supplies. In fact we have been asked to do it so many times, that I thought I would design a relatively simple LED display unit which can be wired together to create large numerical displays. So here it is. There are full instructions for you to build your own (at www.re-innovation.co.uk).
Here are three units built into a power meter prototype: .
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Matt Little from The Curious Electric Company is an electrical and electronic engineer mainly working on renewable energy projects.
He is also a founding member of Nottingham Hackspace in the UK and is interested in helping people make stuff. Every so often there is the need for a device which has not been fulfilled elsewhere. Hence the projects here are a random mixture of things, including LEDs, displays, micro-controller development boards and renewable energy products.