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Serial 4-Digit 7-Segment LED Board
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| Serial LED-4x7 in Action |
Serializes a 4-Digit 7-Segment LED Display: $4.95 each (Bare Board)
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The serial 4-Digit 7-Segment LED Display board was originally designed in conjunction with a 3-part series of articles in the PICAXE Primer column of Nuts & Volts magazine (Dec. '09, Feb. and Apr. '10). If you are interested in the power and simplicity of PICAXE programming, you may want to subscribe to Nuts & Volts as we develop additional PICAXE-based projects in future installments of the Primer. Nuts & Volts subscribers have free access to previous issues back to January of 2004, so you will also be able to access all the earlier PICAXE Primer columns.
The LED-4x7 board implements a PICAXE-08M circuit that provides the necessary hardware and software to accept serial input and convert it to the data format required by the on-board MAX7219 LED-display controller. The complete circuit schematic for the board and photos of the bare board and a completed LED-4X7 are presented below. (Click on either photo to view a larger image of the board.)
| LED-4x7 Schematic |
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| Bare LED-4x7 | Completed LED-4x7 |
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The first aspect of the LED-4x7 that requires clarification is the fact that the LED display is mounted on the back, not the front, of the PC board. This was done to save space and it also explains why the pinouts on the PC board connectors are reversed from what is shown in the schematic. When the LED-4x7 is in use (see the photo at the top of this page) the back of the board (with the LED display) is facing the user, which explains the reversal of the pinouts. As you can see in the photo at the top of this page, I have added a small label to remind myself of the order of the pins.
The Breadboard Connector (in the center of the bottom edge of the LED-4x7 board) also requires explanation. It's a 5-pin right-angle female header, but you use it by inserting a 4-pin straight male header between the female header and the breadboard. The male header pins need to be long enough to make solid connections on both ends. (For an example of a suitable male header, see the first header in the "Headers" link on the left of this page.)
In the above schematic, you can see that both "RxD" pins connect to the same pin on the 08M. This somewhat unusual arrangement is for the purpose of providing some flexibility in how the LCD is connected to a "master" processor. If you are using a 28X1 or 28X2 as your "master" processor, it allows you to connect the RxD input on the LED-4x7 board to either "hserout" (pin 17) or "out 0" (pin 21) on the master processor. This is accomplished by inserting the 4-pin male header toward one end or the other of the 5-pin female header, so that only one of the two "RxD" connections is actually made.
However, the LED-4x7 can also be used with any PICAXE project, even one based on the 08M processor. The important thing to remember is that only one of the two "RxD" pins should be connected. (If you accidentally connect both "RxD" pins, your circuit may function erratically, but resistors R2 and R3 in the schematic prevent any possible damage from occuring.) In the photo at the top of the page you can see that the LED-4x7 is connected to the breadboard via a 4-pin (not a 5-pin) male header, so the "RxD" pin on the left is being interfaced with the 08M in the photo - the "RxD pin on the right is not connected to anything on the breadboard.
The two rows of five holes just above the Breadboard Connector in the photos are for an optional 5x2 straight male header that can be used for a ribbon cable connection if you need to locate the board some distance from your project. On this header, each of the 5 pairs of pins is connected to the I/O pin directly below. As a result, on the other end of the cable you can use either (or both) of the 5-pin rows to connect your project to the LED-4x7 board.
The 4-pin right-angle female connector that is labeled "Program Adapter" is used for in-circuit programming of the on-board 08M processor. For details on its usage, see the "Assembly Instructions" page.