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A simple LED Board

This project used to be an assignment for my BTEC students. They were asked to build the board with LEDs suitably chosen to represent the two traffic lights and a pedestrian crossing at the corner of Market Street and Catherine Street. They then wrote a machine code program to drive the installation and tested the board with all kinds of instruments. This used to be good fun and taught me all kinds of things about teaching hardware testing procedures and low level programming.

The circuit is quite straightforward and if you have a user port, the LEDs can be driven directly from it. If you use the printer port option you'll

Circuit diagramhave to connect an external 6V power source - a battery will suffice. You must however set the link, either by soldering or with a computer type plug. The common is in the middle and has to be linked with either the right of the left pin. The 270 Ohm resistor gives a fairly bright light with standard LEDs and allows about 10mA to flow. This is well within the capacity of the user port VIA.

 The circuit is quite simple and uses an ULN 2801 or ULN 2803 darlington driver. This is quite a useful chip and has internal diode protection, should you want to drive inductive loads, which we don't of course.

The circuit diagram is on the right. Each LED is protected by a 270 Ohm resistor. There is a power ON LED and two test points across the supply should you want to connect a logic probe.

PCBI design all my PCB layouts for half a 180mm by 100mm photo resist board and the layout is quite straight forward. Notice the inclusion of an extra 8 pins between the driver chip and the resistors. These need not be drilled. I included them so that one can connect instruments or additional small loads to the outputs. It is not very difficult to etch your own PCB but feel free to use Vero board. I may even have a few spare PCBs in a drawer somewhere - write to me if you are desperate. The scaled GIF is on the left. For RiscOS users I have included the original draw file.

The finished projectA photograph of the finished project is on the right. This particular board uses green LEDs but the design will obviously work with any mixture of colours except black. Notice the small blue link. It is set to "Printer port". If you use the user port, this link should be to the left.

If you don't want to write your own program, I have written a fairly comprehensive multi-tasking demonstration program that does everything with 8-bit numbers I could think of. You'll find it in the PD section of my web page. It has the fairly obvious name !LED_Board. Should you have any problems with this project feel free to contact me at jochenlueg@freeuk.com.




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