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Arduino Mega Current-Sinking I/O Breakout Shield with LEDs

For those tired of wiring up current drivers and too lazy to wire up their own LEDs!

The Arduino Mega power and prototyping shield provides:

A photo of the board and a small DC motor

So, are you one of those people that actually has a Rev 1.0.0 Mega LED & Power Proto Shield? If so, that is most impressive! I plan on making 10 total, and have only completed 3 so far. I may sell the other 7 once they are made, but that will be a few months. Here is the quick start guide for using them.

Known issues

Before using this, keep in mind that:

Quick Start

Caution

As with most Ardunio shields, misuse of the Mega Power & Proto shield may result in damage to your Arduino. Here is a list of the most common issues that you may encounter.

Removal

Removing the thing is difficult due to the number of pins connecting the shield to the Arduino Mega. If you simply pull them apart there is a good chance of bending a few of the pins. You can straighten them again with small pliers, but they aren't as nice afterword. When pulling it apart, slowly and carefully work it off. I have had some luck partly unplugging it and then using a nylon pry tool to get it the rest of the way off. But I need to find a better solution.

Pins:

A diagram of the pin locations.

The Buttons

There are two buttons included, one wired to pin 52 and the other to pin 22; the other side of each button is wired to ground. They are normally open, so you can use the pin for something else and simply not press the button. This also means they don't have pull up/down resistors. When using them you should enable the pull up resistors using INPUT_PULLUP:

pinMode(22, INPUT_PULLUP);

The buttons are not debounced. If necessary, you could use a software debouncing approach. For many Arduino projects the bounce is not a problem.

A reset button is also included since the on-board reset is covered up. Resetting your system by pounding on it with a hammer is not recommended.


More Details

Each Arduino I/O pin is connected to a Darlington transistor inside a ULN2803A transistor array, as shown in the schematic. When an Arduino I/O pin is high the Darlington transistor connects U3 to ground, allowing current to flow both through the on-board LED and through any attached load. When the I/O is low the Darlington transistor appears as a high impedance and current does not flow. As this may result in a high voltage at U3 there is a protection diode in series with the LED, preventing the high voltage from feeding back into Vcc.

A wiring diagram of the basic setup.

Figure 1: This wiring scheme is used on all I/O pins.

Each array of Darlington transistors should be limited to 2.4 amps total, as imposed by the trace connecting each ULN2803A to ground. The entire system should be limited to 18 amps, which is the current limit on the screw terminal.

Advanced users should review the TI ULN2803A datasheet for details on the Darlington transistor arrays used in this design.

The screw terminal allows easy connection for the higher voltage you plan on controlling, as well as external power for the Arduino. While it could be any voltage up to 30V, I call it +12V for convenience. The ULN2803As will handle up to 50V, but I don't recommend working with voltages that high on this board. Strictly speaking, it is not necessary to wires up the +12V; you only need the shared ground. Wiring up +12V allows for connecting the back EMF suppression diodes and also provides a convenient place to wire up your high power device.

A schematic of the screw terminal and back EMF jumper.

Example

In the sample configuration a potentiometer was wired up to one of the Arduino analog inputs and used to control the speed of two fans.

A sample wiring configuration.

Unknown issues

This board has not yet been fully tested, nor has it been used enough for possible bugs to come out. Some issues that will need investigated:


Possible improvements

As noted above, I do not plan on making more than 10 of the Revision 1.0.0 Mega LED & Power Proto shields. However, depending on demand, I may make an updated version. Here are some of the improvements and bug fixes under consideration:

Another idea is making a version for the Arduino Uno. It may also be possible to make a combo Uno shield and a Mega expansion shield, letting the two work as a pair on a Mega. This would make it easier to use just half of it on an Uno. And you could remove half at a time from the Mega, which would make pulling the thing off a lot easier.


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© David C. Hunter, 2019
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