How to make a very small servo breakout board for the Arduino™ Pro Mini and clones.
Building robots can be very fun and simple, as long as you remember to stick to certain sizes. Too small, and it's tricky to pack all the parts you need inside, and miniaturized parts are more expensive. Too large, and it gets too heavy (mass grows with the cube of size, so if you make something two times bigger, it will be eight times heavier) and you have to use more sturdy materials and stronger motors, which both are more expensive and harder to work with. So there are several "sweet spots", optimal sizes for which the widely available parts are perfect. I discovered that for my robots, the size of a guinea pig, or a small rabbit, is the best.
But there is a problem with that. The Arduino™ board barely fits in that size. Add a servo shield or breakout board, and it becomes hard to keep your robots small. That's why I don't like using the original board. Well, also because it's way overpriced. I use the Pro Mini instead, which is a cheap Chinese clone, which doesn't even have its own USB port – you have to use a separate programmer. But that's fine for me. It's also very small, about 0.7×1.3 inches.
In order to connect the servos to the Pro Mini, you need to somehow provide power for them – so next to the signal pins you need to have two rows of pins with ground and positive voltage. So you need some sort of a breakout board. The traditional approach is somewhat large and clunky – the Pro Mini in the middle, fit into a socket, and the headers for the servos on both sides of it. It becomes almost the size of a regular Arduino™ again. So I came up with my own design. Here's how I make them.
You will need perma-board that has this pattern on it:
First, you need to cut a piece that has 12×5 holes:
Make sure that the edges are even and that it fits between the pins of your Pro Mini:
Next, push the plastic on your male headers to the edge, like this:
And then insert the first header in the second row of holes, like this:
Making sure the pins stay vertical, solder them to the board:
Repeat that for all four headers, leaving the strip in the middle empty. Make sure all the pins are vertical and all of them are properly soldered – no holes around the pins.
Now for the female header.
Remove the second pin from right in the top row, and solder the rest like this:
The two rightmost pins on the bottom row should be connected together, and the three middle pins of the top row should be connected to the rightmost pin of that row. Note that the second from right pin is missing, so they don't connect to it.
Next, we have to remove everything from the back side of our board. I just used the dremel to cut off the pins together with their plastic:
Now, use the knife to make sure that the vertical strips of your board don't have a short circuit between them.
And you can put the female header on your board, like this:
The whole board fits the Pro Mini like so (if it has any exposed metal on the underside, you might need to put a strip of paper or tape between it and the board):
And you connect the servos, the power and any sensors using the servo plugs like this:
For programming, you will remove the female header, to cut the power from the servos – they draw much more than your USB can handle.