The Arduino Nano is a great little board. I use it for most of my small sized projects. But it has two shortcomings that I wanted to address. No easy support for lipo batteries and no built in power switch.
My small projects are almost all powered with lipo batteries. So I wanted a built-in charger in my projects. If you have used lipo batteries you know that you need to use pliers to remove the JST-2 connectors, or risk yanking the wires out of the connector. Then to charge them you need to use a special purpose charger. This involves more plugging and unplugging. The wires and connectors, and for that matter myself, can only take so many cycles. I set out to make board that piggy-backs on the Nano board so it would not take up much room.
Lipo batteries are small and powerful. They provide ~3.7 volts, so they can power LEDS, Arduinos and other devices. Read more below about how best to charge them.
All my projects need a power switch. It seemed that I could use this same board to add a power switch. But I still wanted retain the use of the existing USB jack for the DC in for charging. This results in one limitation — that the board gets power during charging. But once the USB is unplugged, the power switch controls the power to the Nano from the lipo battery pack.
I own an OtherMill, so I can mill my own custom boards. It gives me the ability to control the placement of pins and the ability to create custom board shapes. I have the option of using surface mount components. So the circuit will be compact and low profile.
The main issue is the need to have a connection from the lipo battery to the Nano board while not allowing the voltage to flow back during the charge operation. The lipo must only be charged through a special charge controller. I use the power switch and a Schottkey diode to ensure the lipo is only fed voltage from the MAX1555 charger built into the board. All diodes have a small forward voltage loss, but Schottkeys have low loss, making them good for reverse polarity protection.
The block above containing the pins GND and +5V is the Nano board. The lipo battery connects to the JST-2 on the right side. With the switch in one direction power is carried from the battery to the Nano and if the Nano happens to be connected to USB at the same time, no current can flow back to the battery. In the other switch direction the lipo is only connected to the charger, allowing it to get charged. There is also a charge indicator LED. The MAX1555 has two inputs providing for high or low charging rates. I wanted a high charge rate because I was using batteries with over 500 mAH capacity. If you use smaller batteries you need to switch to the “USB” pin for the voltage input instead of “DC”.
I started this design using “large” surface mount components, i.e. 1206 size. But I have become comfortable using 0603 devices. In some ways they are actually easier. For the record, before I received my OtherMill, I had only soldered components with leads. So I entered a whole new world with surface mount boards after starting to use the OtherMill.
Here is the board layout:
I customarily use the “top” layer in Eagle for routing and the surface mount devices. The connectors and switches go on the bottom, so they need to be mirrored before they are placed. The board only uses a few pins on the Nano board, but there are holes and copper pads to solder all the pins it covers to the Nano board so it has a good mechanical connection to the Nano. There are 2 ground pins available from the Nano board. I opted for connecting them both, at the expense of having a route go under the MAX1555. Although it looks scary at first, routing under devices works quite well.
Here is the Eagle .brd file after reading it in to OtherPlan, the OtherMill interface package
After milling a piece of FR1 this is what I had. I designed the two holes at the top to line up with the holes in the Nano board. I rounded the edges to make the board more pleasing to handle,
Now comes soldering and placing the components. I had been very intimidated my surface mount components. But after using them on boards produced by the OtherMill, I now prefer them to through hole components with leads. One key ingredient is getting a hot air rework station, like the one from SparkFun. I set mine to around 320 degrees and keep the airflow very low.
First solder paste needs to be applied to the board where the components go. In Eagle this geometry is available on the top cream layer. Here is what that layer looks like for this board.
There is a plug-in available for Eagle that will convert this layer to an SVG file so that it can be used to mill a piece of 5 mil mylar, which will serve as a stencil for the solder paste. The plug-in is called eagle2svg-1.1. The mylar is held down to the mill bed by double sided scotch tape.
I used SVG import and chose 1/64th and 1/32 bit to make the openings. I do not want to cut the outline, just the solder stencil openings. This will make it easier to use the stencil later on. Here is what it looks like in OtherPlan before I started the job.
Here is the stencil about to be used. I used double sided tape under part of the board to secure the board to my table. Then I used painter tape to make a hinge after I aligned the stencil with the board. It is important to lift the stencil up cleanly after the solder paste has been applied. The tape hinge works perfectly for this
After applying the solder paste on top of the stencil, a credit card was used to work it into the openings.
Then the stencil is lifted away and you can see how accurate the solder paste placement and thickness is.
This may look complicated, but it beats using a toothpick and dabbing solder paste on all the pads.
Next the components are placed with tweezers. I happen to live near a store that carries surface mount components on spools and sells them “by the inch”. But they are easy to find on ebay and Aliexpress. I used “large” components, size 1206. But I routinely use ones half this size, 0603, and get excellent results. I use small hinged boxes I bought on ebay to hold my components. Here is the board with the surface mount devices placed.
First heat the board from underneath to warm it up. Heat distribution is very important. Then move to the top of the board, keeping a good distance. Also keep the airflow to a minimum to avoid blowing the components around. Too rapid application of heat will rapidly boil the flux in the solder paste and move the components. Once the solder paste has heated up, it will liquify slightly and then turn lighter. Slowly circle the first component you wish to solder. There will be an instant when the paste will turn shinny and melt. Give it a second or two longer so it will flow properly. Components that are not perfectly aligned will often magically snap onto their pads. This is the surface tension of the solder pulling the metal surfaces together.
Next comes attaching this board to the Nano. I am using this for projects that run LED strips containing WS2812B chips. It is easy to get all the LED signals for this from the ICSP connector, so you see the wires soldered on there for the LED strip.
Here is the final result driving an LED strip.