In this Instrcutable we'll cover:
- Design Considerations for the circuit and show the resulting schematic
- The Parts list and give you links to a good parts source and prices.
- The PCB Layout and how we will use that layout to build the circuit onto a Breadboard
- Step by step instructions on placing components onto the breadboard and soldering them into place
- Using the leads of the components and/or wires to help create the traces and solder them into place.
Step 1: Design Considerations
- Input voltage range between 8V and 35V @ 500mA or greater
- Able to accept voltage input from many different sources (i.e. batteries, wall warts, solar panels, ect.)
- Support the USB spec for powering devices (5V @ +/-500mA)
- Small form factor so it could be easily used anywhere.
- Ability to turn it off and on (seams simple enough but i've forgotten to add a power switch to projects in the past, not a very green way to do things)
To allow for the use of different possible input connectors i considered the typical DC jack but decided in the end a set of screw terminals would be best. So i had to include a protection diode in case people connected there power source backwards.
I was able to get the PCB layout down to a 1.5"x1.5" form factor. Small enough to fit inside of an Altoids Smalls tin as long as you lay down the Electrolytic capacitor and voltage regulator flat.
On many of today's smart phones and MP3 players the device won't start charging or use the power from USB unless it is receiving a small voltage on the Data + and Data - lines. These devices , such as all Apple iPods and Phones, are looking for 2V on the D+ and about 2.7V on the D- lines. So voltage dividers are needed to accommodate this. As you will see in the schematic R1, R2, and R3 are feeding the D- line. I found the best resistor values for this where 22K ohms on R1 and a total of 26K between R2 and R3. Two resistors are needed here because 26K Ohm resistors are hard to come by. Then we have a 22k Ohm and 15K Ohm feeding D+. Other resistor values can be used as long as the end result is close to 2V on D+ and 2.7V on D-. I've used this arrangement in past projects and know it works so i'm sticking with it for now. I've tried resistor values under 10K ohms and they don't work. So if you decide to go with a different voltage divider setup make sure the values are greater then 10K ohms.
Here is the Rev 1 Schematic thanks to Upverter.com:
Step 2: Parts List
- 1x 15k ohm resistor $0.07
- 1x 100 ohm resistor $0.07
- 2x 22K ohm resistor $0.07
- 1x 6k ohm resistor $0.13
- 1x 20k ohm resistor $0.07
- 1x SPST slider Switch $1.37
- 1x Fixed Terminal Block $0.54
- 1x USB Type A Connector $0.52
- 1x 0.1 uF Multilayer Ceramic Capacitor $0.16
- 1x 1.0 uF Aluminum Electrolytic Capacitor $0.32
- 1x 5mm Red LED $0.59
- 1x LM7805 Voltage Regulator $0.50
- 1x 1N4004 Diode $0.09
- PCB or Breadboard ($2.19 for a 2 breadboard set at Radio Shack)
Total cost in parts is less then $7 before cost of shipping is added. Making this the most versatile and affordable USB power supply you can find. Its small size allows it to be a lightweight backpack to turn any 8v or larger battery pack into an extended life battery pack for your mobile devices. It also allows you to make use of almost any would be useless wall wart you have lying around.
If you spend another $5 you can get a nice little 8x AA Battery holder and 9V snap connector and have a good 9V power pack for any USB device. Then you have the option of using regular AA batteries or rechargeable AA batteries.
Step 3: Board layout.
With a breadboard layout in mind i have included a PDF file of Rev 2's PCB layout without the copper fill. I picked up a Dual Mini Board Breadboard from Radio Shack for about $2.19. This Breadboard is just slightly bigger then my PCB layout. Best of all the PCB layout lines up to the grid of the Breadboard. So you'll be able to print off the PDF of the PCB layout below and overlay it onto the Breadboard.
Now to get things pieced together.
Once you print the layout cut it down to size and tape it to the side of the board without copper pads. Line up the bottom right corner of the layout with the bottom right corner of the board. Hold the board and layout up to the light so you can see through the paper where the holes are and get everything lined up. To help ensure the holes all line up take one of the components and use it to punch all the component holes out. If things are not quite matching up then make adjustments as needed.
For the most part you will be able to solder the components together similar to the PCB layout but there will be some sections where you may have to make adjustments. We'll get to that in the next steps. I'll break the assembly down into several steps to help insure you get everything plugged into the right places and soldered together.
I've included the PCB files for Rev 2 below in extended Gerber, PDF , and Eagle CAD files if you want to try etching your own board. I've also included a Fritzing file of the REV 1 Board which has a simple prototyping breadboard layout if you want to prototype it before putting everything onto a PCB or peg board. The finished board is only 40mm x 40mm. As you can see there is still a good bit of open space on the board. It can be made much smaller for final PCB production. I did consider having the resisters mounted on end to help shrink things down a little more. But we don't want to make things too small otherwise it becomes difficult for the hobbyist to assemble. I hope to get money together and have a small production run of boards made in the near future. I may get through a couple more of revisions before that happens.
Step 4: Tools Needed
- Soldering iron
- Needle nose pliers
- Wire cutters
- Wire strippers (if you plan to use wire to make the traces)
- Helping hand/PCB clamp
- Small flat head screwdriver or Probe
- Solder braid/pump/bulb
- Magnifying glass
Step 5: Placing the Resistors
Bend the leads of the resistors out so they will not fall out when you turn the board over to solder them into place. When working with breadboard like this i like to use the leads of the components to link the traces from one component to another. So bending you leads in the direction of the nearest component they will be linked to is a good idea.
With the resistors in place flip the board over and solder them in place. Don't worry about soldering the ends of the leads down as traces until we get all the components in place. But go ahead and use your pliers to bend them around to where they need to go and cut off any access so its out of the way.
Step 6: Placing Capacitors, LED, and Diode
With the LED and Diode its important to pay attention to which lead is the Anode and which is the Cathode. On the LED one lead is longer then the other indicating the Anode. So your short lead is the cathode and should be on the right hand side of the board. The Diode's cathode is indicated with a white line around one end of it. That line should be facing towards the left side of the board.
Like before bend the lead out a bit in the direction they need to go. Flip the board over and solder the leads into place. Then rout the leads and trim as needed.
Step 7: Placing the voltage regulator and switch
Flip the board and solder the 2 components.
Step 8: Placing the connectors
Make sure the openings of the terminal block faces out. The terminal block has a couple of plastic legs sticking out just in front of the leads. You can choose to clip these off or widen the holes in the board with a 1/16" drill bit. I suggest widening the holes since they are good for keeping the terminal block in place.
As for the USB connector you'll have to do some added work here. There are 2 clips on either side of the connector. You will need to enlarge the the holes on the board (designated as GND on the reference image above) with a 5/64" drill bit so the clips can snap into the board.
Like the switch the terminal block and USB connector leads will not be long enough to bend over and use as traces but friction should hold both part in place till you solder them. Solder everything in place.
Step 9: Soldering the traces
Take your time and do one trace at a time. Like they always say "measure twice and cut once". In this case check and verify the trace placement twice and solder once.
Step 10: Conclusion
NOTE: Its important to note that any voltage input above 12V will REQUIRE a heat sync on the voltage regulator. Even with only 9V input with a 700mA output i measure 180 degrees from the regulator. If you plan to mount this into a mint tin the body of the tin can be used as the heat sync.
NOTE: When connecting a power source be aware of the polarities as you connect it. If you get the polarities reversed nothing will happen due to the protection diode. So if you get everything connected and the circuit won't turn on then flip the wires around and try again. Remember the screw terminal towards the outside of the board is the ground and the one towards the middle of the board is positive. You may want to mark the polarities on your board.
As mentioned earlier this is the Rev 2 design and future revisions are in the works. I may explore a couple more layout options before moving onto some of the other ideas i have in mind for future revisions. I've included Revision 3 of the PCB layout below. It puts the Power supply within the typical dimensions of a USB thumb drive. The board layout is about 19mm X 56mm.
Plans for future revisions:
- Ability to use both AC and DC voltages
- Remove the need to worry about polarity of the power source being used.
- Dual regulators or change in regulators to allow up to 2A to the USB connector to fulfill the power needs of iPad
- Ability to toggle between 1A and 2A of available current for better efficiency.
- Ability to have a single PCB that is scale able for different version of the kit. (i.e. 1 USB connector with 1A output, 2 or 3 USB connectors with 2A output)