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: