Oak Tech

Well now that the PCB has been designed next up is making the PCB, testing and soldering.

Here’s some pictures of the PCB:

The bare PCB board straight out of the etching tank:

Green version of the PCB , has a protective coating:

Soldering the components onto the GPS board:

Overview of the GPS board it’s finished now:

Next thing up is the software, this requires a bit of work. I need to be able to parse the GPS strings into sensible data.I also need to be able to write the information to the SD-card.
The data I want to write must contain GPS location and the acceleration information. This means I also need to decide on the sample rate I want to have.

Yesterday I placed the design schematics about the GPS-bike project. Today I’ll post about the PCB schematic.

Well first of the image of the design:

GPSBike PCB

The PCB design is built up in the following manner. Power is put in at the right bottom side of the PCB. Almost the entire right area is reserved for the power supply.
It has a separated ground plane, this gives the switching power supply ample chance to interfere with anything else. The accelerometer breakout board is placed on the top right side of the board. Can be fastened with two m3 screws. The GPS receiver is placed in the top left corner, the connection cable can wrap around the board to connect to the SMD header on the bottom side. The SD card is placed on the left side of the board.

I somehow managed to mirror the ISP header without noticing, so the first design had a mirrored header. This new version has the correct header placement:
The schematics and PCB eagle files zipped: GPS_Bike-V2.0
Well next time I’ll show some photos and explain how I’m going to  design the software.

Last time I was talking about why and how I was going to build the GPS Bike.

This post will be about the other two parts of the schematics.

• Microcontroller
• Device peripherals

First of the image of the Microcontroller schematic

This is a very basic microcontroller schematic. It contains the required crystal and capacitors. There are also two switches connected to PD6 and PD7. An indication LED connected to PD5. Sometimes I tend to think ahead, so for future uses I also implemented a header so the switches and led can be externally connected, for example on a outside of a casing.

The accelerometer is connected via the I2C (TWI) bus. The bus requires pull-ups so they are pulled up to the 3.3 voltage supply. The interrupt pin of the accelerometer is connected on PC3(PCINT11).
The GPS is connected by UART. The GPS transmits with 4800 Baudrate. The PPS line if it is available from the GPS is connected to the PD2(INT0) line. This allows an accurate update of a single seconds and can be useful for future uses.

The SD-card requires the SPI bus to work so it is connected to the required pins. Additional to the bus there are two more pins connected the: Write_Protect and (not)Card_Detect lines. The card detect line is an inversed signal hence the “(not)”. This allows to detect additional information about the SD-card if it is write protected or to detect whether an SD-card is there or not.

That is all there is about the brain of the board. Now there is a small schematic left with some leftover headers and connections.

On the right is the accelerometer breakout board mounting headers. It is mounted on a small breakout board so it can be plugged into the two headers. The middle schematic is the SD-card bit. Basically connects all the lines to the micr0controller and has an extra capacitor (about 50uF) for buffering the power supply.

The GPS has is a small SMD header that connects the GPS itself. In addition, remember the GPS is connected to the 5.0 Volt power supply, we need to lower the voltage of the transmit line of the GPS. Maybe not the best solution in some opinions but certainly the cheapest, a simple voltage divider.

Well those are all the electronic designs , next up the PCB design and software development.

I’m currently working on a project that allows a bike to log the GPS location. Hence the name off course.

The reason for this project is to log the location of bumps or bad sections on(in) the road.
So what kind of electronic peripherals do I require to make such a thing.

• A power regulator for the device.
• GPS receiver.
• Accelerometer.
• SD card slot.
• A microcontroller to log/read/do everything required.
• Software

Some parts I already have in possession, so they might not have been the best choice or most efficient one.
But for me it is a lot more useful to use what I have lying around.

I happen to have GPS receiver called the EM406 from Sparkfun runs on 5 volts. (http://www.sparkfun.com/products/465)
Some regular SDCard slots, SD cards require 3.3 volt to work.
An Atmega328P microcontroller, this one can work on 2.5 to 5 volts depending on what speed you want it to run.

I do not have an Accelerometer however most of those sensors work on 3.3 volt or lower so I decided my main voltage would be 3.3 volt.
Next I choose the following sensor: BMA020 with breakout option. (http://jeelabs.com/products/gravity-plug) I’m not that handy (yet) when it comes to soldering LGA devices.

Al right critical components have been decided on to the schematic designs.

Basically I split the board up in several parts.

• Power supply(s).
• Microcontroller and requirements
• Peripheral devices and/or headers for those.

Below here is the power schematic I build

This schematic contains two power regulators, A AMS1117 non adjustable 5 volt linear regulator and a LM2575 Adjustable switching power supply set to 3.3 volt output using two resistors. The switching regulator has two filters on the output. The first RC filter is a 330uH coil with a 470 uF capacitor this will reduce the switching effect of the regulator a lot. The first one is also mandatory else a lot of things will act strange. The second ripple filter is an extra filter but when using it with a microcontroller I think it should always be implemented. This filter can use a coil with values between 25- 50 uH combined with a 100uF capacitor.

Most components on the board are powered with 3.3 V. As mentioned before the main reason for the 3.3 volt is because of the SD card. The selected Accelerometer also works on 3.3 volts. A lot of microcontrollers can work on 3.3 volt so that shouldn’t be any problem.

The only one that does not run on 5.0 volt is the GPS receiver. I’m sure there are plenty that do run on 3.3 volts but I happen to have one that runs on 5.0 volt. That is why I also implemented a simple 5 voltage regulator.

You can find the other schematics in the next post.

If you so happen to get on this site.

Then you should be informed that it is still under construction.

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