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Precision Low Current Measurement w / Feedback Ammeter

If you've been following along on my journey to learn electronics design, you'll know I've been working on a motorcycle power supply. It's getting pretty close to finished and its time to do testing. The last series of blogs were about a constant current load testing device. I needed that to test the power supply under different load conditions. This installment is about the opposite end of that spectrum, what happens when the circuit is "off". Because lots of motorcycles tend to be parked for months without being ridden I need to verify that my device does not contribute to early battery failure. To do that I need to measure the current consumed when it is plugged in but off. We are talking about very small currents though so very high precision measurements are required. Normally, you can use a digital multimeter for this purpose but for measuring small currents this not as straightforward as it seems. Ammeters are connected in series with a circuit an

Constant Current Dummy Load - Complete

The project is finished. The units have been built and sent off for use. Much was learned. Here are some insights. First, don't design a PCB for the lowest cost. That just increases your costs later. I made the the board narrower than was convenient to save $20 and I ended up wanting to put the entire thing a case so I could mount the banana adapters off the PCB where they could be farther apart. Not necessary but preferred for me. Second, putting artwork in copper on your PCB not only works but looks great when you are finished. Take a look at the second photo below. That Freeside logo looks awesome! Third, assembly of through hole components is NOT easier than surface mount parts. It was a bit of a pain to build them all. TH components don't stay in place when you flip it over to solder.  Final PCB I used. Artwork in copper. Queued up for soldering. Delivered and installed. My lab's version with voltmeter and case. It includes a cooling

Raspberry Pi Headless Media Center

Greetings fellow Raspberry Pi enthusiasts. I have something pretty cool to share with you today. I've been wanting to use the Raspberry Pi for a media center since I first heard about it. When Newark finally told me I could order the Pi I jumped at it chance and then promptly left town on a family vacation. My nephew Isaiah, who I met up with in Alaska while we were on vacation, discussed the media center project and how to implement it. Upon his suggestion we settled on running VLC. I had assumed we would have to get some webserver up and running and figure out how to slave VLC to our wishes but that is largely built in already. It is called the VLC Web Interface (or VLC http interface). That made this project all the more simple. Below is the process to install it. This process assumes you have a freshly flashed Raspbian Wheezy (7/15) image that has NEVER BEEN BOOTED BEFORE. This made the most sense for us as we assumed you would plop this guy down somewhere and not use it

Roomba Wii Shield

I've been working on an Arduino compatible Mini USB Host board. Well, I've had it done for a while now. I've slowly been compiling a bunch of projects for it so I can give it a cool Kickstarter one of these days. To that end, I designed a dedicated shield for controlling a Roomba vacuum with a Wii remote controller so that it might make the entirety of the project more compelling. I'd already written the firmware and tested it with a chopped and spliced up cable a few months ago. It worked well for a while but one day the entire thing shot craps. My belief is that the on board voltage regulators could not handle the 16V the Roomba outputs (too much voltage to drop with an LDO without good cooling). So, for the dedicated board I decided to put a TO-220 7805 regulator with a heatsink on it. That should protect the main board from damage. I also had the idea of using optocouplers to isolate the Roomba signals from the shield. Optocouplers are ICs that contain an inte

First flight of the Hexacopter!

It's been a difficult journey, but we finally got the first flight of our DIY Drones Hexacopter . A special thank you goes out to the whole UAV team at Freeside Atlanta for donating their time, money, and resources to this project.  Slade, who did several hours of last-mile troubleshooting to get the UAV in the air, is flying it by hand. Check out the video on Youtube , which shows us testing the pitch, yaw, and roll of the unit and finally getting flight. We were pretty surprised at how smoothly it flies. Stay tuned, as we'll be setting up and configuring the UAV controls so that it can follow GPS waypoint flight paths. We're also going to be posting another video that explains how everything works, how we did the troubleshooting on the initial flight, and some new in-flight footage.

Constant Current Dummy Load - continued

Since the last installment, I've finalized the initial PCB design and sent it off to Seeedstudio  for fabrication. I've never designed a through-hole (TH) construction board before but in general all the same rules apply to SMD and TH boards I believe. 200W 0.1Ω load   30W 1Ω load 10x10Ω 3W in || 25 turn trimmer pot I changed up a few things during the PCB design phase of this project that I thought would be interesting to mention. I added a pair of trim pots to design. One trim pot reduces the maximum voltage going into the first op-amp stage. The second is the in the feedback loop of the first op amp stage. At maximum resistance, 3000Ω to GND (R2 of the voltage divider), with the 820Ω R1 (there for its role as a low pass filter in addition to the voltage divider), I should see a 27% voltage gain. Turned down to 0Ω, that pot creates a situation where the output is the high output voltage of the op-amp (essentially infinite gain). Something like 8V for the op

Arduino Rotary Encoder Library with Velocity Sense

While working on the constant current load project I found that while nice, the multiple turns required to turn up the power up was a little annoying when you just wanted to hurry up and get to a high value. The standard solution is to have a "fine" and "course" knob. Since I designed the input in the digital realm it seemed like software was the obvious solution. Why not sense the speed the user is turning the knob and extrapolate the pace of change based on that input. It seems intuitive to me. I implemented it as a little C++ library that you can drop into your arduino/libraries directory. Here is the most trivial implementation of the library. #include <RotaryEncoder.h>; RotaryEncoder encoder(A0,A1,5,6,1000); void setup() { Serial.begin(57600); } void loop() { int enc = encoder.readEncoder(); if(enc != 0) { Serial.println(enc); } delayMicroseconds(5); } Inside the library, the code counts the number of the sequential clicks in one

Tonight (July 14th, 2012): Chili fundraiser for the Offroad Wheelchair Project

The Off-Road Wheelchair is a device that will enable a person of limited mobility to experience the outdoors. Freeside Atlanta is interested in putting our engineering skills to the test, and showing the world we can create anything we put our minds to. We have the space, the tools, and the talented people to make this happen - we just need your help to purchase the materials and make this project a reality. The idea originated with Robin Beattie, who participates in regional Burning Man events, like the Alchemy Arts Festival here in Georgia. The festival takes place on a farm, with terrain features like gravel, light mud, paths with tree roots, and moderate changes in elevation. The Off-Road Wheelchair will be designed to traverse this environment, enabling Robin to get around in a place that would normally be closed off to her and others with limited mobility. Upon completion, the Off-Road Wheelchair designs and notes will be released under a Creative Commons (by-nc) license, w

Constant Current Dummy Load

I just got my first oscilloscope. Complex analog circuits are now possible. As I mentioned in my last post, I working on a power distribution unit (PDU) for motorcycles, cars, and other DC applications. The project is very close to the testing phase and that means I will need to run this device at precise power levels and at precise temperatures. The testing protocol also includes vibration studies, but today we are just going to talk about precision current testing. This can be done many different ways. You can just put a resistor to ground and use Ohm's Law to tell you how much current you are burning. If you want to change the current in use though, you will have to change the resistor. That is not very handy if you want to test your power supply at multiple currents as you will need a lot of different resistors. Since my system is running at about 14V and I am talking about 15A of current, I'll need some pretty beefy resistors and those are expensive. Instead

"Making lemonade with lemons" or "Reworking your bad PCBs"

I've been working on a large project for a few months now. It's a DC power distribution unit, and as you can imagine it has need for relays. I've got the thing setup to accept cards to can perform many uses both input and output. One of the many cards I've designed for this system (dual low-side switching relay, 5V, 12V, audio sensor etc..) is a dual high-speed 12V solid state relay card. Rendering of PDU and fresh pile of PCBs from Circuitmart This is a photo of the relay on a breadboard.  single channel of dual high-speed 12V solid state relay on breadboard This is the card from both sides. The "empty" space is used for thick and wide traces to carry current. These will be built with 6oz copper and can source 23A @ 330W in theory. I've only pushed them to 100W so far but they showed no meaningful rise in temperature so I think I am on the right track. By the numbers, they are right where they should be. Because these cards are small and