mechg

Here is some shameless self-promotion: I make and sell gauss meters that use MSP430G series chips: http://gravitastech.weebly.com/index.html My customers include slot-car racers who use them to tune motors, guitar makers who build magnetic pickups, and Model-T Ford owners who rebuild their own generators. The design is not open source.

If you are able to record both acceleration data and gyro data, it will be interesting to see if the gyro chip on the sensor tag can accurately measure rotation while under the high acceleration of a rocket.

Heads up to anyone trying the lower shunt resistors - I was just helping someone diagnose a problem where current readings in the mA range were almost twice what they should be, and we discovered that his range switches had more contact resistance than the ones I purchased. Identical brand and model of switch, but his have a resistance of almost the same value as the shunt resistor for the mA range (.1 ohm). This effectively doubled the voltage seen by the op-amp. On the PowerScope that I have been using in my daily work, I had to set the calibration constant all the way down to 811 t

It was pointed out to me that the 4-pin male header on my parts list, 3M9323-ND, is actually 2mm pitch, not .1" as is needed for the PowerScope. This one has the correct pitch: 3M9449-ND I have updated the parts list on the project site with this change. Anyone who purchased the 2mm header can certainly use it if they can break the pins apart and solder them individually.

Here is a good article on various approaches to current measurement. Of the approaches discussed, the PowerScope falls into the category of low-side current sensing using an instrumentation amp. http://www.eetimes.com/design/industrial-control/4236380/A-Current-Sensing-Tutorial-Part-II--Devices

There is a new C source zip file v1.1 on the download tab here: https://code.google.com/p/powerscope/ that fixes the bug where calibration constants are not stored in flash rom. Also, yesterday I was able to test the PowerScope with lower shunt resistors of .1ohm for mA range and 100 ohm for uA range. It is working well, although it does waver by 10-20 nanoAmps when touching the metal buttons or putting your hand near the meter, due to body capacitive effects. This does not surprise me since the smaller shunts mean that the voltage being measured is very tiny and is easily disturbed b

Ok, I found the issue. When I originally tested the storage/retrieval of the calibration constants, I must have been working at a lower clock speed. It appears that the "information" flash segment, where I store the calibration constants, does not like being written-to at 16MHz. By dropping down to 1MHz, just while data is being saved, all of the constants are being stored and retrieved correctly. I had to add this code around the call to storeCalibrationData() in main() : --------------------------------------------------------------------------------------------- BCSCTL1 =

For anyone who has built or is building a PowerScope, I've discovered a bug that is keeping some of the calibration constants from being saved between power cycles. I will post an update here, and update the source on the google code site when I have a fix. For now, the workaround is to calibrate the PowerScope using your trusted DVM, then write down the calibration constants you see on the PowerScope display, and copy them into the default values in Calibrate.c on this line int calibrationConstant[NUMBER_OF_CONSTANTS] = {993,1007,996,1000,998,973}; They are in this ord

PCBs are sold out already. Wow.

Very nice! I need one - for all those times that someone asks me for the time, date and voltage. ;-)

I think the cost expectation is set by the fact that decent multimeters can be had for $25 these days. Most of them cannot measure in the sub-microAmp range, though.

...... Burden voltage is always relevant if the drop impacts how your circuit draws current. When I was developing the PowerScope, I tried using smaller shunt resistors of .1ohm and 100 ohms to reduce the burden voltage to .03v max. Unfortunately, the tiny voltages were swamped by the contact resistance of my solderless breadboard connections. So I stuck with the 1 ohm and 1000 ohm shunts for the moment and lived with the .3v max burden. Now that I have it all on PCB, I intend to try lowering the shunt values again to see if it can handle it. The tiny voltages across the shunts

I had considered that, but I like to keep things modular. Maybe we can lower the cost a bit if I can convince 43oH to sell the PCBs individually.

The instrumentation amp is pretty good at measuring small voltage differences across a shunt resistor. Here's another device that does something similar, although it outputs to a separate voltmeter: http://www.adafruit.com/products/882 The Analog Front-End chip would suffice without the instrumentation amp, but it has fairly low maximum voltage input ratings, so for this application I thought it best to add the instrumentation amp to protect the AFE.

Oh, yeah... I had the link to OSHPark wrong. It's fixed now. It was formerly known as dorkbotpcb.