2014-06-18

wireless power - status update 2

So what happened in the last two weeks? As already mentioned, I finalized the PCB and send it away. I did go with Würth in the end. They were not the cheapest of the lot, but provided the most bang for the buck. I got 5 day manufacturing time (and since they deliver with courier, probably even from Germany, delivery also takes only one day). Also, they did not charge extra for silkscreen or solder resist (as may happen with cheaper manufacturers). And last but not least I got Ni/Au plating for still the same price, which is not only better for long-term usage, but also better than HASL for SMD soldering. So I send the board files off to WEdirekt (no need to convert them to Gerbers), and thanks to Simon Leuz I even got a 10 percent rebate

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2014-06-10

Wireless Power - the full schematic explained

Since the contest is now nearing its completion, I gather its about time to actually show and explain the whole circuit. I already went through some of the parts of it, and how I decided how to build it. But I never showed how these parts work together. So lets have a look.

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2014-06-06

wireless power - status update

So the last couple of weeks was silent with regards to my project - I was just too busy. Being on a business trip left not so much time for doing electronic projects (and I forgot to take the Eagle project with me). And the aftermath was then rather busy at work.

But I got new transmitter coils from Würth in the mean time, and did some experiments with them (which need a write-up).

Apart from that I have finalized my schematic, and added all the missing details I already explained the most important parts of it, but there are still some minor tweaks here and there. So there will be a separate post about it.

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2014-05-24

Building a multi-voltage reference for my DMMs

A while ago I saw in the Dangerousprototypes forums a nice project: MrRef. Its a small voltage reference, intended to verify that a digital multimeter is still within its spec. Normally one would use either a real high-precision meter (like the Agilent 3458A) or a really good, calibrated voltage reference. But these are not exactly cheap, and probably overkill for a 50$ (or cheaper) multimeter. So matseng took a not so expensive voltage reference (a MAX6350, but it onto small breakout board, together with 4 batteries, and had something which is at least more accurate than the normal hobbyists multimeter.

Seeing this I did something similar - but I used a REF5020 from TI instead:

The REF5020 voltage reference

but it was not as accurate as I would have liked (the initial accuracy is 0.1%), and the 2.048 Vv output voltage meant that its just over the range for a 2000 counts multimeter (so it read 2.05).

So when I bought a new multimeter last fall (two UT61E actually), I looked into building a new, more accurate reference, with a wider range of voltages.

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2014-05-15

wireless power - battery under-voltage lockout with multiple cells

When designing systems powered by rechargeable batteries, especially LiIon batteries, its important to ensure that the batteries are not discharged too much. For primary cells this is not a problem (except that the device won’t work anymore), but secondary cells can be permanently damaged by such a deep discharge. normally this just results in permanently reduced capacity (which is bad enough). But a LiIon battery that has been discharged too deep can actually catch fire (that happens when its charged again, since during discharge internal shorts can form inside the battery). So an under-voltage safety mechanism has to be used to ensure that never happens. Basically all single cell LiIon batteries already come with such a circuit (most times inside the case already). Many battery packs too come with it, but especially pack for RC models do not, since the high current there make that difficult. A typical cut-off voltage is about 2.5 V, sometimes as low as 2.3 V. That is a voltage where the capacity might be permanently reduced already. So I would like to have the cut-off voltage higher, at about 2.9-3.0 V. That means building my own under-voltage-lockout (or UVLO)

Such a safety mechanism gets a little bit more complicated in the case of a battery pack, since there the cut-off needs to happen when just one cell gets discharged below this voltage. So lets look how we might implement that.

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2014-05-13

AXP configurable logic design contest wrapup

So its time to finalize my project for the AXP Logic design contest. After having looked at the evaluation board, explained the electronic circuit as well as how I build the rest of the project, the last part the rules are calling for is a small essay describing my experiences with the AXP logic gates, the evaluation board and the project in general.

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2014-05-07

How to build a "most useless machine"

As promised last time, this article is about finally building the most useless machine of them all. When explaining the electronic / logic side of this project, I already talked a little bit about how it works. But now its time for some details.

The basic idea is rather simple: there a motor, connected to an arm. On the outside of the enclosure is a switch, which turns the motor on. The arm then moves the switch into its original position, and returns into the enclosure afterwards:

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