I made a Volumio music player

My goal in this project was to make a music player for my living room on which I can stream music from a server or radio and using open source software so I can upgrade it in the future without being dependant on a manufacturer. Also I wanted it to be remote controllable from a smartphone.

After some research I decided to build it based on a Raspberry Pi running Volumio which does everything I wanted out of the box. I was in for some surprises though by not using a setup that was fully supported by the developers so some coding ensued. I also wanted to spin my own housing design to give it a unique look. Here is how it turned out:

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Electrical design

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Advanced capacitive sensing with Arduino

…or any other microcontroller.

There are several methods to do capacitive sensing with a microcontroller. The easiest and most common way in the Arduino world is to charge the capacitive probe through a high value reseistor and measuring the time it takes to charge it. While this is indeed a simple method it is highly susceptable to interferance and noise as the wire acts as an antenna due to the high impedance. Also it is not very sensitive compared to other methods but good enough for a simple button.

Here is a video comparing the resistor method to the two methods discussed in this blog post:

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Nanowatt Pulse Motor

This post is about ultra low power motors you can build yourself with a little help from a 3D-printer. Put inside a glass display it will run for years.

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The motor is not indended to drive anything, how could it at such low power… just to put this into perspective: if you put a drop of cold water in your hand and let it warm up, the energy transferred to the water could run the motor for hours. I hooked mine up to an amorphous silicon solar cell Continue reading “Nanowatt Pulse Motor”

Grinding Ruby bearings

Ruby (or Corundum) bearings are used as small low friction bearings in watches or high quality measurement equipment. I wanted to have some bearings for my nanowatt power motors but I was unable to find any for a reasonable price which is somewhat ironic as there are plenty of watch manufacturers here in Switzerland. I asked around and nobody was able to tell me where I could buy a few. These things are also available cheaply from China but there are minimum order quantities of a hundred pieces or so for a doller per piece.

What I was able to find on aliexpress though was 3mm Corundum crystals used for jewelery. You can get them in small quantities for less than ten bucks. I learned quickly that they are useless for needle bearings because the needle jumps off the small flat surface all the time: it must have a dent in the center.

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DIY LoRa Antenna

One of the simpler designs for an antenna is a 1/4 lambda antenna with ground reflectors. Without the right equipment it can be hard to get it just right as the assembly has to be quite precise to get it to operate at the correct frequency.

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It would be a lot easier if there was some kind of jig to build it and get the ground reflectors to the correct angle and all the rods at the right length. That is what I set out to do. The design is for a 868MHz antenna but in theory it can just be scaled for other frequencies. Update: there is now also a 915MHz version of the jig available on thingiverse. I did not build and measure that one though.

Design and verification

First I thought I’d just do some simulations and ‘get it right the first time’. Continue reading “DIY LoRa Antenna”

Zero burden voltage current amplifier

When measuring currents of a circuit there is always one issue to consider: the burden voltage or in other words the voltage drop accross the sensing resistor. The way current is usually measured is using a shunt resistor and amplifying the voltage drop accross that resistor which is simply Ohm’s law: U = R*I. For most applications this works fine. If you want a really low burden voltage Dave Jones from the EEVblog built a very nice circuit called the µCurrent which uses much smaller resistor values than a multimeter does and the output can be hooked up to an oscilloscope. When I did that to measure the current drawn from a microcontroller I noticed that the noise on the highest setting of the µCurrent (up to 1.25A) was just horrible in the low milliamp range. At the next lower setting I was unable to measure the currents of 20mA drawn by the circuit. So I set out to build my own amplifier for this application.

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Testing the final circuit with an Arduino as a load

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