A PC Watchdog

Project Features

• Automatically resets a crashed, frozen Linux PC
• Simple
• Uses a cheap USB-to-serial adapter

Introduction

For several years I have been using a MythTV Linux server to record television shows. It works very well and I like it a lot. It has one major problem though: Sometimes it locks up. I suspect that this happens when it changes channels and signals are weak (e.g. due to bad weather). It seems the drivers for my DVB-S receiver cards don't like that.

This is annoying, because it remains in that state until the PC is reset manually. No recordings are made in that state. It gets particularly annoying when I am on holiday, because then the recordings of a week or more can be lost. The box needs to be reset via the reset button. Not even ctrl-alt-del will work.

The aim of this project was to automate the process with an external watchdog. The idea was that the PC had to send a "heartbeat signal", i.e. a fixed character via a USB-to-serial adapter regularly. If it doesn't do so, the watchdog resets the PC after five minutes.

Since this is a Linux box, a simple cron job can be used to create such a heartbeat signal. If the signal is there, this means that the operating system is alive, the cron daemon is working and the USB driver is good.

The Eyes - A Last Minute Halloween Effect

Introduction
Halloween is a rather new tradition in our part of the world, but my family is very enthusiastic about it. Tombstones are built, bones are scattered in our front garden and a fog machine is kept in the basement for just this one day of the year. This time I thought that I could contribute a little LED effect. My idea was to use a couple of red LEDs to create a red-glowing eye effect, like some evil little monsters lurking in the dark.

The Morse Thermometer - Part 3

Putting it all together
In part 1 and part 2 of this posts I investigated and designed the components for a solar-powered Morse thermometer. In part 3 everything gets connected.

In order to get the most out of the collected sunlight it is important to use as little power as possible. A red LED is a good choice here, because it only has a forward voltage of 1.7 Volts. So it will consume only about half the power of a white LED with a forward voltage of about 3.3 Volts.

Perhaps you remember that I had found out that even when the LED is not lit, the boost circuit from the solar lamp draws about 7mA. So it was important to switch it off during the time the thermometer is in stand-by.

Let's have a look at the original lamp circuit again.

Another simple electronics project for kids

 

Project Features

• Simple
• Cheap
• Doesn't take long
• Can be done by children (with a little help)

Introduction

One of my sons recently started to nag me. He wanted to solder something. Initially I had no idea what he could solder and I thought he was simply bored. But he kept going and I knew I had to find something. Finally I remembered that it would be "Blue Night" soon and that I wanted to take my kids there. So we could build some coloured light object to be worn at the event.

I looked through my unused hoard of components and found the bag with one hundred rainbow LEDs (i.e. slow colour changing LEDs). I had ordered them from China for little money - for no good reason really... 

And so was born the idea to build a little pendant of 3x3 LEDs.

A Beginner's Guide to Driving LEDs - Part 4

The transistor as current source

In part 3 of this tutorial we looked briefly at several current sources, which contain integrated circuits. In this part, we will look in more detail at a simple current source built from a transistor, a zener diode and two resistors.

A beginner's Guide to Driving LEDs - Part 3

 
Actively Stabilizing the Current
In part 2 of this tutorial we found that there are situations, when a simple resistor is not good enough to keep the current through an LED stable. There are countless ways to stabilise the electrical current actively. Some are simple, some are complicated. I will show a few here.

A Beginner's Guide to Driving LEDs - Part 2

 
Resistance is not futile...

Part 1 of this tutorial showed why using an LED is not as straight forward as using a light bulb. Now we have a look at different ways to operate an LED.
 
The simplest and well known way to operate an LED is to add a series resistor. It is quite easy to determine the value for this resistor:

A Beginner's Guide to Driving LEDs - Part 1

Introduction

LEDs are just great. Nowadays they are cheap, durable and power efficient. You get them in all colours, shapes and sizes.

But along with all the advantages comes a disadvantage, too. LEDs have special demands when it comes to their power supply. In most cases, this is not too bad. But many beginners in electronics have problems to understand those requirements. With this post I would like to help those people to get an intuitive understanding of how to use LEDs. I will keep it simple, but hopefully still useful.

The Morse Thermometer - Part 1

Project Features

• Extremely simple hardware
• Low-Cost
• Uses the internal temperature sensor of the ATtiny25 

Introduction

I used to have one of those electronic thermometers with an outdoor sensor. That sensor transmits its readings through an RF link. Except that it didn't really work. Every now and then I had to reset the thermometer so that it would re-connect to the sensor. This annoyed me and I thought maybe I could make something that actually works.

Well, that was my excuse for this project...

A Simple Electronics Project for Kids

 

Project Features

• Simple
• Cheap
• Doesn't take long
• Can be done by children (with a little help)

Introduction

When I was working on one of my projects recently, my 8-year-old son came along and wanted to "help me". I was working on something really complicated and there was no way I would let him meddle with it.

On the other hand I wanted to take the opportunity to show him that electronics can be fun. So I decided to build a small 9-Volt torch light with him.

The Micro POV

Electronics design in my opinion is the art of finding the best compromise. And this can be quite a complex art. There are many different and sometimes contradicting requirements to a project: Cost, time, space, weight, power, technology, market, availability of materials and tools, knowledge. This is why the all singing all dancing solution isn't always a good solution. Sometimes less is more. And I don't know why that is, but I have a special affection for minimalistic solutions.

I know, there are POV projects on the net in abundance. And most of them are some kind of propeller clock. There are a few really nice examples and I wanted to have one. I even started to build such a work of art and it was going to be really great with lots of colour and such. But very soon I gave up, because I just don't have the skills or equipment to build more complicated mechanical objects.

I realised that I had to reduce my expectations - a lot. And looking at various projects that I found on the net, I decided that maybe I should go for something really ... compact. And so the idea for a micro POV was born.

The name is of course a reference to the open source project MiniPOV by ladyada. But I wanted to go one step smaller - something to wear e.g. on the wrist. Of course that generates a few problems, which is just what made the whole thing the more interesting. Every component for this project needs special attention.

LED Photos

OK, this one isn't really a project. But I wanted to share it anyway. One day a bought this little ...light thingy in a junk shop. I didn't really know what it was, but it had LEDs and anyway it was only 1 €...

This is what it looks like:

It turns out that it has three very bright LEDs in red, green and blue and it works in different modes; one of them is a colour change mode. But it does this by using a PWM scheme. So when you swing it around fast enough, you see circles in different colours.

And when I got my new camera with a night shot mode that allows to do an 8 second exposure time, I had to try photographing this. The pictures turned out quite well.