Category Archives: Electronics
Last week I bought one of those new Raspberry Pi computers. It is a small single-board computer about the size of a deck of playing cards. It was developed in the UK by the Raspberry Pi Foundation with the intention of making a cheap computer to be used in schools for teaching about computers as opposed to teaching about Word, Excel and other applications.
There are two main versions. The one I got (and I think the most popular) is the slightly more expensive Model-B which I bought in Australia for about $38.00 (without SD card). While it doesn’t look like it could do much it has a 700MHz ARM processor with 512MB inbuilt RAM (like those used in some mobile phones), 2 x USB ports, composite video output, HDMI output, a 3.5mm audio output, a SD card slot, an Ethernet socket and a micro USB socket used to connect a 5v power supply. It can run Linux (I have a version designed for it called Raspbian) which is installed on a SD card. All that’s needed to start off is a keyboard, mouse and screen (a TV will do) and a connection to the internet (I use a USB wireless network card). The other thing it has of interest is a GPIO (General Purpose Input Output) header that lets you connect sensors and other things.
The first thing that stood out is the box. It looks like a box of tablets. I opened it up and removed the small circuit board and spent the next 15 minutes admiring it and passing on all the “interesting” facts I knew about it to my wife until I thought she could take no more. The next day when I had more time I inserted the SD card, connected my phone charger as a power supply, a keyboard and wireless mouse and the TV via a HDMI cable and switched it on. Lots of text came up on the screen followed by a prompt for a username and then password. I read the instructions that came with the SD card that I bought on ebay and got the Linux GUI desktop running. I felt like I did when I got my first Commodore 64 computer. I was excited that I could get it to boot and realised that there was so much to learn. Eventually I got an old USB wireless networking stick to connect to our network so I can put that LAN cable away again.
The thing that is perhaps the most exciting other than the value for money it offers is the potential it holds for hobbyists. Already there is a community sharing tips and project ideas. They are cheap… cheap enough that money is not really a reason to avoid using them. Cheap enough that it doesn’t matter if you destroy one and cheap enough that you can buy a few if you want to have several projects going.
So what are my plans? I’m finding the learning curve is a bit steep at the moment and I’m not very confident that I can get very far with it. Initially I want to get it to run as a server and be able to log into it remotely. Next I would like to see if I could connect a sensor or two – maybe a seismic sensor and be able to monitor that remotely. After that I would like data from the sensor to be stored and remotely accessible.
I recently thought about some of the things I did when I was young and wondered how I have not received permanent injuries from some of the things I did. Perhaps I have. It’s not that I was particularly wild, but as I child I didn’t take too much notice of safety – I guess I thought I was indestructible.
I planned to write about some of the dangerous things I did but when I started putting them all together I risked looking like a complete twit if put it in all one article, so I’ve decided to just mention one incident. Maybe I’ll mention some others later.
When I was in High School I loved to fiddle with electronics. I wanted a TV in my bedroom but I certainly couldn’t afford a new one. Colour TVs had only been available in Australia for six or seven years so old colour ones were not easily available either. My Mum managed to get a broken black and white valve set (or tubes as that is what I believe they are called in some parts) and a few old chassis. The TV was in a large wooden cabinet with the speaker at the bottom a bit like this one. It didn’t need a stand – it was big, or at least the box was. The screen wasn’t particularly big.
I regularly read “The Serviceman” articles in electronics magazines. This probably sounds quite dull, but you may be quite surprised if you read them. Like all good writing, pleasure gained is more to do with the writing style than the subject and these articles where often quite funny detailing interesting customer interactions as well as how tricky technical problems were solved.
Using information I learned from these I began swapping components from the broken set with bits in the other chassis I had from areas of the TV where I suspected the problem was. Basically just using trial and error although one great feature that it had like many electrical things of that time was a schematic diagram stuck on the inside of the cabinet which helped. I eventually suspected the high voltage flyback transformer that ran the picture tube. These things apparently put out around 15 000 to 20 000 volts and need lots of care when they are running. I found a similar one in a chassis and swapped it. The TV I was fixing had a metal guard covering the transformer and a valve that a big metal connector on top with a wire that ran to the transformer. I couldn’t get the top of the guard back on, so I just left it off – after all most of the other chassis I had didn’t even have one. Switching the TV on brought it to life. Placing the tip of a screwdriver near the connector on the valve that ran to the transformer would produce a nice constant spark about 1cm long. I was pretty pleased with myself, even though I had used trial an error more than proper diagnosis. The first lesson I learned is fixing things is very rewarding.
I used the TV for quite a while and I even remember watching the Poseidon Adventure on it. One day I decided to adjust the horizontal hold while I had the back off the set. I reached around the back to adjust the knob while watching the screen to see what difference I was making. It was then I touched the metal connector on top of the valve. I got one hell of a whack. In some things I’ve read they have said that these are not that dangerous because even though it is high voltage the current is low and someone even said that when he did it he didn’t even notice except for the burning smell. That was not my experience – I felt pain and was left with tingling in my arm for the next half an hour. The second lesson I learned was respect electricity, because it won’t respect you.
It was about that time that we got a special protection gadget wired into our fuse box.
If you want to see what these transformers can do just go to YouTube and do a search for flyback transformer.
Detecting when an electric blankets is switched on
I have often wondered about the effect that electricity may have on our bodies. Years ago I discovered that I could tell when Mrs Piffle’s electric blanket was switched on while she was laying on it by running my finger very gently down her arm. When the blanket was switched off it felt smooth. When it was on I could feel a slight vibration which is hard to describe. It certainly felt less smooth. We did a series of tests with her turning it through the different settings – sometimes on sometimes off, and I could always tell when it was on or off. Just for the record, the electric blanket was on a foam mattress on a wooden base.
Do I have super powers? Am I particularly gentle person? Am I just awesome? Probably not. If I can detect this when some others can’t it is more likely that I benefit from baby bottom soft skin on my hands from years of office work. A few other people who have tried it, including Mrs Piffle have been able to detect the effect too, so it seems to be common, but due to the weakness of the effect maybe we just don’t usually notice it.
Electricity from a TV antenna
I had also noticed a similar, but stronger effect when I rubbed my finger along the rabbit ears antenna that was connected to our previous television but only when the TV was turned on. I’ve read that leakage can occur within TVs resulting in some power being released into the antenna socket. So, that probably explains that one.
Power from a laptop
I had forgotten about it until recently after my wife got a Macbook Pro. She was sitting on the bed one day with the laptop connected to the charger and I touched her arm and got exactly the same vibration. We tried unplugging the power connecter from the side of the laptop and the effect stopped, plugging it back in and it returned. Another series of blind tests showed that I could tell every time if it was connected or not. We did another series later and at that time the effect felt much weaker, so there must be something else at play. These laptops are in an aluminium case which is probably relevant. It’s interesting to note that the effect only happens when it is plugged into mains power. We then tried with the lid closed in sleep mode and the effect was still there, and then with it shut down, but still plugged into the power adapter and again the effect was still there.
Mrs Piffle can even notice it herself through the tips of her fingers sometimes when she is resting them on the metal case in front of the keyboard.
The power adapter for the Macbook has a fairly low output voltage of just 16.5v – not what I would have thought could be detected with your finger.
Nothing from an iPad
We also have an iPad so I did a few tests with that – after all it also has an aluminium body, but I could not detect any difference.
Is it a form of mains hum?
The mains power her is 240v 50Hz. Perhaps the higher mains voltage is relevant, perhaps not. The frequency of the vibrations seems identical in all tests so perhaps it is 50Hz from our mains electricity, but I can’t think of away to measure it with the equipment I have. It’s odd and very interesting, at least to me.
I found I got best results when you rub an area of skin that has no hair as the roughness of the hair makes it more difficult. Inside arms and the side of the neck seems to work well (except with the rabbit ears antenna – I rubbed my fingers up along its … ears?:)). Sometimes I use a fingertip, other times I find a knuckle works better.