tim   .


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.   hunkin




Electricity is mysterious stuff, though it is possible to get an intuitive feel for it by working with it for a while. It is much less dangerous than people think, particularly since modern trip switches have replaced traditional fuses. I still try to use low voltage systems for museums and public places because its one less thing to worry about.

My favourite type of motor is a car windscreen wiper motor. DC motors have a particularly high starting torque, which makes them perfect for driving crap mechanisms that have a lot of friction. Wiper motors are powerful (up to 30kg cm), can run at a variety of speeds, from 5-180rpm, according to the voltage used (from 3-12) volts, quiet, and relatively cheap (free from the scrapyard, about 60 new), and relatively safe being low voltage and not usually powerful enough to cause serious injury. I buy 24v ones direct from Bosch. They sell ones with a proper shaft rather than the taper and screw usually found on car wiper motors and the 24v allows an even greater range of speeds.

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To control the motors and other mechanisms, when I was young I bought cam timers from industrial surplus stores. A small motor on the end drives round the drum with a series of cams in contact with microswitches. By adjusting the cams, any circuit connected to the microswitches can be made to switch on and off as the drum rotates. Cam timers are now obsolete (though they can still sometimes be found on ebay). Its still satisfying to be able to see how it's working, particularly if you are a bit intimidated by electronics. 

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More flexible and reliable than a cam timer is a Programmable Logic Controller. These industrial devices are primarily made for factory automation, controlling pneumatics, but are very adaptable. You can read a piece I wrote about them for Make magazine here. I use a Mitsubishi fx range that new cost from 100 (4 outputs) to 350 (14 outputs) but increasingly buy mine from Ebay, which sometimes has them much cheaper.  They are programmed in ladder logic either with a hand-held programmer or on a PC. It took me a while to get the hang of the programming language, but the struggle was definitely worth it. They are amazingly robust and reliable. I've fried a few outputs connecting them to switched mode power supplies (with the inrush current - most notably a 15watt DVD player connected to a 5 amp rated output). However despite having over 50 PLCs running every day in different locations (most running for over ten years) I've only ever had one fail completely (which was replaced free under Mitsubishi's lifetime guarantee). 

  PIC chips can also be used for control. They are very cheap, and basic programming of PIC chips is increasingly being taught in schools. They are more versatile than PLCs - they cycle much faster (so they can do things like multiplexed displays). They can also easily accept analogue inputs and outputs (making them suitable for motion control). I've done a few things with them but its easy to get bogged down with their added speed and analogue capacity. I've tried several different ones. PIC (by arizona microchip) were the first microcontrollers but I don't find their software intuitive. 

Arduinos are easier to use and because of their success, there's a great selection of 'shield' boards to plug in. These provide relay outputs, servo drivers, mp3 players, accelerometers, the list is endless. There are also large online libraries of programs so its often possible just to modify something that already exists. If you get stuck there's also a big online community to ask for help.    

Raspberry Pies are more powerful and can run HD video as well as  everything else. I haven't used them myself. This is partly because its useful for all my machines to run from the same system. Its also because years ago when I first thought of learning C++ a friend advised me to stick with what I understand if it does what I need. Its better to understand one language thoroughly than several languages partially.  I don't enjoy learning new programming languages so I followed his advice. 

For people like me who aren't really into learning programming languages, it makes sense to do the most with the least possible programming time, so I stick to my PLCs where possible. PLCs are also constantly getting faster and more adaptable so I've never really had the incentive to change completely.  

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There is now a great site for learning about transistors, the heart of all electronics.
 If I had to start from scratch now, I would make lots of the circuits shown there.

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I did quite a lot of electronics for my engineering degree, but it was so theoretical it was no help in actually building anything. I eventually bought the children's kit above - which was brilliant. Then I found the booklets by Forest Mimms.
I recommend his  'Getting started in electronics' ISBN 40293 10515  
If you want a bible of Electronics, its 'The Art of Electronics' by Paul Horowitz and Winfield Hill - CUP - ISBN 0 521 37095 7

There's a guide to making simple electronics for coin operated machines on my Under the Pier Show site





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