CAM TIMERS
For controlling the motors and other mechanisms, I used to buy
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 still made (see the Radiospares catalogue). They are still the simplest way
of controlling a sequence of electrical devices. In
this age where most technology is hidden, they have increasing attraction
as a control system that can be understood simply by watching them work.
Programming languages are constantly changing, so if you need a control
system to last hundreds of years, cam timers could be a good solution! |
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PLCs
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. I use a Mitsubishi fx range that
new cost
from £100 (4 outputs) to £350 (14 outputs) but increasingly buy mine from
Ebay, which always has a good selection 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) but 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. 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. PLCs are also
constantly getting faster and more adaptable so I've never really had
enough incentive to change. |
