Dave Althoff Jr wrote:

>For Christmas, my brother gave me a neat toy...a KNex Ferris wheel kit. 
>Nice.  But the drive system on it is absolutely horrible.  It comes with a
>little 3v geared motor with a battery box containing two AAA-size
>batteries and a switch.  Trouble is, the motor has a worm gear, so it
>can't slip, and the wheel has no clutch on it.  Lousy engineering.
>
>But then, I have these ][e's lying about.  It seems to me that with a
>simple bit of machine code, I could turn the paddle port into a neat
>little DC motor controller, using the annunciator instead of the battery
>box to power the wheel.
>
>So the question is...How much current can I draw off of the annunciator?? 
>Not being much of an electronics geek, but knowing that the motor won't
>complain at being fed 5v instead of 3v...any advice for protecting the
>computer?  

A worthy project, from which you can learn much and have a lot of
fun in the process.  ;-)

The logic output can't supply any significant power, so driving a motor
directly won't work.  Instead, use the logic output to drive a transistor,
or perhaps a power Darlington transistor (two transistors connected as
emitter followers, one driving the other).  Another good choice would
be a power MOSFET, since it can control a lot of current with very
little drive.  Either way, you want to put a reverse-connected diode
across the motor to avoid inductive switching spikes from killing the
transistor/FET.

You will no doubt try just switching the motor on and off initially.  But
you may also wish to try variable speed control, by switching the motor
on and off at a very high speed (at least a hundred times per second)
while varying the percent of the time that the motor is "on" (duty cycle).
By varying the duty cycle smoothly, you will find that you can control
the speed of the motor quite nicely and realistically.  Of course, reversing
the motor will require more complexity--a so-called "H bridge" of four
transistors or FETs to allow current to flow in either direction through
the motor.

Check the web for motor control circuits and algorithms. (Scott Edwards
has some info if I recall on his BASIC Stamp applications site, but there's
a lot available. For example, look at http://www.stampsinclass.com/)

>Of course the next step is to add a shaft encoder and hook it up to the
>paddle buttons so that the computer can handle a "load/unload" cycle......
> 8-)

-michael

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