On Fri, 23 Dec 2005, Peter Todd wrote:

> On Tue, Dec 20, 2005 at 08:12:06AM -0800, Neil Baylis wrote:
>> You don't need to mount the laser on the shaft of the stepping motor.
>> All you need is a tiny mirror mounted on the end of the shaft at 45
>> degrees. Point the laser into it, i.e. along the axis of the motor,
>> and it will scan the beam through 360 degrees if you want. This will
>> also work with a clock movement, even a very small one. The mirror
>> only needs to be big enough to intercept the laser. This lets you use
>> any laser.
>
> As someone else mentioned I'll need a front-silvered mirror for that. I
> thought of this option initially, and decided against it as the mirror
> would eventually get dusty. Cleaning is problimatic as it's high up on a
> ceiling and would requier special training.

You do not need a front silvered mirror. The double reflexion produced 
by the usual mirror is not a problem as it is weak (<10%) of the main 
beam and very close to it (at most sqrt(glass thickness) for angles of 
+/-45 degrees).

You may also want to give up on the stepper and use an analog 50uA meter 
movement from an analog DVM. You can glue the mirror on that and drive 
it with PWM from a pic as I said.

You may find that the stepper will not be able to stop on positions you 
like. For a reasonable sized room even a 0.5 degree per step stepper 
will cause your spot to stop far away from any reasonable-sized stops on 
a wall. The analog movement can be driven with say 16-bit pwm and will 
easily be settable for such situations.

good luck,
Peter
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