Hey everyone! Love the forum, I've been a lurker for a bit too long to be honest!

Anyways, my name is Stuart- I'm a technology student at the University of Glasgow. And like I'm sure a lot of you guys have done, I've recently started getting electronics lectures, tutorials etc. So I have a VERY basic knowledge but I'm eager to learn (hence joining).

I was actually wondering if anyone here could give me a hand with a project I'm working on just now, it should be REALLY simple for you guys looking at some of the depth that some things have gone into!

I'm currently designing a clock in my design class, and we have to make a working prototype of our model. The design that I have is basically a laser like >>this<< one which is on a unit on the ceiling which slowly turns, the time is then indicated by where the laser dot is pointing to on the wall.

The problem I have had is that clock mechanisms are very fragile, and dont have enough torque to turn the weight of the motor round. So I decided that a DC motor would be far better to do this. The problem I have however is that they turn severeal hundred revolutions per minute! Not the 0.08 revolutions per hour that I need it to do in order to tell the time! Now I know that if you half the voltage going into a stepper motor, it halfs the RPM. So could I just have a circuit containing only a power supply, a big enough resistor, and a motor and set the speed from that?

Because it is only a prototype, it does not need to extremely accurately be able to tell the time, however I cant have it spinning a laser round at 200rpm! I just need to slow it down enough to give our lecturer a picture in his head of how exactly the product would work.

Someone suggested a stepper motor to me, but are they not very expensive and do they not need to be computer programmed?

Thanks for reading guys, any help will be really appreciated!

i would solve it mechanicaly and reduce the rpm (if the difference is verry big)

with 2 disc s and a rubber string like in the old tape recorders or turntables,
if you don't want to have anny mechanicaly slipp work with sprocket chain or sprocket to sprocet conection
still a chance that you have a fall back from the top dead point problem

if you don't want that work with a sprocket wormweel configuration

with all methodes you will increase torque so you don't have to worry about that

sucsess with your project
Robert-Jan

You could try using a mains synchronous motor. These are usually found in mechanical mains timers such as this one:
http://www.maplin.co.uk/Module.aspx?...=timer&doy=4m2

They should be accurate enough for what you need because they sync with the mains frequency and they are geared-down when used in timers etc.

Originally Posted by stuz

I'm currently designing a clock in my design class, and we have to make a working prototype of our model. The design that I have is basically a laser like >>this<< one which is on a unit on the ceiling which slowly turns, the time is then indicated by where the laser dot is pointing to on the wall.

The problem I have had is that clock mechanisms are very fragile, and dont have enough torque to turn the weight of the motor round. So I decided that a DC motor would be far better to do this. The problem I have however is that they turn severeal hundred revolutions per minute! Not the 0.08 revolutions per hour that I need it to do in order to tell the time! Now I know that if you half the voltage going into a stepper motor, it halfs the RPM. So could I just have a circuit containing only a power supply, a big enough resistor, and a motor and set the speed from that?

Maybe I'm misunderstanding this, but could you perhaps just mount a little mirror on the spindle and bounce the laser beam off that? Laser (and other aimable lighting) effects units don't usually move the heavy lamp but rather just move a lightweight mirror.

If you can find a tiny little chunk of mirror which the motor can spin then you don't have to mess around with the mechanics of the thing and can just use it as-is.

Then again, I've only had the one cup of coffee today so take that into account.


Torben

Originally Posted by picasm

You could try using a mains synchronous motor. These are usually found in mechanical mains timers such as this one:
7 Day Plug-in Mechanical Timer : Timers : Maplin

They should be accurate enough for what you need because they sync with the mains frequency and they are geared-down when used in timers etc.

Mechanical timers like this are extremely accurate, simply because they are AC motors tied to the mains, which are kept at a precise 60Hz (over the course of 24 hours they average 60 cycles per second) and keep better time than a $10,000 Swiss movement watch!! Of course, they keep better time than say, a computer, which uses a crystal time base which has an accumulated error over time (unlike the mains). You may not notice it on your computer, as they sync up when you get online and correct the time. But a standalone machine will accumulate an error that will probably be noticable within a week.

Stepper motors are driven by 'stepping' through a series of coils (either 2, 3, or 4, depending on the motor) at the frequency you want to run the motor at. Trying to drive a DC motor at the correct speed is hard enough to do at full voltage, let alone when you try to tweek it down to run really slow, not a good idea. They do have stepper motor controllers that take a DC input so you wouldn't necessarily need a micro to run one, but I don't know how slow or how accurately they'll go without researching it.

I like the rotating mirror design myself, but good luck figuring out the angles and dangles of having a fixed light source and projecting the dot around this perimeter over there!!!! Seems the light source would have to be in the middle and deflected around to get the positioning correct. Maybe two mirrors, one to deflect the light source from a side to the center, and then one rotating to reflect it out to where it needs to be.

Interesting project. Keep us posted...

er, um, YA... I see that now... I just stumbled onto this forum last night... looking for injector control circuits... THANKS for the insight...

besides, better late than never...