How to stop a dc motor automatically

[Elektro_geek]

Hey guys, I'm a complete noob to any of this but I want to make a retractable license plate holder as found on ebay, and in the near future start making them for a few of my car forums (for car shows, etc.)

But I don't know how to make the motor stop when it fully retracts the plate/when it fully reveals the plate... again I have no clue about any of the tech lingo, but would like to learn...any help would be appreciated!

This is an example of what I'm talking about:
https://www.ebay.ca/itm/05-09-Shelby-Show-N-Go-Retractable-License-Plate-Frame-/370503806262?_trksid=p5197.m7&_trkparms=algo%3DLVI%26itu%3DUCI%26otn%3D5%26po%3D LVI%26ps%3D63%26clkid%3D4801862684931333689

 

[alec_t]

Use limit switches.

 

[audioguru]

It sounds illegal to me.
You robbed a bank and are driving away. Then you retract your licence plate so the cops don't know who you are and how many times you have done it before.

 

[Elektro_geek]

Use limit switches.

Thanks, I'll probably end up using these.

It sounds illegal to me.
You robbed a bank and are driving away. Then you retract your licence plate so the cops don't know who you are and how many times you have done it before.

Well...if used for that i guess it would be...
I'd be using it for car shows to prevent people from seeing my licence plate and increasing the chance of someone stealing my car. My team currently removes our plates and replace them with our club plates when we enter the shows...i think it would look a lot more cleaner if the plate tucked itself under the bumper or grill....and it would be a lot less hassle..

P.S. someone driving around without a plate would look a lot more suspicious than other cars lol

 

[Reloadron]

I would also vote for limit switches as a simple starting point. The mechanics would be a worse nightmare then the electrical for non large production I would think.

Additionally I always remove my plates before I rob a bank.

Ron

 

[duffy]

^ LOL! Always find helpful advice here on electro-tech-online.

 

[Diver300]

I have used a circuit that measures current and stops the motor when the current increases when the motor stalls at the end of travel.

However, you need a timer so that the motor can start, or the current peak when you first energise the motor causes it to stop immediately. You also need a timer to stop it eventually if the current doesn't peak. The normal operation relies on the current peak being significantly larger than the running current. That has been a problem on car windows where there is loads of friction, and that is worse in the winter than the summer, and I don't have any way of testing a whole car at low temperatures.

It ends up fairly complicated, so I did it all inside a microcontroller. It's not got a large component count, but if you haven't programmed microcontrollers before, there is a lot to learn.

If you did it with OP-amps and timers, there is less to learn but it is a more complicated circuit. Either way needs quite a bit of adjustment for your application.

 

[Diver300]

You might be able to use limit switches to measure the torque. If you have the motor / gearbox on a rubber mount where it can twist, the twist will be dependent on the torque. You could use the limit switches to stop the motor once it has twisted far enough.

(Think of a power screwdriver kicking back when the screw bottoms out, as in https://www.youtube.com/watch?v=5jvOTsi3i64 at 0:44)

That will the inertia of the motor, and the time taken to twist the motor in its mounts means that you shouldn't get any problems with it tripping at the start.

 

[4pyros]

If you look at the Show N Go Retractable License Plate Frame install instructions;
https://www.pfyc.com/supportfiles/GN1046.pdf
You will see thay just let the little motor stall, infact there is a warning about holding the switch to long.

 

[shortbus=]

Check out GM style power door locks. They have a motor and a rack and pinion to move the lock. They stop and shut off at the end of the stroke. By taking one apart you can reverse engineer it. This is the type - https://www.allelectronics.com/make-a-store/item/DLA-1/DOOR-LOCK-ACTUATOR/1.html But you can get them at a wrecking yard too.

 

[KeepItSimpleStupid]

Many times these will stall, because they are just designed to be pulsed. I believe the pulse is usually derived, so holding down the button doesn't do any real damage. A Polyswitch might also be used to protect the motor from continuous engagement,

Dual, directional limit switches can easily be implemented with two limit switches and two diodes.