Amp-controlled cutoff switch

[Frank of Oz]

Hello Gurus,
I wonder if anyone can help me, please.
I need a schematic of a current based cutoff switch.
It's for a "Curtain opener/closer. The idea is to have the (small) 12 volt motor stop once the curtains are either closed or open.

I have the LDR switch, which will initiate the motor action at dusk and dawn and rather then use limit switches, would like to have some automatic stopping build into the system.
Any help appreciated.
Best regards
Frank

 

[Nigel Goodwin]

Use limit switches! - simplest and best!.

 

[Frank of Oz]

current- cutoff switch

Thanks for the reply Nigel,
You're probably right, it would be simplest. I use limit switches on my remote operated gate in the driveway. Limit switches work like a charm.
The idea is that the curtains open and shut depending on day/night conditions, automatically.
Limit switches are a bit hard to install, (not impossible), but I thought that there must be a way to use an amp-comparator or such, which can be adapted to use as an current-draw off-switch.
Thanks again.
Frank of Oz

 

[Nigel Goodwin]

You 'could' monitor the current to the motor, but it's going to be more complicated, and far less reliable - and you should still install limit switches for safety anyway.

 

[Hero999]

A simple circuit that cuts power to the motor when the current exceeds a certain level won't work because it will cut out when the motor starts. You need to ignore the current surge when the motor starts, then stop it when the current rises again. This easiest way of implementing this is with a microcontroller or even a PLC.

You still need to have a fuse or circuit breaker if your current sensing circuit fails but make sure it's a slow blow not fast blow or it'll go when the motor starts.

 

[Frank of Oz]

overcurrent

Hello Gurus,

We are talking only about very small motors with a low gear ratio.
DC 12V and 10 revs/min. I have a very nice motor made in Taiwan (Jye Maw-JM600-3540)
I have seen small motors which "stall" and then stop due to overcurrent protection, so it must be feasable.
If I find something on this, I will let everybody know.
Maybe I need a micro-controller, but I thought that a amp-comperator or similar could be utilised.
Thanks you for your input.

Best regards
Frank

 

[Hero999]

It wouldn't surprise me if even those little motors have microcontrollers inside.

You could make it so the cut out has too longe time constant to cut off when the motor starts but still short enough to protect it from overheating when it stalls.

It's also possible that the gear box might not be able to take the motor's full torque but it's probably unlikely in this case as it's only a small motor.

 

[Frank of Oz]

Hello Gurus,

I bought a "curtain opener/closer" device. Works good.
It has a micro processor, Atmel AtTiny45 8 pin Micro Processor.
Plus another 3 pin device, which I think is a voltage regulator and some small resistors & capacitors on a double sided PCB.
Far above my capabilities to draw.

It supposedly measures to initial starting current and when the curtains are at the end of travel and start to stall, the current is compared to the start-current. If it exceeds it by factor X, (maybe 15%) the motor switches off.

Ingenious.

Why am I not as smart

Gurus, where can I learn more about micro-processors.
I know the internet is full of info, but to find the beginning of the story is more difficult.
What MP is preferable to learn? PIC or Atmel or something else?

Ahhh, so many questions so little time
Regards
Frank

 

[Torben]

Hello Gurus,
Why am I not as smart

Not as smart, or just didn't spend years studying it and then years doing it professionally? I don't think your smarts are the problem here. You'll get the hang of it. I sure hope I do...I am, but slowly, it feels. I need to find a mentor.

Gurus, where can I learn more about micro-processors.
I know the internet is full of info, but to find the beginning of the story is more difficult.
What MP is preferable to learn? PIC or Atmel or something else?

Ahhh, so many questions so little time
Regards
Frank

Check Nigel's PIC tutorials (linked in his signature). Atmel AVRs and PICs are both good ones to learn, but it does seem that more people on this board use the PICs, plus Nigel has those handy tutorials. Other than that, I don't think there's really that much to choose between them. I learned 8051s first, but that's because I had a couple in an old bin I picked up somewhere. PICs and AVRs can do more, faster, than most 8051s that I know of.


Torben

 

[Frank of Oz]

Thanks Torben,
Microprocessors are a different animal altogether.

My brand-new micro-controlled curtain controller is already doing some wrong things, not completely unexpected, though.

So, back to the drawing board. Limit switches are definitely being re-considered.
Thanks gurus for the good advise.

Regards
Frank

 

[rjvh]

maby this is where you're looking for ??
if you have some questions ask them, the language is dutch but I can translate some for you (don't ask for the whole articel)


sucses
Robert-Jan

 

[steev]

yes,
the current drawn by a motor in different conditions is not straight line(graph)
when the motor starts it need more current to over come the weight of rotor and the friction etc...once it reached the required RPM it will lower the current.current it the thing which most devices maintains automatically(not like voltage)
so i think just for closing and opening safe and best id limit switches or some sort of proximity switches.

 

[Frank of Oz]

Thanks rjvh and steev,

I have a dutch friend and I also speak geman so the translation won't be too difficult.

thanks again for the help

Regards
Frank