Best way to use DC current rise as off switch?

My car has the feature on the driver's side electric window that lowers the window automatically if the switch is pressed hard enough. However, none of the other windows has this feature and nor does this feature work in reverse, raising the window. So I was thinking that I could use a 555 circuit that would sense the switch being on and after it was on for a few seconds the timer would keep the window motor energized until power was removed. So I built the timer circuit but now need some way to shut the thing off and all I can think of is some sort of current sensing switch that would turn off the power when the current rose to a certain point. For the driver's side window I could use whatever is there for the down motion but that won't work for any of the other windows. Anybody have any advice? I'm gonna search the web too but I value the instruction I have received here in the past. Thanks, Eric

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etpm
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Which is a good thing; children are hurt (and even killed!) by those.

This will be annoying to the users who just want to open the window part of the way, but the darned thing won't stop! You have a great solution; just perhaps not a problem to go with it. It's simply not a big problem to hold a switch until the window rolls down. Some cars stil have manual cranks!

Still, nice little project, what the heck.

How about a microcontroller which interprets the switch closure as an abstract command? If you press it and hold it, the window rolls down while you are holding it, then stops when you release it. However, a fast double-click (just like with a mouse button) could mean "roll down all the way by yourself". Bonus: a click while it is doing that would cancel the roll-down. (Handling double-clicks means there will be a small delay in the response before the continuous hold is detected and the window begins to roll.)

All that logic done in firmware, and the real switch is now some transistors driven by GPIO pins.

An important requirement to think about would be safety: that the system never goes into a mode where the window unconditionally rolls all the way up, either through a fault in the software or hardware. (Stuck GPIO pins, shorted transistors ...).

I would even make the roll-up "piecewise". If you pull the switch up and hold it, it rolls up for two or three seconds, and then stops. You have to release the switch and activate it again to keep closing.

Reply to
Kaz Kylheku

For simplicity and isolation of HV kick backs from the motor, you can use a reed switch with the proper number of turns of wire that runs the motor, around it. When the contacts close from excessive current, it pulls the RESET pin low on the 555 timer.

There's more to be done but that should get you started.

Jamie

Reply to
Maynard A. Philbrook Jr.

Well Kaz, a microcontroller would be just too much overkill, too expensive, and besides, as I mentioned above, I already built the 555 timer device. Which is simple and robust. The device shuts down when power is removed of course, and since it switches the power to whatever it is controlling whatever shuts it down shuts down the controlled device, in this case the window motor. The 555 timer device needs to be powered from more than just a momentary switch closure, so someone just stabbing at the window switch won't cause it to continue up, or down, on its own. Eric

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etpm

LM555 in 8-pin DIP: $0.33 Two resistors and a cap: $0.05 board space for it all: $0.53 total: $0.91

PIC12 in 8-pin DIP: $0.70 Board space for it all: $0.27 total: $0.97

Damn. You're right -- the microprocessor _is_ more expensive!

Unless you need more than two resistors and a cap for the 555 solution.

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Tim Wescott

Greetings Jamie, I like that idea. The timer I put together, once power is removed, will not cycle back on unless power is supplied for at least two seconds. I wonder if reed switches are available that are normally on? That would be simple and best. It would be best, from a safety standpoint, for the shutting off to be accomplished by power being removed from something, not supplied instead. This way a broken wire won't defeat the shutoff. I'm used to limit switches, emergency switches, contactors and the like operating this way on the machine toold I use every day. Cheers, Eric

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etpm

Plus the cost of the programmer, possible need for a USB-RS233 etc..

Jamie

Reply to
Maynard A. Philbrook Jr.

Yes they have them in NC operation too.

There is also another idea you can add to that, if you take the reed switch and instead of mounting it on the PCB you mount it near a mechanical limit that has a magnet there. Doing this will kill two birds with one stone, you have current limiting via the coil wrapped around the switch and the switch is mounted in such a way that it can come close to a limit and still turn off for that direction.

But the NO types are very common and cheap in comparison, you should be able to reset the timer using this instead of opening a circuit.. Lets face it, the timer must open the circuit when it times out right?

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If you're near a Radio Shack, I think they still have them or, you can get one from a magnetic door alarm kit.

Jamie

Reply to
Maynard A. Philbrook Jr.

snipped-for-privacy@whidbey.com laid this down on his screen :

The window switches were designed that way to protect your children from your stupidity. You might also notice that to get the windows to go UP you have to hold the switch UP Early on there were some injuries from putting objects on the up end of the switch then they were changed so you had to hold them up to close the window.

Darwin is looking for candidates for this years award :-?

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John G
Reply to
John G

Picky picky. Of course, you only buy one programmer, and then you have it the next time you feel tempted to use a 555 where a microprocessor will do.

(Sez Tim, who just designed a two-transistor Schmitt trigger-based timer because he's TOO LAZY TO PROGRAM A PIC!!!!)

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Tim Wescott 
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Tim Wescott

"Maynard A. Philbrook Jr."

** I hope " more to be done " includes a start up delay so WHEN ( not if ) the current surge in the DC motor t switch on closes the reed relay momentarily - it gets ignored.

Been there, seen that.

... Phil

Reply to
Phil Allison

"Phil Allison"

... surge in the DC motor at switch on

Reply to
Phil Allison

Yes, we must practice by example! :)

Jamie

Reply to
Maynard A. Philbrook Jr.

PIC sent me a Microstick II kit with extra PIC24's included, USB-cable and all, for, what was it? $35 bucks, shipped FedEx! IDE and compiler: free download from website.

The biggest cost here is your time. $35 for a microcontroller kit is an expensive part of the project if you consider your time to be worth nothing. (If your time actually *is* worth nothing, then $35 is a lot of money in an absolute sense, also.)

Reply to
Kaz Kylheku

Yes, but it can be designed not to provide a mechanical advantage over bare fingers. :)

Reply to
Kaz Kylheku

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John Fields

He needs to put on a full second shift, and maybe a third. They are breeding too fast. :(

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Reply to
Michael A. Terrell

Your idea sounds good to me. Simple and robust. So I guess I need a resistor relay combination where the relay coil draws little current and the resistor doesn't use much current before the relay pulls in. Thanks, Eric

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etpm

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