How to interface with a strange "switch"

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For a small summer project I obtained a rotary switch at a car scrap-yard. Or so I thought.

The thing was originally for adjusting the height of the beam from the drivers seat and has 11 distinct positions. There where only four terminals on the back so I figured that it was some kind of rheostat, pot-as-voltage-divider or something like that.

I tried to measure with the DVM on the different terminals, but everything was rock steady.

So I opened it up, and it turns out that two of the terminals are for a small bulb to light up symbols on the panel.

The two other go to a coil. Inside this coil is what looks like a permanent magnet or a ferrite cylinder. When Turning the knob on the thing this cylinder it moved in or out of the coil.

Questions:

How does this work? (I suppose it was used to read out the L of it in some kind of way, but how?)

Is there any realistic chance of hooking this up to
an AVR to read the positions (through eg the ADC)?

I always tell you that I am primarily a software person, and this is outside of what I can manage electrically.

Background: The Old Volvo has an interval wiper with only one setting, and it is driving me mad. You either have too much water on the winshield, or you have the screeeeching of wipers on a too dry shield. Only alternative is to "tap" the wiper lever by hand, which also sucks.

I thought I'd arrange something a bit more adjustable with an AVR and some surrounding stuff. I got the thing out of another old Volvo, so it fits perfectly in a free place on the dash-board, both spatially and aestethically. I'd love to be able to use it with as little modification as possible.

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Yes, I'd agree that in order to get the actual position, you'd have to measure the total L of the "thing". As a first approach, I'd apply an AC voltage via an R and see how the amplitude changes with the 11 positions. Or you could place this L within an LC oscillator and measure the frequency. Or you could issue a single pulse onto the L and measure the time it takes the voltage over the L below a certain point of discharge.

But then, i'm only a hobbyist with lots of crazy ideas, and not proven to be an engineer in the sense of the word... ;-)

Einstein was right: "Two things are unlimited: the universe and the human stupidity. But i'm not quite sure about the former..."

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Hi Johan,
I wonder whether on the original application, there was a similar reluctance device on the headlight which is in a AC bridge circuit with the device you have and would drive a positioning motor until the required position is achieved.
An advantage with that is there are no variable resistors to wear out or to get noisy.
One way, you could probably use it, is to build an LC oscillator with it and then measure the period with Input Capture.
Did it come of a German car by any chance. Only Germans would come up with clever stuff like that! :)

Edit.
Like Cliff I need to type faster.

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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For a fast test put the L with a R in seriel.
Flip the two outputs (one high and onr low) to make a tone, R should be in the ballpark of 10-100 times the DC resistens of L. The timing of the center point's zero crossing will give the possion.

If you just ground one end you have to be carefull about big burb's back from L (perhaps use some extern clamp diodes)

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JohanEkdahl wrote:
For a small summer project I obtained a rotary switch at a car scrap-yard. Or so I thought.

The thing was originally for adjusting the height of the beam from the drivers seat and has 11 distinct positions. There where only four terminals on the back so I figured that it was some kind of rheostat, pot-as-voltage-divider or something like that.

I tried to measure with the DVM on the different terminals, but everything was rock steady.

So I opened it up, and it turns out that two of the terminals are for a small bulb to light up symbols on the panel.

The two other go to a coil. Inside this coil is what looks like a permanent magnet or a ferrite cylinder. When Turning the knob on the thing this cylinder it moved in or out of the coil.

Questions:

How does this work? (I suppose it was used to read out the L of it in some kind of way, but how?)

Is there any realistic chance of hooking this up to
an AVR to read the positions (through eg the ADC)?

I always tell you that I am primarily a software person, and this is outside of what I can manage electrically.

Background: The Old Volvo has an interval wiper with only one setting, and it is driving me mad. You either have too much water on the winshield, or you have the screeeeching of wipers on a too dry shield. Only alternative is to "tap" the wiper lever by hand, which also sucks.

I thought I'd arrange something a bit more adjustable with an AVR and some surrounding stuff. I got the thing out of another old Volvo, so it fits perfectly in a free place on the dash-board, both spatially and aestethically. I'd love to be able to use it with as little modification as possible.

Try a 555, Astabil, Rb

HM

HM

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I rented Citroen a couple of years ago in France, and it had automatic windscreen wipers. Does anyone know how they work? My car also has only one intermittent speed, and it IS German...

Four legs good, two legs bad, three legs stable.

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Four legs good, two legs bad, three legs stable.

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Quote:
it had automatic windscreen wipers

20 years back I had to make a lot of long journeys through snow and rain.
I tried to envision something that could be mounted to any car (even an old Volvo).
Wasn't really into electronics at that time...
With all sensors available today should be doable.
Quite a market.
Sorry for hijacking your thread Johan.

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Quote:

Sorry for hijacking your thread Johan.

Don't worry. I am the last person allowed to complain over threadjacking..

My friend has been fooled to fall for the BMW hype, and his latest has auto-wipers. Works OK most of the time, but drove me crazy due to lack of rythm. "Spülung durch technik", though. :D

As of January 15, 2018, Site fix-up work has begun! Now do your part and report any bugs or deficiencies here

No guarantees, but if we don't report problems they won't get much of  a chance to be fixed! Details/discussions at link given just above.

 

"Some questions have no answers."[C Baird] "There comes a point where the spoon-feeding has to stop and the independent thinking has to start." [C Lawson] "There are always ways to disagree, without being disagreeable."[E Weddington] "Words represent concepts. Use the wrong words, communicate the wrong concept." [J Morin] "Persistence only goes so far if you set yourself up for failure." [Kartman]

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I've seen loads of sucessful capacitive touch screen projects using just a simple counter and comparator - to me, it feels like a more robust solution than detecting zero-crossings etc.
What they did was to connect a resistor above a cap, switch the "up" side to VCC and connect the comparator to the middle node. With a known R, that gives C. If I recall correctly, they could measure accurately down to single-pF if not better.
Of course, this is not a cap, but an inductance. The same principle should work though. Wikipedia has the timing graph of interest. Just remember to "reset" the circuit before the next measurement (connecting both sides of the inductor to ground).

Of course, one solution is to simply get a new car. But where's the fun in that? I've owned cars older than me. =) It's a challenge, and the reliability is somewhat lacking. I once got stuck in the woods near nothing because the generator died, and the car had no indication light for this.

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I am more a fan of adjustable intervals, just use a pot or a couple of buttons to set the time. You should have no problem connecting to the wiper relay either. On the other hand i see why you would want it to look pretty, could you simply remove the internals and replace with a pot?

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Check out Plons Elmcie schematic. The circuit around the LM311 is a generates a square wave with frequency dependent of the inductance of the DUT.

Felipe Maimon

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Here is the basic capacitance meter using a Tiny2313...
Swap the cap(DUT) for the inductor and adjust the resistor...

The idea being, setup up the analogue comp, start timer, set pin to input, charge until trigger, stop timer, set pin to output to discharge, repeat...

I have gcc for it if you want it...

Attachment(s): 

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fmaimon wrote:
Check out Plons Elmcie schematic. The circuit around the LM311 is a generates a square wave with frequency dependent of the inductance of the DUT.

Most certainly will. Thanks!

krazatchu wrote:
Here is the basic capacitance meter using a Tiny2313...
Swap the cap(DUT) for the inductor and adjust the resistor...

The idea being, setup up the analogue comp, start timer, set pin to input, charge until trigger, stop timer, set pin to output to discharge, repeat...

I have gcc for it if you want it...


I'll re-read that when my brain is fully charged, and see if I understand that in detail. As I started out - I'm a half-idiot when it comes to electronics.

As of January 15, 2018, Site fix-up work has begun! Now do your part and report any bugs or deficiencies here

No guarantees, but if we don't report problems they won't get much of  a chance to be fixed! Details/discussions at link given just above.

 

"Some questions have no answers."[C Baird] "There comes a point where the spoon-feeding has to stop and the independent thinking has to start." [C Lawson] "There are always ways to disagree, without being disagreeable."[E Weddington] "Words represent concepts. Use the wrong words, communicate the wrong concept." [J Morin] "Persistence only goes so far if you set yourself up for failure." [Kartman]

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Rain detector. Just an idea. Detection of back-scatter from modulated light source coupled into the inside of the windscreen?

Charles Darwin, Lord Kelvin & Murphy are always lurking about!
Lee -.-
Riddle me this...How did the serpent move around before the fall?

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LDEVRIES wrote:
Rain detector. Just an idea. Detection of back-scatter from modulated light source coupled into the inside of the windscreen?

Great idea! Why didn't somebody think of that before?

Four legs good, two legs bad, three legs stable.

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John, you turn back the clock a few years when you come to Australia!

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Dear Johan,

think it's worth to take a look to:
MCS Electronics AN #147 - Car Windscreen Wiper Control

Such an option was installed in few cars I drove, did proved to be way handy and very intuitive in control.
Have no intention hijacking original thread, indeed.

Best regards!

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John_A_Brown wrote:
LDEVRIES wrote:
Rain detector. Just an idea. Detection of back-scatter from modulated light source coupled into the inside of the windscreen?

Great idea! Why didn't somebody think of that before?
back scatter rain detection/measurement ... was used in Vaisala products 20+ years ago.

Cheers,

Ross

Ross McKenzie ValuSoft Melbourne Australia

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... And in widespread use today! ;) I think it was a joke. My Mazda3 has it.