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Ford Open Circuit Door Pins

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Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: February 02, 2015 at 6:03 AM / IP Logged  
I'm curious about - and probably hijacking - SNS's OP "... doorpins not being positive or negative...".
That to me implies floating switches as often used for non-vehicle alarms - typically series chained NC triggers (window foil; reed or other switches; IR outputs, etc) which usually have dispersed resistors for impedance sensing circuits. (Forget the Hollywood cut or short of sensor & alarm wires - that's as valid as cutting bomb blue or red wires, or blowing keypads to open doors!).
For such systems, unless switch voltages are known, short or open detection circuits are required. The closest I can think of in common(?) automotive applications are blown-fuse detection circuits tho often they require or assume near +12V voltages at the fuse. (Blown bulb detectors are usually current sensing else assume grounded bulbs.)
The DEI techtip however refers to NC switches which AFAICSee are grounded.
Of course the techtip may work for floating switches that have a low enough impedance or reference to GND.
So I'm curious - are there "floating" switches - ie, 2-terminal door etc switches where neither is (direct) connected to GND; nor +12V for that matter?
Or is it a case of jargon where some refer to the floating output when the switch is open and that is confused with a floating switch?
[ The term floating itself means neither +ve or -ve. IE there is no reference to any rail/voltage except as thru pull up/down resistors else attached circuits like alarms which are usually relatively high impedance. ]
My curiosity is also partly a worry - did I recently reply crap in a recent thread? (No, not that thread.)
However that was about NC versus NO door etc switches - not floating - unless I misread or the OP was also confused as to what system was involved. (Maybe I'll revisit and check or ask if the problem was solved.)   
If there are no "floating" switches and it's merely a case of NC versus NO, then IMO there is no issue and I have indeed hijacked... tho maybe also forewarned some of other implementations or possible automotive system yet to come.   
Thanks in anticipation.
Silver - Posts: 618
Silver spacespace
Joined: July 11, 2004
Location: United States
Posted: February 05, 2015 at 1:13 PM / IP Logged  
newer fords are N/C switches. ground when the door is closed and then go to either + some kind of voltage or an open circuit. a majority of the fords i've done, they go to +12V when a door is opened which allows me to use a positive door trigger input on the alarm and then diode isolate as required. however i recently worked on a newer mazda 6 and they only went to +4-5 volt when a door was opened. just did the pain in the butt tech doc with resistors/diodes for it
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: February 05, 2015 at 4:09 PM / IP Logged  
Thanks. However that sounds like they are still NOT floating - ie, they still have one end connected to GND else +12V.
Most (traditional) car door etc switches are GND connected. IE - one end is GND.
For NO they - ie, the other end of the switch - only grounds when the door (etc) is open.   Otherwise that end is "floating" noting that in practice that means it's at +12V since it's usually connected via a dome light (low resistance) or high impedance sensing circuit which must be +V to detect when it is grounded.
A dome switch can mean a +12V impedance (resistance to +12V) if 1 Ohm or lower whereas a FET sensing circuit cam mean several mega-Ohms.
And it may be +3.3V or +5V or +8V etc if it is some sort of sensing circuit attached without a +12V doom light etc...
If diode isolation is used on all outgoing switch feeds then it will essentially be floating - ie, you will not read any voltage on the switch's open output.
An NC switch is the same as above but swapped in sense - ie, it will be GND with the door closed and floating (hence +V) when open.
NC switching is typically used for "fail safe" alarms and systems - ie, you know immediately if a switch or circuit has opened thru a fault or from something or someone cutting the wires. (For alarms you add series resistance and use impedance sensitive circuits to counter bypassing attempts whether shoring NC switches or cutting NO switches.)
The above however are not floating switches, merely floating outputs.
The resistive alarm input switch may be considered floating depending on where the resistances are placed AND what you define is "floating" ie, not referenced to a voltage except for thru 10M or 1M or 100k etc resistor/impedance. IE - "floating" in a circuit carries some definition of impedance; it may be 100 Ohm for a headlight or over 100k for an electronic circuit.
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