convert ground when closed triggers

My 2012 Ford Fusion's trunk and hood triggers are ground when closed, but the Autopage RS730 alarm I installed accepts ground when open trigger input, so I used Bosch Style relays to convert them. Pin 85 and 86 to +12v and (-) factory trigger. Pin 30 to ground, and pin 87a to alarm trigger input. This works fine as long as the relays and battery are good. However, today when I was not home, the battery died and the alarm went off more than 10 times until the backup battery also died. I wonder if there is a better way to wire up this kind of triggers without wiring new switch pins.

On the note my neighbor left on my door, besides complaining about what I mentioned above, he seems to be unhappy about the warn-away of the motion sensor. He parked his car next to mine. The warn-away of the motion sensor is supposed to be triggered when he gets on or off his car. If he is unhappy about the three chirps, I think I have to give up the function.

There's always this way :  https://www.the12volt.com/installbay/file.asp?ID=1121

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Soldering is fun!

kreg357 wrote:

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There's always this way :  https://www.the12volt.com/installbay/file.asp?ID=1121

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Thanks. However, I think that is for (+) trigger input. Should I use relays to wire them to (-) trigger?

You would use the positive door trigger instead of the negative one. The link converts the rest at ground (normally closed) to a positive trigger one. The diodes provide isolation, resistors allow a positive signal when the door is opened. The circuit isn't intended to operate a relay.

kreg357 wrote:

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There's always this way :  https://www.the12volt.com/installbay/file.asp?ID=1121

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Another question about the wiring scheme is whether the resistors can become very hot.

With the resistors being 10K ohms, no. 0.02 watts per resistor approximately.

Ween] wrote:

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You would use the positive door trigger instead of the negative one. The link converts the rest at ground (normally closed) to a positive trigger one. The diodes provide isolation, resistors allow a positive signal when the door is opened. The circuit isn't intended to operate a relay.

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Thanks, but this question is not about door trigger. For door trigger (+), I used dome light output from SJB. This question is about hood and trunk triggers, which are only available as (-) trigger input.

The same link would apply, add extra diode from your domelight. Doors, hood, and trunk would all report as positive door trigger.

Ween] wrote:

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The same link would apply, add extra diode from your domelight. Doors, hood, and trunk would all report as positive door trigger.

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You mean wire factory hood and trunk triggers to alarm's (+) door trigger input? The alarm has dedicated hood and trunk (-) trigger input wires. It's better that I can use them, so that I can know from the LCD remote. For hood trigger input, it also serve as safety shutdown mechanism.

Ok. There are relays available that trigger on much less current than the bosch type. You'd need to look for a 12 volt coil, spdt contacts, with a minimal coil current. Use one on the hood pin. I'd still wire the trunk with the diodes and resistor, converting it to positive. Unless of course you need to know specifically if the trunk is triggered. Maybe some members will chime in with other suggestions, other than the obvious addition of independent hood and trunk switches.

Ween] wrote:

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Ok. There are relays available that trigger on much less current than the bosch type. You'd need to look for a 12 volt coil, spdt contacts, with a minimal coil current. Use one on the hood pin. I'd still wire the trunk with the diodes and resistor, converting it to positive. Unless of course you need to know specifically if the trunk is triggered. Maybe some members will chime in with other suggestions, other than the obvious addition of independent hood and trunk switches.

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The wiring scheme basically convert open circuit to positive input. Can I just use a relay to convert the positive input to negative input?

Sure, but as you found out, there is an additional drain on the battery. The relay coil is activated when the vehicle is off. That's why I recommended finding a relay with a coil that draws the least amount of current.

Ween] wrote:

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Sure, but as you found out, there is an additional drain on the battery. The relay coil is activated when the vehicle is off. That's why I recommended finding a relay with a coil that draws the least amount of current.

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I mean use the wiring scheme posted by kreg357: https://www.the12volt.com/installbay/file.asp?ID=1121
but instead of connecting to positive trigger, connecting to pin 86 of a relay. Pin 85 and pin 30 to ground, pin 87 to (-) trigger of alarm. I guess a relay with much lower coil voltage is required.

Can I buy a 6v coil relay and measure its resistance and then use a resistor parallel with the coil so that the total resistance for them is 10 kohm. This way the coil should get 6v when the circuit of the factory trigger wire is open. Right?

a reed relay is what you're looking for. Personally i would use positive door triggers (use of diodes and resistors) and for trunk and hood pin use mercury switches.

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Ted
2nd Year Tier 1 Medical School
Still installing as a hobby...pays for groceries
Compustar Expert

tedmond wrote:

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a reed relay is what you're looking for. Personally i would use positive door triggers (use of diodes and resistors) and for trunk and hood pin use mercury switches.

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Would you please explain how I should use the reed relay?

This one seems to be low consumption: https://www.westfloridacomponents.com/REL052APH02/12.5mA+12V+Low+Signal+Relay+Omron+G5V-1-DC12.html

Can you not still get a DEI DTI Mazda for trunk and hood?

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Aren't your door switches normally open - they only close with the door open?
That's what the DEI techtip refers to.
Use a normal relay; there is no drain unless a door is open (and BCM etc is active).

And re that TechTip 1921 diagram, I don't see why ONE 10k (or 1k) resistor from the (+) Door trigger input to +12v fused isn't used instead of its four resistors...

howie ll wrote:

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Can you not still get a DEI DTI Mazda for trunk and hood?

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The problem is the alarm system uses negative triggers for those, but the tech tip and DTIMAZDA are for positive trigger. In fact, when trunk and hood are opened, the wires read +12v, but when I tried, they cannot work with the relays, maybe too less current, so I doubt DTIMAZDA output can power relays.

oldspark wrote:

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Aren't your door switches normally open - they only close with the door open?
That's what the DEI techtip refers to.
Use a normal relay; there is no drain unless a door is open (and BCM etc is active).

And re that TechTip 1921 diagram, I don't see why ONE 10k (or 1k) resistor from the (+) Door trigger input to +12v fused isn't used instead of its four resistors...

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My question is about normal closed factory hood and trunk triggers and negative triggers for the alarm system. I don't understand how a normal relay can be used with the TechTip.

NO RELAYS!! You will drain your battery.
Just use a DTI Mazda on each.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

howie ll wrote:

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NO RELAYS!! You will drain your battery.
Just use a DTI Mazda on each.

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The triggers for hood and trunk are negative. You mean connecting to positive door trigger?

No the DTI Mazda simply converts the normally closed circuits to the more common type normally open, pos. trigger, just use that as explained above for the doors.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

sam369 wrote:

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My question is about normal closed factory hood and trunk triggers and negative triggers for the alarm system.

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The tech tip says "Some vehicles have door triggers that rest at ground when the door is closed..."- ie, Normally Open switches that are GND whe the trunk/door is open.
That DEI TechTip is not applicable to +ve triggers nor NC door/trunk switches.

If you do indeed have "triggers are ground when closed" switches (which IMO are generally not used in vehicles), then you'll need a FET to turn on the relay, or a transistor if a small drain of typically under 1mA is acceptable.
Using a relay means it is normally energised and thereby its coil drains the battery, but DEI TechTip 1921 has nothing to do with that.

oldspark wrote:

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sam369 wrote:

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My question is about normal closed factory hood and trunk triggers and negative triggers for the alarm system.

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The tech tip says "Some vehicles have door triggers that rest at ground when the door is closed..."- ie, Normally Open switches that are GND whe the trunk/door is open.
That DEI TechTip is not applicable to +ve triggers nor NC door/trunk switches.

If you do indeed have "triggers are ground when closed" switches (which IMO are generally not used in vehicles), then you'll need a FET to turn on the relay, or a transistor if a small drain of typically under 1mA is acceptable.
Using a relay means it is normally energised and thereby its coil drains the battery, but DEI TechTip 1921 has nothing to do with that.

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Sadly, what are generally not used in vehicles are used for Ford Fusions.

So use a relay & diode if needed, or transistor, for the Luggage Compartment and a transistor for the Anti-theft switch or swap its GND output to #2 and use an NC (SPDT) relay to GND the "hood ajar" line.

oldspark wrote:

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So use a relay & diode if needed, or transistor, for the Luggage Compartment and a transistor for the Anti-theft switch or swap its GND output to #2 and use an NC (SPDT) relay to GND the "hood ajar" line.

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Transistors are quite new to me. I haven't figure out which kine to use and how to wire it up.

If I connect Gate to factory hood or trunk trigger wire (ground when not triggered), Source to vehicle ground, Drain to alarm trigger input, which kind resistor can break the circuit when Gate get ground?

If I use one of this kind of relay for each trigger, is it a concern 25 mA drain when car is off?

I mentioned that the trigger wires read +12v but cannot power 12v relays when hood or trunk is opened. Maybe they can be Gate input to close the circuit (n-channel enhancement)?

New to you? You nailed the (MOS)FET wiring!! The only additions - a 1M or 100k resistor from Gate to GND (assuming an N-ch FET) to ensure it turns off (it's too easy for the Gate to get its nA from floating outputs, so pull it down to GND).
And add a pull-up resistor (10k? 100k - but significantly smaller than the Gds (G to GND) 1M etc resistor) if the trigger input is is foating etc (not normally "hi" = +12V).
A series resistor between the source (switch) and Gate is often added but that's usually to protect the source (switch) from overcurrent in case of FET breakdown, but that's usually irrelevant for switches. (Sorry about "source" vs FET Source... maybe the latter should be referred to only as S?)

However I'd prefer to swap the Hood Switch's #1 output to #2 so its GND is only connected when the Hood is open. Then add a normal car SPDT (changeover; 5-pin) relay to disconnects its 30-87a between GND and the HOOD AJAR #10 C2280F input.
Otherwise the FET can drainlessly invert the standard OEM wiring/signal.


There is no "resistor" to break any current. Only the relay, else as above for a MOSFET.

I missed your last reply.
Normally Open GND switches will almost always measure +12V on their outputs. More than a DMM voltage test is required to know what switching is involved.

And the fact that that +12V cannot energise a grounded relay coil might prove my point.
Instead try connecting the relay coil between +12V and the switch - that's how you'd use a relay on the Luggage Compartment switch (and how I reckon you should change your Hood switch wiring).

oldspark wrote:

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New to you? You nailed the (MOS)FET wiring!! The only additions - a 1M or 100k resistor from Gate to GND (assuming an N-ch FET) to ensure it turns off (it's too easy for the Gate to get its nA from floating outputs, so pull it down to GND).
And add a pull-up resistor (10k? 100k - but significantly smaller than the Gds (G to GND) 1M etc resistor) if the trigger input is is foating etc (not normally "hi" = +12V).
A series resistor between the source (switch) and Gate is often added but that's usually to protect the source (switch) from overcurrent in case of FET breakdown, but that's usually irrelevant for switches. (Sorry about "source" vs FET Source... maybe the latter should be referred to only as S?)

However I'd prefer to swap the Hood Switch's #1 output to #2 so its GND is only connected when the Hood is open. Then add a normal car SPDT (changeover; 5-pin) relay to disconnects its 30-87a between GND and the HOOD AJAR #10 C2280F input.
Otherwise the FET can drainlessly invert the standard OEM wiring/signal.


There is no "resistor" to break any current. Only the relay, else as above for a MOSFET.

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Thank you very much for the help. I'll buy the things needed, and report back how things go.

oldspark wrote:

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I missed your last reply.
Normally Open GND switches will almost always measure +12V on their outputs. More than a DMM voltage test is required to know what switching is involved.

And the fact that that +12V cannot energise a grounded relay coil might prove my point.
Instead try connecting the relay coil between +12V and the switch - that's how you'd use a relay on the Luggage Compartment switch (and how I reckon you should change your Hood switch wiring).

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The door triggers look like wired the same way as trunk and hood triggers, but I can use +12 from them to power door edge LED lights when doors opened. Not sure if they are differently regulated inside SJB.

Well, if they were wired like the trunk/boot switch, or if they were wired upside-down - ie, switch output to the SJB interchanged with switch pin #1 - then switch #2 powers extra loads/relays as normal.

Otherwise ye olde transistor idea... (probably just one thru diodes).

Received MOSFET transistors today, but haven't received resistors yet. I tested with a 10k resistor and it works. I wonder if I can just use a 10k resistor or if I should wait until I receive 1M ones.

The MOSFET does not "need" any resistor.
They are added to:
- ensure MOSFET turn off when input (Gate) is "not ON".
- limit damage to external "input" circuitry given the rare occurrence of a Drain-Gate breakdown thereby shorting +12V or whatever to the Gate hence shorting your input source to the MOSFET's supply (eg, +12V).
- a resistor to pull up grounding inputs if required.
And if more than one of the above (resistors) is used, consideration must be given to voltage-divider effects, ie the resistance of each inter-connected resistor should differ by some minimum ratio, eg one order of magnitude.

oldspark wrote:

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The MOSFET does not "need" any resistor.
They are added to:
- ensure MOSFET turn off when input (Gate) is "not ON".
- limit damage to external "input" circuitry given the rare occurrence of a Drain-Gate breakdown thereby shorting +12V or whatever to the Gate hence shorting your input source to the MOSFET's supply (eg, +12V).
- a resistor to pull up grounding inputs if required.
And if more than one of the above (resistors) is used, consideration must be given to voltage-divider effects, ie the resistance of each inter-connected resistor should differ by some minimum ratio, eg one order of magnitude.

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Thanks. I'll just use a 100k or 1M resistor for the gate. In this application, I think short is not a concern since drain is connected to a negative trigger wire similar to the the wire connected to gate.

I used a 100k resistor with a MOSFET for each of the two triggers. It's been several days, and there have been working well. Thanks you all for the suggestions, especially oldspark, who help me solved the problem.

Thanks appreciated.
And thank you for the reply.


That's one thing I like about MOSFETs - resistors are not critical.

The main "optional" resistor is from G to GND to ensure the MOSFET stays off when the input (Gate) is not >+5V etc and that resistor merely needs to be less than 10 MOhm or maybe <1M - ie low enough to ensure any floating Gate voltage is grounded - and the currents involved (namely the Gate turn-on current (Ig?) with Vgs = ON) are typically well under 100nA.

The only other resistor is "series" Gate resistor which is to protect the whatever that is connected to the Gate in case of a D-G short circuit.

The 3rd resistor is an input issue - if the input to the Gate cannot pull the Gate voltage above Vgs-ON - eg, to ~+5V or higher - then a pull up resistor is required.

The only complication or critical aspect is if the resistors from voltage dividers (which they will of more than one resistor is used). Then calcs will have to be done to confirm that Vg is high or low enough with the input ON and not ON.   
However separating them by an order of magnitude or more (ie, at least 10 times bigger or 10 times smaller) is usually the simple solution since it means less than a ~10% voltage variance thru voltage divider activity. Simple "logic" does the rest...
RG, so if the "stay OFF unless on" resistor from Gate to GND is 1M, then the series Gate "let's protect the switch/alarm/circuit input" resistor can be 100k or smaller. Sanity check... yes, the 100k current will swamp the 1M current, ie the 1M current will keep the MOSFET OFF until the 1M/100k = 10x higher input current turns it on.
I'll avoid the pull-up resistor.   


With BJT transistors (ie, not FETs) you have to derive the series-Base resistance range from transistor specs (ie, its gain which is usually specified as a range eg 80-500) and from the load size.
FETS do not need input (Gate) current limiting whereas BJT (Bipolar Junction Transistors - commonly referred to as "transistors" even tho transistors include FETS & other types - needs to be of some minimum value (that limits current) but some minimum to ensure its resulting (Base) current times its gain (aka β = Beta) supplies enough current to the load. Then you add a Base to GND resistor and Base to input resistor that provide a Vb of about 0.7V blah blah blah....
Nah - transistors (BJTs) for simple amplifier designs, but (MOS)FETs for switching or digital applications like this. (The bonus is then the very high impedance (negligible current) of the FET's Gate etc.)


PS - because vehicles are typically 12V or higher, same sized resistors may work.
EG, if both our Gate-GND & input-Gate resistors were 1M and therefore a +12V "on" input (or 14.4V etc) means a Gate voltage of +6V (or +7.2V etc) which may be high enough to turn the MOSFET (fully) on. Certainly an L-type would - ie Logic-types needing input voltages under 5V for full turn on.