how to wire garage door into car alarm?

Spark is correct, the resistor value is not critical, anything above 300 ohms should be fine.  Just monitor it for heat when the garage door switch is grounded.

oldspark wrote: IAAI can confirm, but the resistor value shouldn't be too critical. Maybe 10k is ok, or higher. (The upper limit depends on the impedance of the alarm (ie, what input sensing resistors it uses); the lower resistance limit is minimise current with ignition off and the switch closed, ie, 1k is ~12mA; 10k = 1.2mA, 100k = 120uA etc.) I'd prefer higher (100k or more) to reduce current drain.

"(The upper limit depends on the impedance of the alarm." I checked the alarm manual but it doesnt specify this, it only says this: "Positive door switching input (typical Ford Motor Co.)"

i am an idiot wrote: Spark is correct, the resistor value is not critical, anything above 300 ohms should be fine. Just monitor it for heat when the garage door switch is grounded.

monitor for heat? oh boy.. cant have any fire hazzards here! should I add a fuse somewhere and what about my previous question, putting a switch in for when the garage door is unplugged, without a switch the alarm wouldnt operate correctly because of this circuit, no? can a resistor be used either way or is it one way (orientation)? and it seems from Oldsparks latest post that the higher the resistor, the less current drain is this correct and also how can I determine exactly what would be the best resistor to use for this in terms of heat and current drain? as I posted, all the manual indicates is that this positive trigger is for FORD type door pin switchs input but I dont have a Ford to check if this, if it is a pulse or a constant voltage and @ what amperage. any idea? thanks

Actually one should get in the habit of occasionally checking switches, connectors and wiring for heat - it's usually a sign of too low a wire gauge (high resistance) or failing connectors and switches (bad contacts -> higher resistance). I should get in the habit too. (Might have saved a burnt foot when my 30 year old floor mounted dip-switch melted!) The above AND your resistor heat issue are solved by 2 basic formulas, namely Ohm's Law V=IR, & the Power formula P=VI. ie, V=IR means the voltage across something equals its resistance times the current thru it. (Volts, Ohms, Amps) P=VI means the Power dissipated by something equals the Voltage across it times the current (I) thru it. (Watts, Volts, Amps) Note that a load can convert that power - eg, a tungsten lamp converts ~5% to light and (wastes) ~95% as heat. A resistor is 100% heat. So your (say) 1k resistor. Assume 15V (typical worst case for a 12V system). V=IR. Divide both sides by resistance R and we get V/R = I. So I=V/R = 15V/1,000R (where I use "R" instead of having to specially paste in the Ω symbol (since I couldn't be bothered learning which of the Alt-whatever codes is appropriate for hatemail (html) pages etc)). So I = 15/1000 = 0.015A = 15mA. How much heat is that? P=VI = 15V x .015A = 0.225W. That's ~.23W which is just under 0.25 = 1/4 Watt. So a 1/4W resistor should be fine (especially since we over-estimated a little by using 15V instead of 14.4V or 14.2V or 13.8V or 12.7V etc - those being typical alternator else battery voltages). I'd prefer a 1/2W resistor but that may be an old's cool thing when 1/2W resistors were physically more robust than 1/4W. However. a 1/2W resistor can dissipate twice the heat that a 1/4W can, and it should run cooler. (Resistors can be quite warm or hot at their rated power.) Can you do the same calculations for a 300R resistor? BTW - resistors come in "preferred values" and ratings eg, 1/8W, 1/4W., 1/2W, 1W, 2W 5W, 10W, 20W etc, and Ohmages that are "multiples" based on 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 & 82 - eg, 10R, 100R, 1,000R etc 120R, 120k etc. (That's the traditionally common "E12" range so named because there are 12 "base values". There are also E24 & other ranges. And "multiples" and "base values" are not the correct terminology - I forget that sort of basic stuff!) 1/2W used to be the "standard" resistor size unless higher power was required or miniturisation was important. But modern 1/2W resistors are the same size as 1/4W resistors were a few decades ago (yes, I was (emotionally) shocked!).    I can do the 300R example, but I do it quicker than above. I happen to remember that P = VV/R or IIR (aka V**2/R or V^2/R, or I^2R etc) where VV = V-squared usually written as V-superscript-2 or V², but often as V**2 or V^2 or V-e2 for mathematicians etc with poor Alt-keyboard memories.) So for 300R & 15V, P = 15V x 15V / 300 = 225/300 = 0.75W, ... hence a 1W resistor (or larger). Maybe you can check that? FYI - the VV/R & IIR comes from combining V=IR & P=VI. ie, P=VI = (IR)I - IIR (I x I x R). or V=IR; divide both sides by R to get V/R = I, so P=VI => P=V x (V/R) = VV/R. [Yes, I remember my Year-3 maths!) POST-EDIT - that VV is V V (meaning V times V or VxV etc), it's not a double-U (W) (or double-V as some logical languages call our W). Incidentally, it is the P=IIR version that is the "easy key" to the heat check theory. Ignoring current, we see that P is proportional to R, ie, the heat given off (by switch or connector contacts or a wire) is proportional to its resistance. Bad contacts are a higher resistance as are thin wires, hence R increases so P - the power or heat given off - increases. Alternatively, for P=IIR, if the R is constant, then P is proportional to I² (I x I). So if P is whatever at current I, then doubling the current means the new "P" increases by (2I) x (2I) = 4I, ie, the Power/heat quadruples. If 3x the current, it's 3x3 = 9x the heat. And I'm not sure why I went into that - it isn't that relevant for your resistor, but it shows how increasing the current has a much larger effect on the heat generated. Doubling the current requires 1/4 the resistance for the same heat output or wastage; 3x required 1/9th the resistance, etc. That impacts wire gauges, component ratings, heatsinks, etc. Oh well, another OldFart crazy ramble....

oldspark wrote: Can you do the same calculations for a 300R resistor? Oh well, another OldFart crazy ramble....

I didnt want to quote your complete post but I apreciate youre in depth explanations but I will need to reread it once or twice at least. but can I do the same calculations a 300R? not yet.. and I cant seem to determine what the "R" is in 300R from your post. I did stop by the local radio shack (before reading the last post) I was suprised to see that they had 1k resistors in different wattages! I currently have in hand some 1k ohm resistors.. both 1 watt and .5 watt versions, they had smaller (less than 1k) but nothing larger. which of these should I go with here, the 1 watt or .5 watt one? also still wondering if I should add a switch between the 12v source and the resistor here? seems like it would have to have one to work correctly when the garage is not connected. I might go ahead and try this temporary like with a switch and fuse before the resistor but I still need to find out if the resistor's orientation matters (guessing it doesnt) I would hate to fry this alarm system so IAAI, if you can provide just a little bit more input on the subject it would be a nice boost of confidence for me. Ive allready wired up the garage door switch to a connector and should be able to take it from here but I'll be sweating a good bit while making the final connection on this deal. I'll think it over and have everything ready but not gonna risk it until you can give me the go ahead ;-) thanks

The "R" is a common substitute for the Ω or Ohm symbol. It is often used in place of decimal points for resistances - eg, 1.2Ω would be written as 1R2, 1.2k = 1.2kΩ as 1R2k. (That's more of an FYI in case you ever strike it.) I wrote why "we" usually use R instead of Ω in my last reply, but I didn't quote it in this reply because it is just one of the things you'll pick up upon a re-read - and then probably D'oh yourself LOL. But that's my fault - too much detail, and in a single reply. My replies should be segmented, but instead I assume people will re-read a few times AND do the "one bit at a time" learning. [ Note that quoting a section of one's own previous reply can be interpreted as a bit of a dig at later queries. Of course, I would never intend it that way would I?     I obviously quote my own replies to highlight that I have already answered something and that better reading is required. (Or better writing. But that would put the onus me!) Normally I provide info in small chunks and assess understanding before continuing. But on webforums, I do "all in one" single replies, usually because some implement after a small chunk before their full understanding and thereby cause damage or waste money etc. (Hmmm - that "bulb out" circuit that someone implemented despite my further writings, and totally missed the diagram's "specialised component" qualification and substituted some other device... I must see how they went, but after I complete (or start?) my own bulb-out indicators.)     BTW - adding resistors in series ADDS their resistances - ie, 2x 1k in series - 2k. (The formula is R = R1 + R2 + R3 + R4... etc.) In parallel lowers the resistance - ie, two 1k's in parallel = 0.5k = 500R = 500Ω. Note - the actual formula is more complex, but for EQUAL Ohmage resistors, it's the resistor Ohms divided by the number of them in parallel. [The parallel formula is "the reciprocal of the sum of the reciprocals" which reduces to (R1 * R2)/(R1 + R2) (where * = x = times or multiplied by). For 2 equal resistors that's (R*R)/(2R) = R/2. For n equal resistors, it's R/n. LOL - now you know four formulae - V=IR, P=VI, and the series and parallel resistor formulae. [ Inductors (coils; units = H = Henrys) use the same formulae, and capacitors BUT swapped around, ie, parallel capacitances (farads) increase capacitance and series reduces, so parallel caps use the series resistance/inductor formula while series caps used the parallel resistor/inductor formula. ] Yeah, I know, too much detail. But in a few years when you start designing your own alarms... PS: Your earlier question... Resistors are NOT polar; they can be used in either direction.

oldspark wrote: But in a few years when you start designing your own alarms...

cant thank you enough Oldspark, I wish we could meet.. something tells me you have alot of valuable info within. in a few years... I hope so but there are many indications the world will not be the same at that time. off topic but while you are here there is a guy on u-tube that has apparently rigged an alert system that goes off when sombody touches the glass or metal on the vehicle. here is a link: http://www.youtube.com/watch?v=LCnZqb3adWE&feature=relmfu skip the first 2/3rds, he demonstrates it towards the end. I think he may be pulling a fast one here but if such a system is possible then maybe you know how it works since he doesnt explain or provide details. BTW Ive wired up this IAAI suggested circuit with the 1 watt/1k ohm resistor, its ready, Im just waiting for IAAI to consider my plan and confirm I wont fry the alarm before I test it out. I have a 3a fuse>switch>1watt/1k ohm resistor>alarm pos trigger with ground from garage door swich intersecting this circuit @ the connection with resistor and alarm trigger. thanks

Ah, the world isn't the same as it was a few years or decades back, yet so much remains the same - eg, Ohm's Law, battery and electrical basics. Greed has improved though (as in gotten bigger and stronger) LOL. Or not LOL AFAIAConcerned. And I come from the days of "what's the point, we'll all be dead tomorrow". Bless those Cold War years, at least the Commies kept Capitalistic greed in check. Mind you, I now see the loss of 90-99% of the world's population in the near future (ha - a sea level rise of 1m by 2100CE? Try 7m!)... But it's not as if I'm one to ramble nor try a bit of social enlightenment, so back on track. I'll check that youtube later (hopefully I'm starting my ~10 hour drive for an Easter car meeting tonite instead of yesterday...), but there were ways of detecting touches. Often (body) capacitance was used, though glass generally had to be metalised (eg, window tints etc). Microwave detectors was another method, though they were more of a proximity alert and are still often used. (I chose uwave (microwave) for internal sensing rather than voltages or ultrasonic and other often false triggering methods, not that a uwave that detects outside the vehicle should trigger an alarm - just an "I know you're there" buzzer or lighting etc.    But I'm keen for a peek of that possible scam, though others on here are probably more qualified etc - you already know IAAI (he's a fool if he thinks he's an idiot, but I definitely know what he means by his IAAI name). I like scams - like voltage sensing battery isolators for non "stator" systems, and dc-dc converters to charge aux batteries - noting that both do have some legitimacy, ie, ~ 5% of cases. Oops, I'm point scoring or stirring again.     As to me having valuable info (thanks!), I'll see how the next few days turn out. The gathering includes many from a car forum I once contributed to (I think over 3,000 posts!) and all for a car I have never owned nor knew that much about. (LOL!) Mind you, it may be a few weeks before I get out of hospital ha ha. [ Which car? I'll give you a hint - it's the only car in the world that can't have a fire in the boot (trunk) because its holes and gaps are small enough to vent hydrogen from batteries but too small to let oxygen in. It must be true - I read it on the web, coincidentally on their site (after I asked what's preferable - a boot or engine bay fire? The fools reckoned boot!). Oops, egotistical point scoring yet again. ]

OK, I couldnt stand waiting any longer so I tested it and... no success! first with the 1 watt resistor, as soon as I armed the alarm (two-way autopage) the door open icon flashes on the remote. I went ahead and opened the garage door anyway to see if it would trigger alarm, nope.. nothing. then I tried with the half a watt resistor and got the same result. any other ideas? I'd love to get this to work, I'll keep thinking it over myself too. It would be great to have this ability for many reasons. thanks anyway IAAI, let me know if you have an alternate idea. Oldspark, have fun with the car club. I have been involved with many of these clubs, they can be fun but most have politics that make not as fun as it could be. I prefer the disorgainised clubs most for this reason LoL

Arm it with the door open, then wait the 20 seconds and close the door.  Does that set the alarm off?  I gave you bad info earlier.  I said to use the normally open contact.  You need to use the normally closed contact.  I forgot about the magnet being by it reverses the state of the switch.

IAAI, great! I reversed the mag switch wires and IT WORKS! so for anybody wanting to do this, I ended up with the 1kohm/half watt resistor. the cool thing about it is this circuit could be used in a trailor harness too except instead of a using magnetic door (reed) switch, just loop it back. now if the vehicle is armed and the trailor is disconnected, the alarm goes off. you could also incorporate it into the trailor plug itself if you upgrade to one with two extra terminals. thanks again IAAI & Oldspark