car alarm

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car alarm

Post by notstock78 » Wed Jan 15, 2003 6:05 am

I am a student at Northwest Tech and I am trying to build a car alarm for my advanced lab progect. I want to use the idea of a touch lamp so when someone touches the door handle it will trip the alarm. could anyone give me some idea to start from?

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Re: car alarm

Post by chessman » Wed Jan 15, 2003 9:09 am

not sure if this helps much...
I do remember reading touch lamps do something with detecting changes in capacitive levels..i think... i don't know how you could apply that to a car, i'm way outta my league here but i thought i'd contribute my thought.<p>~Kyle

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Re: car alarm

Post by russlk » Wed Jan 15, 2003 12:49 pm

Touch lamps use an IC made for that purpose. You could buy one and take it apart (a touch lamp), or US Digital makes the LS7232 IC which you may be able to find. These sites will provide more info:<p><p> ... type=store<p><p>

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Re: car alarm

Post by Edd » Wed Jan 15, 2003 4:39 pm

notstock78……..= 25 yr old hotrod(der) ???<p>Was the detection mode assigned by your instructor, or your idea ?, if him , I think he planned to challenge you !
Two techniques come to mind, the suggested utilization of “touch lamp circuits” or one of their dedicated design IC’s would be circumvented by lack of two of their requirements, if attempted to be used in an auto environment. Specifically, the circuits are used connected to an AC power source where a good ground(ing) is also attained. The basic principle on which they work is the human body being in a field rich environment of AC house wiring and the proximity induction of a weak field of 60~ AC into same body. Touching the lamps sensing connection transfers some of that AC into a hiZ hi gain amp that then conditions it and passes it on to further circuitry which drives power switching circuitry (or a couple of latching F/F’s and them engaging/providing drive scaling to a power control triac if multiple lamp drive levels are involved).
You’re just not going to get that grounding requirement on that car sitting out on the parking medium perched up on those four insulators. Also, you just lost your 60~ field out there.<p>Some of the first touch circuits used two contacts and one’s fingers across it presented a resistance between the elements and adjoined circuitry detected it and engaged switching action. Environmental considerations presented to an auto….. like rain, sleet or even the overnight accumulation of dew should preclude continual foolproof reliability using this technique.<p>Another system was a proximity detector where there were two RF oscillators running close in frequency and an insulated nearby “detector” wire/plate/contact was connected to the freq determining L/C circuitry of one osc ckt while the other ocs ckt was maintained as a stable/fixed osc. A touch of the sensing unit’s contact would skew the resonating freq of that osc ckt and the heterodyned result of the two mixed signals output would produce an audio range output which was then passed thru a selective R/C filter then conditioned and passed on to power control/switching circuitry. I wouldn’t expect this to work exceptionally well either or be rather insensitive/touchy, also due to the grounding situation or an equivalent ground counterpoise.Plus the aforementioned wet sensor problems presented.<p>Then there’s the actual validity of the situation of “touch” detection in the confirmation of a breach of security of the vehicles door handle. How abut does activation of the unit by an adjacent parked occupant accidentally touching the handle trying to extricate himself from a close by parking clearance. Or even the fat lady squeezing through between vehicles with her gluuuuuutamus maaaaaaaximus.?<p>We certainly know the fallibility of shock detection sensors from high winds,earth tremors,and
even adjacent hi powered car AUDIO. Seems like the most reliable detection has been the internal sensing of either battery drain sensing or the lighting circuitry pull, engaged by door, hood and trunk switches.<p>On different handles, the recessed pull handle, the finger latch, the lateral cantilevered model or excepting , possibly, the OLD rotary model, seems like all seem to have offered some degree of mechanical movement even in the locked mode, so the most reliable detection might be to get behind the doors side panel and mount a Pushbutton or microswitch to detect that movement and tie it into the common sensor line just like all of the other nodes. That way when a detection is keyed, at least one knows that a definite, concerted action was made to try entry to the vehicle. <p>73's de Edd
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Chris Smith
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Re: car alarm

Post by Chris Smith » Wed Jan 15, 2003 11:35 pm

The problem with touch mode switches is static air on hot days will set it off. Indoors you don’t have this problem, but out doors static charges can and do travel with hot wind like packages of static charge or capacitance. <p>The 555 is one of the easiest ways to build a touch switch.

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Re: car alarm

Post by bodgy » Thu Jan 16, 2003 2:38 am

Okay I like going way out on a limb, before coming back to reality, how about a cheap strain gauge on the inside of the door handle. When a hand is put on the handle to open the door the gauge will be strained :) and can set the alarm off. So anyone just bumping the door or 2 fat Tenors wobbling alongside, or rain shouldn't affect it. Heat might be another matter as well as severe cold.<p>I reckon it could work.<p>Or to really show off obtain samples of those new biometric sensors and go from there. I still like the strain gauge theory.<p><reality mode back on><p>colin
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Re: car alarm

Post by Chris Foley » Thu Jan 16, 2003 3:52 am

A good place to start learning about touchswitches, and digital logic in general, is CMOS Cookbook by Don Lancaster, which should be available through your library. Mr. Lancaster's web site is [url=,],[/url] and is worthwhile for any technical student. You can buy the book on the web site, if you want.

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