moroney@dec-jon.UUCP (05/01/86)
>> What's the circuitry behind the buried-wire car detectors at >> stoplights? Technical details please. > The buried wire vehicle detectors used with traffic lights work on the >principle of an inductance bridge. The buried loop is one arm of a low >frequency inductance bridge (generally 100 KHz to 200 KHz excitation). When a >vehicle is present within the loop, the inductance increases from its free air >value by virtue of the ferrous metal in the vehicle acting as a "core". This >change in inductance is detected by a bridge circuit in some signal devices, >and by detuning the frequency of an LC tank in an oscillator in other devices. To pick a minor nit, the "inductive" type actually senses the CONductivity of metal over it, as a kind of transformer with a shorted turn when a car showed up. I found this out when they installed such a sensor in front of the house I used to live at the corner of a busy city street and a very minor city street. This sensor could be activated by a ~1 foot square of (non-ferrous) aluminum foil. This light would turn red ONLY when the sensor was activated (or a crosswalk button was pushed) so I knew I triggered it. One thing amusing about this particular setup, the sensor started about 2 feet behind a stop line, which itself was behind a marked crosswalk. Some fools in small cars didn't seem to know what stop lines or crosswalks were, and stopped right in the crosswalk, not over the sensor at all (which required a car to be on it ~5 seconds to trigger) and they would wait for the light to turn green, which it didn't... eventually they would run the red. If they stopped behind behind the stop line in the first place... Some of the "ultrasonic" sensors are <=20kHz - there used to be one that I could hear. It got replaced by the buried wire kind when that intersection got rebuilt. Still LOTS of old ultrasonic sensors in the Buffalo, NY area. -Mike
zeek@pyramid.UUCP (Jim Zeek) (05/01/86)
One thing about these sensors is that most of the time motorcycles can not trip them. I have sat at lights for several minutes waiting for them to change. You find yourself praying for a car to pull up behind you to set the thing off. I even tried putting the kickstand down even though this is not safe to do. I'm surprised more cycle riders havn't spoken up. Jim Zeek @ Pyramid Technology (pyramid!zeek)
smillie@leadsv.UUCP (05/02/86)
In article <329@pyramid.UUCP>, zeek@pyramid.UUCP (Jim Zeek) writes: > One thing about these sensors is that most of the time motorcycles > can not trip them. If you can find the cut marks for the sensor, position the bike over them. If not, the sensors are usually to either side of the lane so if you keep to one side or the other the motorcycle should triger the signal no problem.
kds@intelca.UUCP (05/02/86)
> One thing about these sensors is that most of the time motorcycles > can not trip them. I have sat at lights for several minutes waiting > for them to change. You find yourself praying for a car to pull up > behind you to set the thing off. I even tried putting the kickstand > down even though this is not safe to do. I'm surprised more cycle > riders havn't spoken up. > Jim Zeek @ Pyramid Technology > (pyramid!zeek) quite the contrary, I have had (to me) a suprising amount of luck tripping these puppies with my *bicycle*. You just need to know where to sit. Many of them are in a sideways figure 8 (sortof), and to trip it you just have to sit on the middle of the pattern. For simple loops, just park it on one of the sides. In most locations you can tell where the loops are, they seem to cut them out when they pave the streets; in other locations, they paint the pattern on the street! -- If you don't like the answer, then ask another question! Everything is the answer to something... Ken Shoemaker, Microprocessor Design, Intel Corp., Santa Clara, Ca. {pur-ee,hplabs,amdcad,scgvaxd,oliveb,qantel}!intelca!kds ---the above views are personal.
chip@vaxwaller.UUCP (05/08/86)
> > One thing about these sensors is that most of the time motorcycles > > can not trip them. I have sat at lights for several minutes waiting > > for them to change. You find yourself praying for a car to pull up > > behind you to set the thing off. I even tried putting the kickstand > > down even though this is not safe to do. I'm surprised more cycle > > riders havn't spoken up. > quite the contrary, I have had (to me) a suprising amount of luck > tripping these puppies with my *bicycle*. You just need to know where > to sit. Many of them are in a sideways figure 8 (sortof), and to > trip it you just have to sit on the middle of the pattern. For > simple loops, just park it on one of the sides. In most locations > you can tell where the loops are, they seem to cut them out when > they pave the streets; in other locations, they paint the pattern > on the street! It is true that you can sometimes find the pattern, but that doesn't always mean that the damned things will "see" you. I've also been able to trip some with my bicycle, and others I can't trip with my motorcycle. Even the trick of setting the kickstand down on the traces in the street (assuming you can find them) sometimes doesn't work. What I've done in the past is to call the responsible agency (city, county, etc.) and notify them about the matter. USUALLY they will correct the situation, however, sometimes they don't. That usually prompts a second call, and if that doesn't work, I send them a registered letter (receipt requested) telling them of the situation, and follow that up with a call telling them just how liable my "lawyer" thinks they would be if I ran the sucker and got into an accident. That ALWAYS works. Try it. You'll like it. Happiness; Chip -- ,, *** SOLIDARNOSC *** Chip Kozy (415) 939-2400 x-2048 Varian Inst. Grp. 2700 Mitchell Dr. Walnut Creek, Calif. 94598 {zehntel,amd,fortune,rtgvax,rtech}!varian!chip