Triggering traffic lights with a magnet - does it work?

**Steam** · 3rd March 2008, 17:20

I read you can trigger the traffic lights with a big loudspeaker-magnet glued to the underside of your bike, because the induction loop sensor under the road picks you up as a sodding great truck with a huge magnetic field.
Or something.

Will this really work? Someone with some electrickery knowledge...?

How big does the magnet need to be, and how close to the ground?

**Mikkel** · 3rd March 2008, 17:23

Originally Posted by Steam

I read you can trigger the traffic lights with a big loudspeaker-magnet glued to the underside of your bike, because the induction loop sensor under the road picks you up as a sodding great truck with a huge magnetic field.
Or something.

Will this really work? Someone with some electrickery knowledge...?

How big does the magnet need to be, and how close to the ground?

Hmmm, I thought they were just regular pressure sensors...

That, with the common kiwi flair for premonition, have been placed where you stop to wait for the lights. No, not a distance before the intersection so they'd actually allow you to cruise through - you have to stop!

**Steam** · 3rd March 2008, 17:27

Originally Posted by Mikkel

Hmmm, I thought they were just regular pressure sensors..

No, they are induction loops, they have a magnetic field. Metal passing over them /through their magnetic field induces an electric current in the sensors, which triggers the lights.
Will a big magnet do the same? Or does it have to be metal passing through the magnetic field? I need to raise the "inductance", which metal does, but I don't know if a magnet will do the same.

**Mikkel** · 3rd March 2008, 17:31

Originally Posted by Steam

No, they are induction loops, they have a magnetic field. Metal passing over them /through their magnetic field induces an electric current in the sensors, which triggers the lights.
Will a big magnet do the same? Or does it have to be metal passing through the magnetic field?

Most magnets are metal...

Are you positive that's how it works? Seems a bit odd to me...

Especially since if you move metal through a magnetic field then a current can be induced in the metal (i.e. the vehicle - not to worry we're talking small currents here).

On the other hand a car shouldn't have a large magnetic field - but if they did they would be able to induce an induction current in a coil hidden beneath the tarmac, yes.

Edit: I recall some advice somewhere about leaning your bike on the stand onto the road surface if you got caught behind a light that is unwilling to change... Would indicate a pressure sensor me thinks...

**twotyred** · 3rd March 2008, 17:34

It may work,I believe most of the commercial traffic light triggers are basicly a high flux magnet... have thought about getting one a few times but your idea is certainly worth a punt in the name of science!

**gijoe1313** · 3rd March 2008, 17:38

My RZ Daisy has no probs with the lights, but my little ol'hornet does ... I have to flick my kickstand down and then just flick it back up and no probs after that!

**Steam** · 3rd March 2008, 17:40

Originally Posted by Mikkel

Are you positive that's how it works?

Yes.

Originally Posted by Mikkel

Edit: I recall some advice somewhere about leaning your bike on the stand onto the road surface if you got caught behind a light that is unwilling to change... Would indicate a pressure sensor me thinks...

That's because leaning it on the stand brings that lump of metal (the stand) to the road surface, hopefully triggering the sensor.

Oh, here's a thing I found by an electrician dude, that shows a magnet will not work any better than a similar weight of steel:

Will not work, at least not better than any non-magnetic metal lump.

Traffic light sensors are inductive loops in the road surface (sometimes you can see the cuts in the asphalt where it is). An alternating current is sent through the loops, and if metal passes over it, the magnetic field from the loop causes eddy currents in the metal, which will alter the electrical characteristics of the loop. These altered characteristics can be detected. It makes no difference if the metal a magnet or not, just that it is an electrical conductor. In a way, it's just a big metal detector. So a magnet will make no difference.

So maybe I have started and ended my own thread.

**twotyred** · 3rd March 2008, 17:40

Originally Posted by Steam

How big does the magnet need to be, and how close to the ground?

commercial units are usually only a 100mm in size or so and are just attached to the bottom of the fairing or frame although they may be gruntier than a speaker magnet... with magnets,size is no guarantee of power

**Steam** · 3rd March 2008, 17:44

Originally Posted by twotyred

commercial units are usually only a 100mm in size or so and are just attached to the bottom of the fairing or frame although they may be gruntier than a speaker magnet... with magnets,size is no guarantee of power

WOnder how that works? Are they powered by the bike's electrics itself? I suppose an alternating-current electromagnet would do the trick to induce a current in the loop...

**mowgli** · 3rd March 2008, 17:49

Originally Posted by Steam

No, they are induction loops, they have a magnetic field. Metal passing over them /through their magnetic field induces an electric current in the sensors, which triggers the lights.
Will a big magnet do the same? Or does it have to be metal passing through the magnetic field? I need to raise the "inductance", which metal does, but I don't know if a magnet will do the same.

Yes, a large speaker magnet will increase the chances of triggering the lights. For best results you'll need to orient the magnet with the poles in the vertical sense so that you get the greatest number of flux lines cutting the induction loop. Being closer to the road will increase the induced signal as will moving over the sensor quicker. I make a point of stopping smartly over the sensor rather than creeping up to the lights.

Edit: See post #7. I stand corrected.

**davereid** · 3rd March 2008, 17:55

Yep, my scooter won't trigger them - BUT its OK to apply right hand rule if they don't work. ie you don't need to sit there stuck.

Thats because you are allowed to use right hand rule at faulty lights... and if they don't detect a vehicle but are supposed to they are faulty.

(I got a ticket for it years ago).

**McJim** · 3rd March 2008, 18:08

This is the reason the congestion is so bad in NZ cities. In the UK the lights are generally phased so you stand a fair chance of getting a green light as you approach but in NZ you have to stop at every fucken light. Daft. Proper phasing is cheaper and more efficient than embedding induction loops.

Moron planners.