Traffic light changers: and a free gift to New Plymouth peoples

[thehollowmen]

A friend at the scooter shop and I have used Neodymium magnets to change traffic lights. They trigger the induction loop in the ground and make the lights change. Yes they really work, and surprisingly quickly at night.

Try it, they'll make night time riding a lot easier.

I've got a new bike, and forgot to take the magnets off the old one.
So I've ordered 40 more from trademe (seller=magneticworld) and only need a few, if you're in NP I'll happily give you a couple to stick to the bottom of your bike / scooter. Give me a message and I'll sort you out when this batch arrive.
Colin

[niero]

Hey, I am still somewhat sceprical about this magnet business. So let me get this right, There is a magnetic sensor below every white (double or single) line where the lights are... it sences a car because I guess the inductor coil that produces the spark generates a magnetic field or something else on the car... OK, fair enough but how on earth does it make the lights trigger? Does it have a brain that goes, "oooooh strong magnetic field, must be many cars waiting, Il better switch the lights!"?!?!? Just does not make sence. Anywone care to explain. Cheers

[thehollowmen]

What it does is it detects a change in the magnetic field: Basically it is a glorified metal detector. But they often aren't sensitive enough for some of our bikes.

Reving your engine to high heaven sometimes generates enough of a change in the magnetic field in your alternator, but I don't like doing this late at night as I don't want my neighbors to vandalize my bike.

Those cut out squares you see in front of an intersection are loops of wire below the road surface. What it does is tell the computer at the lights that a car is waiting there.

Lights at night, and sundays are often set up in a 'lazy' method. Basically the route with the most traffic stays green until a sensor on the side road is tripped. This can also happen with some filters, turning right across into a high volume traffic can get you stuck for quite some time if the sensor hasn't been tripped. Actually, in Dunedin there is one across the southern motorway (just before the speed camera); turn right from south Dunedin to go north and on a sunday you can be stuck there for an hour or so waiting for another car to come up behind you and trip the sensor. And that would be one very dangerous red light to run.

Or, just looking at your geographical location, go down barbados st at 2am and try to cross bealey ave. The lights just won't change unless a car comes up beside you or a pedestrian hits the button.

I'm not selling anything, just sharing the knowledge of something I know works. I've even walked up to the lights waving the magnets around with no bike and seen the lights change.


Hey, I am still somewhat sceprical about this magnet business. So let me get this right, There is a magnetic sensor below every white (double or single) line where the lights are...

[Horse]

I've seen it suggested (probably on KB) that putting your sidestand down over the induction loop will trigger the sensor. Gets the metal closer to the loop I guess...

[Reido]

i've had this problem coming home from work at 9pm. got stuck at a set of lights where it was very unlikely any car would come up and join me in cue. waited probably about 5 mins before i got frustrated and ran the red. (safely, just followed the traffic to the right turning right)

Very annoying

can you give a TM link? im interested.. but in Auckland

[Dave-]

Hey, I am still somewhat sceprical about this magnet business. So let me get this right, There is a magnetic sensor below every white (double or single) line where the lights are... it sences a car because I guess the inductor coil that produces the spark generates a magnetic field or something else on the car... OK, fair enough but how on earth does it make the lights trigger? Does it have a brain that goes, "oooooh strong magnetic field, must be many cars waiting, Il better switch the lights!"?!?!? Just does not make sence. Anywone care to explain. Cheers

its science dont be sceptical.

everything has a magnetic field...you, me, the empty glass on your desk, your mouse, CDs, your house, trees outside, earth is the largest magnet on earth its how a compass works....in many cases these cannot be practically measured because they're so small.

a car has lots of mass, 1-2 tonne, so it has a larger magnetic field than a motorcycle which has less mass.

however magnets have much larger magnetic fields for their size so by slapping one on the bottom, of your bike (close as possible to the road) you drasticly increase your magnetic field to the same size as a car.

the way the loop works is actually very common, its the same principle as we get electricity, a magnetic field passed through or even near a loop creates electricity (or i should say converts energy) but what happens is a wee circuit sits there waiting to get a jolt of electricity, when it does it knows a magnetic field has passed the loop and tells the lights to change.

[breakaway]

http://www.dealextreme.com/details.dx/sku.13507

[xwhatsit]

I keep telling people you don't need these things. You can trigger the sensor every time by directly parking on top of it -- most motorcyclists don't, they park off to the RHS of the lane.

Look for a rectangular box sort of carved into the road with a vertical line through the middle. Park your bike along the middle vertical line, right on top of it. You don't need to put your side stand down, you don't need to rev -- it just triggers the light and away you go. Even works on a tiny little Honda Cub.

Check out the picture, once you know what you're looking for you'll see them everywhere.

[breakaway]

I'm gonna have to agree with Tom here, I've never had a problem with a light not coming on, and I commute at ungodly hours all the time. Worse come to worst, slam your side stand down.

[hayd3n]

I'm gonna have to agree with Tom here, I've never had a problem with a light not coming on, and I commute at ungodly hours all the time. Worse come to worst, slam your side stand down.

hymm but then my bike would stall as it has a stand switch and i aint removing it coz it can be a lifesaver

[CookMySock]

soooo, what if your bike is completely made of plastic ?? then you are screwed aren't ya!! :Pokey:

DB

[KiwiRat]

I keep telling people you don't need these things. You can trigger the sensor every time by directly parking on top of it -- most motorcyclists don't, they park off to the RHS of the lane.

Look for a rectangular box sort of carved into the road with a vertical line through the middle. Park your bike along the middle vertical line, right on top of it. You don't need to put your side stand down, you don't need to rev -- it just triggers the light and away you go. Even works on a tiny little Honda Cub.

Check out the picture, once you know what you're looking for you'll see them everywhere.

Exacty buddy. Stop right in the centre of the loop.

Hey nonny nonny, lights change when traffic allows.

Peace.

[breakaway]

hymm but then my bike would stall as it has a stand switch and i aint removing it coz it can be a lifesaver

Even in neutral?

[awayatc]

soooo, what if your bike is completely made of plastic ?? then you are screwed aren't ya!! :Pokey:

DB


You can get metal training wheels.......

[Monsterbishi]

The 'things' in the ground, are called 'loops' - since they are exactly that, just a big double, kinda figure 8 loop of wire.

When they're freshly installed, even a steel toe in a boot will trigger them, but as they get older, vibration, etc affects the resistance in the wire and makes them less sensitive - a couple of well placed magnets will help compensate in this instance, plus if you're riding over any of the traffic counter loops that are out there, it makes you look like a much larger vehicle on the data sheets :Punk:

[<Rhino>]

I keep telling people you don't need these things. You can trigger the sensor every time by directly parking on top of it -- most motorcyclists don't, they park off to the RHS of the lane.

Look for a rectangular box sort of carved into the road with a vertical line through the middle. Park your bike along the middle vertical line, right on top of it. You don't need to put your side stand down, you don't need to rev -- it just triggers the light and away you go. Even works on a tiny little Honda Cub.

Check out the picture, once you know what you're looking for you'll see them everywhere.

I did what you suggested and a set of lights that didn't seem to change for me(is an intersection with SH2) and parking in the middle works. I used to park on the side, so what you said does work :yes:.

[niero]

I am not trying to be a smart ass here but in order to generate a current yes you need an inductor coil a magnet and something else... something that is not present when you have got a red light (and no you cant be a pretend bogan ere)... ooh movement! In order to achive electricity you need movement, moving a magnet through inductor coil only creates microamps of current, that is very small even if you are talking circuits, + the magnet is right inside the inductor coil, how on earth can that chip sence through CONCRETE + no movement. I just cant understand this, sorry if that sounds so dumb.

[Monsterbishi]

I am not trying to be a smart ass here but in order to generate a current yes you need an inductor coil a magnet and something else... something that is not present when you have got a red light (and no you cant be a pretend bogan ere)... ooh movement! In order to achive electricity you need movement, moving a magnet through inductor coil only creates microamps of current, that is very small even if you are talking circuits, + the magnet is right inside the inductor coil, how on earth can that chip sence through CONCRETE + no movement. I just cant understand this, sorry if that sounds so dumb.

ok, for starters, you don't need a magnet, or movement you just need a ferrous metal that will interfere with the existing fields around it.

and secondly, they're not underneath the concrete, they're in shallow channels covered by a rubber compound.

lastly, yes, the circuitry is sensitive enough to detect the changes.

[thehollowmen]

Thank you, you seem to know what you're talking about.
I agree that any metal will work to cause magnetic flux, but a magnet will cause greater magnetic flux.

A lot of these plastic scooters or super-light alloy bikes don't have enough mass to trigger SOME of the worst ones, ones that have miscalibrated or aren't working. I have one a block from my house and if I come home after dark I can get stuck for ages. This is a fix for my problem.

My DL650 wouldn't change some of the badly calibrated / worn / plain not working ones, and I rang the council in Dunedin and CHCH, but they and the contractors didn't want to hear from me about the problem.

I'm just offering a solution if everything else fails; if you regularly encounter one and bike placement and the kickstand trick doesn't work. I'm even giving these out to some pushbike riders because even the sensitive well calibrated ones don't change lights for them.

Please people, use what works for you. But if it fails, I'm handing out another solution for free here.


ok, for starters, you don't need a magnet, or movement you just need a ferrous metal that will interfere with the existing fields around it.

and secondly, they're not underneath the concrete, they're in shallow channels covered by a rubber compound.

lastly, yes, the circuitry is sensitive enough to detect the changes.

[swbarnett]

I have tried parking in the middle of the inductive loop. Normally it works. However, I was at the lights in the centre of the Albany township yesterday (end of the shortcut from Oteha Valley Road). I waited for a good two or three minutes with no reaction from the lights. As soon as a car arrived in the lane beside me they changed. I think not all lights are sensitive enough even for this technique.

[Monsterbishi]

I have one a block from my house and if I come home after dark I can get stuck for ages. This is a fix for my problem.

My DL650 wouldn't change some of the badly calibrated / worn / plain not working ones, and I rang the council in Dunedin and CHCH, but they and the contractors didn't want to hear from me about the problem.

If you live in Christchurch I can point you in the right direction as to who to contact to get results, Christchurch is broken into 5 pieces as far as maintenance contracts go, and the transit network is different again!

[pritch]

I'm gonna have to agree with Tom here, I've never had a problem with a light not coming on,

There was a thing about this on TV a while back. The sensors are adjustable for sensitivity and some councils (I think Dunedin was one?) have them adjusted too high to pick up smaller vehicles particularly scooters.

The other day on my way home from work I was second in the queue at the lights which were taking their time to change, when I looked the woman in the car in front of me was stopped so far back that she wasn't even on the sensors. So I rode around her on the SJ50, which seemed to offend her, but at least we got a green otherwise we could still be sitting there.:confused:

[xwhatsit]

There was a thing about this on TV a while back. The sensors are adjustable for sensitivity and some councils (I think Dunedin was one?) have them adjusted too high to pick up smaller vehicles particularly scooters.
Perhaps somebody involved with the roading (Monsterbishi? I know there's a few others as well) could give a few reasons otherwise, but I can't think of anything particularly obvious as to why the sensors could not just be adjusted to be over-sensitive anyway. Having a `false positive' in this situation is not a great problem -- mebbe somebody coming from what's considered the primary direction late at night has to stop and wait for a few seconds, but that's not a great hassle compared to people going through red lights just to get around these things (something I did on occasion on my late-night commute until I realised how to trigger the things).

[skidMark]

Hey, I am still somewhat sceprical about this magnet business. So let me get this right, There is a magnetic sensor below every white (double or single) line where the lights are... it sences a car because I guess the inductor coil that produces the spark generates a magnetic field or something else on the car... OK, fair enough but how on earth does it make the lights trigger? Does it have a brain that goes, "oooooh strong magnetic field, must be many cars waiting, Il better switch the lights!"?!?!? Just does not make sence. Anywone care to explain. Cheers

Look on the ground as you get to traffic lights about 2 metres prior to the white line is a figure 8 on the ground... but in a squarish shape...

[] <<like that
[]


ride onto the tar lines.....

Some bikes have trouble setting them off especially newer ones as they are completely alloy...

detects steel more easily...aka cars.

[portokiwi]

:rolleyes:If you are giving them away I dont mind trying 1 out here in Auckland.

[awayatc]

I keep telling people you don't need these things. You can trigger the sensor every time by directly parking on top of it -- most motorcyclists don't, they park off to the RHS of the lane.

Look for a rectangular box sort of carved into the road with a vertical line through the middle. Park your bike along the middle vertical line, right on top of it. You don't need to put your side stand down, you don't need to rev -- it just triggers the light and away you go. Even works on a tiny little Honda Cub.

Check out the picture, once you know what you're looking for you'll see them everywhere.


I didn't even know I didn't know that......
kept an eye out for those things on the ground......Found em!
Thanks for that info....
Good post

[awayatc]

Marginally related,:
many moons ago one uncle was an ambulance driver in a big German city.
The Ambo had a transmitter to change the traffic lights.......
Somehow some clever people managed to make/get their own transmitters.....

This was before the interweb and all, so it was all hush hush haha

:scooter:

[Badjelly]

Marginally related: many moons ago one uncle was an ambulance driver in a big German city.

That's what I thought the title thread meant: one of those things the fire engines and ambulances use to force the lights to change. Now that would be a cool thing to have!

[firefighter]

That's what I thought the title thread meant: one of those things the fire engines and ambulances use to force the lights to change. Now that would be a cool thing to have!

would be nice to have them here!

[Irontusk]

ooh movement!

Your skepticism when it comes to physics has me a little annoyed, so are you trying to say that you aren't moving when you ride up to a set of lights??

[Monsterbishi]

Marginally related,:
many moons ago one uncle was an ambulance driver in a big German city.
The Ambo had a transmitter to change the traffic lights.......
Somehow some clever people managed to make/get their own transmitters.....

This was before the interweb and all, so it was all hush hush haha

:scooter:

They still use them in many countries, even had them here in New Zealand for a while, TCS(the company whom we contract to install loops for us), installed them for the council at nothing short of great expense, and then they just sat unused for a few years, were then removed at yet more expense!

And to answer one of the earlier questions, yes, sensitivity can be adjusted on the loops.

[cooneyr]

I keep telling people you don't need these things. You can trigger the sensor every time by directly parking on top of it -- most motorcyclists don't, they park off to the RHS of the lane.


Exacty buddy. Stop right in the centre of the loop.



parking in the middle works. I used to park on the side, so what you said does work :yes:.


Look on the ground as you get to traffic lights about 2 metres prior to the white line is a figure 8 on the ground... but in a squarish shape...

[] <<like that
[]


ride onto the tar lines.....



Finally some people who get it. I've noticed a few of these threads of the past couple of years and even drew and posted a picture (see this post (http://www.kiwibiker.co.nz/forums/showpost.php?p=1694823&postcount=4)).

You dont need magnets just stop in the correct place. The middle line of the loops is the most sensitive - it even works with an alloy framed treadly.

Cheers R

[thehollowmen]

Left hand lane, barbados st heading south across bealey in the early hours of the morning. That loop doesn't even pick up cars half the time.

Yes, for 99% or whatever of loops, positioning in the center of the loop works fine. But if there is one that always gets you stuck for ages on the way home, it might be byorked so try a magnet, and I've got some going spare.

:Oi: god damn it is getting frustrating saying this again and again, if positioning doesn't work because the loop is screwed up, here is a solution.

Please, get with the program. You're talking about how to trigger responsive loops, I'm talking about how to attempt to trigger non-responsive loops.


Finally some people who get it.

[xwhatsit]

Left hand lane, barbados st heading south across bealey in the early hours of the morning. That loop doesn't even pick up cars half the time.
If that's the problem, it's probably worthwhile writing a (polite, sensible) letter to the appropriate council and alerting them to the fact the sensor is on the blink. I've heard of the councils in Wellington (didn't Jim2 write a swag of letters?) and up in Auckland being quite willing to calibrate them appropriately when told about the problem.

@cooneyr -- yup, I think you're the one who informed me about this in the first place. Thanks for informing us!

[cooneyr]

Left hand lane, barbados st heading south across bealey in the early hours of the morning. That loop doesn't even pick up cars half the time.....

......Please, get with the program. You're talking about how to trigger responsive loops, I'm talking about how to attempt to trigger non-responsive loops.

I'd defiantly be letting the Chch city council know if there is a problem. The induction loops are just a cable wound in a figure 8 a few times in the road surfacing. If the road surfacing breaks up for whatever reason the cable could be broken and the loops fail. However - there is a couple of guys sitting in the Chch city council offices continuously monitoring and modifying the system (for almost all of the signals in the upper half of the south island) so they should notice when loops fail or are unresponsive. Still if you think there is a problem then fire a letter or email at the council - CCC contact us page (http://www.ccc.govt.nz/Contact/) with an email addy.

It is likely that the phasings between about 8pm and 6am are on "on demand" for the side roads and the phase lengths will be very very bias towards the main road. If the lights change to allow the side road to go no matter how long you have to wait when the phasings are "on demand" the loops are working it is just that the phase times may not be very favorable. It is not unusual to have to wait up to around 30 seconds (with can seem like forever) on the side roads. Try waiting and timing it one time and see how long it actually takes - you may be surprised how short it really is.

Cheers R

[Hoon]

Yeah I have this problem not with my bike but when driving Army Unimogs. Because they are so high off the ground and the tyres so large, it often doesn't trigger the sensors. There are lights around our base which we avoid (or make sure we aren't first in queue) otherwise they don't change.

[Badjelly]

Yeah I have this problem not with my bike but when driving Army Unimogs. Because they are so high off the ground and the tyres so large, it often doesn't trigger the sensors. There are lights around our base which we avoid (or make sure we aren't first in queue) otherwise they don't change.

Call me a miserable SOB, but somehow the fact that someone else has this problem makes me feel better :-).

Perhaps you should contact the local authority and ask them to increase the sensitivity. Be polite and don't make any reference to automatic rifles or rocket launchers, initially.

[hayd3n]

Even in neutral?:clap::clap:
what now i gotta go out of gear
what happens if some moron comes hooning behind me and i need to get the fuck out of the way?
vroom ohh fuck im in neutral ,clunck, arghhh to late!!!!!!!!!!! :clap::clap:

[rwh]

You dont need magnets just stop in the correct place. The middle line of the loops is the most sensitive - it even works with an alloy framed treadly.


I do that most of the time, when I'm stopping for the lights. If I'm not planning to stop (ie when the light is green), I have other priorities for choosing my lane position. So there are occasions when I get the timing just wrong, and don't want to be thinking about changing my line just because the light has gone orange, or some other incident has caused me to change my plans.

Think I might try this (already know where to get some magnets).

Richard

[Big Dave]

I have an objection in principle to adding weight to my bike when the stand works just as well.

[Molly]

I suppose if it works and doesn't disadvantage other road users then there's no harm in trying it.

[Blackshear]

Dunno if anyone in the North shore has offered or anything, but I've got about a hundred of these cylindrical Neodymium magnets just sitting here. I WAS going to invent perpetual motion, but that was before the Laws of thermodynamics 101.
One per person!
I never really had a problem with lights, owing to the massive gait of my 250, but if it changed one light, I'd be happy. Not that i'd notice or anything.

[DougB]

I have just read on a USA site that the only way they can trigger their loops is to have a magnet attached to the instep of thier boot and when over a loop put the magnetic foot down.

[Mystic13]

Thanks "thehollowmen" for the thread and "blackshear" for the offer for North.

PM'd you, I'm in for one.

Just to comment on the discussion.

The loops don't always trigger. I have one near my home that mostly NEVER triggers. I blast up, look around and shoot the red multiple times a week. I figure if I ever get stopped I can talk my way out. Other than that there are many others I come across. I never sit for too long.

As for current flowing through the loop etc and moving bikes it's not quite like that as far as I'm aware. The loop doesn't need a moving object. An object that suddenly appears will trigger the electronics (although it would be hard to imagine that happening)

The thing in the ground is an inductive loop. So when it's connected to the electronics there is a certain impedance measured (not resistance). The arrival of a vehicle changes the impedance and if it's above a set level the electronics do their thing.

I'm guessing it would be similar to the hand held scanners customs guys use at airports. Except it's a portable inductive loop.

[vifferman]

I have an objection in principle to the stupidity of the traffic light controllers, particularly on the North Shore. Many of them are a simple program running with a timer for different days of the week and times of the day, taking no account of traffic volumes or sensors (apart from pedestrian buttons - go figure!)
Auckland City and North Shore City could help save the country $$$$ in fuel and ease traffic congestion heaps by replacing many of the controllers with 'intelligent' ones that are co-ordinated with one another, and that sense traffic volumes rather than just operate mechanically and stupidly.
[/rant]
Late at night, I sometimes ignore the lights (particularly red turn arrows) if there's no traffic - just pause, look around, and go.
Think about it: traffic lights are supposed to replace road signs and people making decisions based on road rules, traffic, etc. They should make traffic flow better and more safely. If they're being stupid, and actually hindering traffic flow, what's the point of them?

[firefighter]

Late at night, I sometimes ignore the lights (particularly red turn arrows) if there's no traffic - just pause, look around, and go.
Think about it: traffic lights are supposed to replace road signs and people making decisions based on road rules, traffic, etc. They should make traffic flow better and more safely. If they're being stupid, and actually hindering traffic flow, what's the point of them?


in the U.S it's legal to turn right on a red, you give way but if it's clear you go (obviously) which is the same as us going left, which I think is bloody awesome, and we should do the same here.

[Badjelly]

As for current flowing through the loop etc and moving bikes it's not quite like that as far as I'm aware. The loop doesn't need a moving object. An object that suddenly appears will trigger the electronics (although it would be hard to imagine that happening)

The thing in the ground is an inductive loop. So when it's connected to the electronics there is a certain impedance measured (not resistance). The arrival of a vehicle changes the impedance and if it's above a set level the electronics do their thing.

Interesting. To do that there would have to be an alternating current flowing through the loop. A metallic object in the vicinity would then change the impedance of the loop, which would change the amplitude or phase of the AC, which could be detected.

The other way they could work would be passive. A moving magnet (or a moving metal object disturbing the Earth's magnetic field??) would generate a current in the loop. The method would not require any pre-existing current in the loop. But you say this is not the way they work.

Either way a magnet on your bike should improve its detectability, I think.

I don't know that these musings have any practical relevance, but I like to know this stuff. (Just like countersteering, really.) Time for some research...

Edit: Wikipaedia says they work like metal detectors:

http://en.wikipedia.org/wiki/Metal_detector

[ColonelKFC]

I always thought it was a weight related thing. They put temporary lights on Bryants bridge in Aokautere in Palmy when it collapsed and went to one lane. they also put in sensors below the road for a couple of months. Being me, I parked right on top of the bastard thing to change the lights. 10 minutes later I pulled forward to let the solitary car behind me trigger it, and with no oncoming traffic it changed almost instantly.I still don't know to this day if i was weight or magnetic field that changed it. Ambos and fire engines have light changers these days but i thought they worked like a remote control (most traffic lights have a small antenna on top if you look hard enough)
but the magnets may work in a similar way

[Badjelly]

I always thought it was a weight related thing...

Not since they got rid of the rubber-tube detectors. (How old do you have to be remember those? It's a long, long time since I've seen one.) You could usually trigger them with a well-directed stomp.

These days it's more of a volume-of-conductive-metal thing.

[Mystic13]

Not since they got rid of the rubber-tube detectors. (How old do you have to be remember those? It's a long, long time since I've seen one.) You could usually trigger them with a well-directed stomp.

These days it's more of a volume-of-conductive-metal thing.

lol.. rubber tube detectors... that sounds interesting.

Next thing you'll be trying to tell us about punch cards for computers or machines that swallow long lines of paper tape with holes in it.

I told my kids that microwave ovens didn't exist when i was a kid and they're shocked to hear that. "what? how did you heat things up?".

And here I am seeing the first new generation Samsung mobile phones with video projectors. I so want one of these. I have no idea what I'd use it for but...

http://www.gizmag.com/samsung-pico-projector-mobile-phone/10773/

[Naki Rat]

lol.. rubber tube detectors... that sounds interesting.

Next thing you'll be trying to tell us about punch cards for computers or machines that swallow long lines of paper tape with holes in it.

I told my kids that microwave ovens didn't exist when i was a kid and they're shocked to hear that. "what? how did you heat things up?".

And here I am seeing the first new generation Samsung mobile phones with video projectors. I so want one of these. I have no idea what I'd use it for but...

http://www.gizmag.com/samsung-pico-projector-mobile-phone/10773/

And rotary dial telephones, floor mounted headlight dipswitches and manual toll calls.

[Bass]

I don't know that these musings have any practical relevance, but I like to know this stuff. (Just like countersteering, really.) Time for some research...

Edit: Wikipaedia says they work like metal detectors:



Commercial metal detectors typically have an excitation coil and 2 detection coils. Metal coming into the field changes the phasing between the exciter and the detectors.
They also play around with the frequency used (and it's always way above 50 hz) according to what they are looking for. In this way, they can actually persuade the machine to ignore some stuff e.g. the metallised film used for packing potato chips and the like, so will can look only at the bag contents.
Obviously then, the magnetic properties of the metal to be detected, come into play, with magnetic materials e.g. mild steel, being the easiest to find, followed by ferrous-but-nonmagnetic e.g. austenitic stainless and then other alloys.
Lastly, shape and motion also play a part. A needle shaped object can be easily located in some orientations wrt the field but almost invisible in others.