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Magnetic field array

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LAN_403

Simon

Mon May 29 2006, 01:32AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

Jaycar sells bi-colour LEDs. IIRC, (and I'm pretty sure, although I've never used them) their colour depends on the polarity.

In any case, I'm sure there's a simple way to make
no field -> no LEDs
N field -> red LEDs
S field -> green LEDs.

Still a cool toy.

Tesladownunder

Mon May 29 2006, 03:11AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

I also have tricolour LED's but although this will mean only one LED to fill the space, you still need the dropping resistors (I think). They are also more expensive.

What is probably more useful to me now is a separate indicator after a certain threshold is reached such as 0.1 Tesla. I can still access all the outputs separately so am open to suggestions.

Peter

Carbon_Rod

Mon May 29 2006, 06:03AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Well, I tend to find the Hall-effect sensors are rather slow for some things (convenient and cheap internal preamps.)

Perhaps a stack of old multi-platter hard drive read heads with the super sensitive resistive driver-chip pickups (no its not an error â€“ its an old technique thatâ€™s several times more sensitive than just plain passive coils etc.)

Mind you I would be curious to know how fast those cheap magnetic colour changing plastic sheets would respond to flux changes (or expose an impression like film.) The only electromagnet around this place is a little 3Kwatt job that gets too hot and would likely burn anything with plastic =(.

Good luck,

Tesladownunder

Mon May 29 2006, 04:57PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Actually it is the green LED's that are slow compared with the Hall device. Here is a comment from Terry Fritz about LED persistence with the phosphor not decaying for 66ms for the white LEDs and 0.5us for the red LED's (ie 100,000 times faster). Compare that with 50us (23kHz) for the Hall device.

I am not after great sensitivity and really am only interested in fields over 0.1 Tesla so not really looking for ultrasensitive giant magnetoresistive heads.

Peter

Carbon_Rod

Tue May 30 2006, 03:05AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

I donâ€™t know what kind of resources are available too you,

If you get those little devices cheap enough perhaps have dedicated North sensors (super bright Red LEDs), and flip another sensor over right beside it for South sensors (super bright Blue Leds). However, both off when neutral is not much of a bonus with twice the sensor cost.

Or have a single resister from the output fed into two LEDs and have two switched power rails for the LEDs (1 NPN 1 PNP and a 555 clocking a few KHz.) As the potential across the LED increases it will turn on one and turn off the other reversed biased LED.

Or a cheap virtual ground made out of an opamp like an LM380, one drive resister, and two LEDs back to back. A few volts above virtual ground will turn on one, and below turns on the other. Only drawback is only low output LEDs could be used (high output units may not work with only +- ~2.5V.)

Edit: Maybe the same divider trick with two resisters (200ohm each should not be too wasteful for a low number of pixels) in series across the rails, two back to front Diodes connected to the node and the sensor output.

Still very neat,
Cheers,

Bored Chemist

Tue May 30 2006, 08:11AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I couldn't get that link to work. Anyway, why would there be a phosphor in green leds? Green leds have been arond much longer than blue ones and to drive a green emiting phosphor would need a blue (or UV) led.

Bjørn

Tue May 30 2006, 09:13AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Green LEDs using phosphor are very rare, I have never seen one.

I would have used a microcontroller to read the values and to drive the LEDs. It is more relaxing to fiddle with code on a large screen than to fiddle with a soldering iron were there is no room.

Tesladownunder

Tue May 30 2006, 05:56PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

BjÃ¸rn BÃ¦verfjord wrote ...

Green LEDs using phosphor are very rare...I would have used a microcontroller to read the values and to drive the LEDs. It is more relaxing to fiddle with code on a large screen than to fiddle with a soldering iron were there is no room.

Agreed. Interestingly I was dividing up the red and green LEDs on the chip appearance. The red LED has a small black chip and I thought the green one looked like a white phosphor LED with the light yellow phosphor. I wasn't really thinking about it at the time.
Fiddling with code is not easy for me so I guess I am stuck with the iron. This might be taking some nasty pulsed fields so best not too close to a computer. The visualisation of an invisible field in realtime is always fascinating with the output next to the input.

Carbon_Rod wrote ...

Edit: Maybe the same divider trick with two resisters (200ohm each should not be too wasteful for a low number of pixels) in series across the rails, two back to front Diodes connected to the node and the sensor output.

I thought about a divider but it still added to the parts count. The Hall devices are about AUD$1 each.

Still, an interesting toy at present. I may be able to adjust thresholds by running it with a different supply rail voltage as it will tolerate 4.5 - 6.0 V. I will try to extend this to complete the project, then off to the next one.

Peter

Conundrum

Thu Jun 01 2006, 07:57PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

I have a load of similar SMD sensors from old broken video recorders which may well work. Also they output bipolar signals on two pins so it might be possible to connect them directly to bicolour LED's with suitable AC biasing of one pin..

regards, -A

"Bother" said Pooh, as his phone cut off 2 seconds into a call...

Tesladownunder

Fri Jun 02 2006, 04:44PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

The array now has 44 sensors and gives more detail.

First pic shows side on view.
Second shows mapping of 2 ring magnets with south pole up and one NIB with northpole up. And why are the hot spots yellow and not green? Because it is overlapped by red and the two colours make yellow. (add some blue and they would be white just like in an TV screen)
Third shows a more complex magnet (multisectored magnet out of a video motor) Note that the fields do not line up properly as the red and green LED's are physically separated by 5mm or so.

Peter

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