Hall sensor for reading magnetic tape - Page 1
Forum section: electronics / projects

author: petert
author_rank: Regular Contributor
timestamp: August 03, 2019, 05:29:19 pm
content: Hello,I was trying to read a magnetic audio tape with a hall sensor (by swiping it along the tape), but it seems the sensor I have (A1302) is not sensitive enough to pick up the changing magnetic flux on the tape. I don't need a high spatial resolution, I just want to magnetize the tape with flux of alternating polarity and then pick it up again by swiping a sensor along it. Similar to a magnetic stripe reader.Does anybody have a suggestion for a hall sensor (or similar) that reacts to small magnetic fields? (Tape heads give even smaller signals, i.e., the readings are undetectable and may be somewhere in the noise floor.)I tested that the sensor (and the tape head) reacts to magnets, it just seems to be not sensitive enough for magnetic tapes to give any reading. A sensor that could pick up small magnetic fields of a few Gauss/Tesla would be ideal.Does anybody have some experience regarding what works well?

author: guenthert
author_rank: Frequent Contributor
timestamp: August 03, 2019, 05:51:06 pm
content: Quote from: petert on August 03, 2019, 05:29:19 pmGiven the popularity of tape heads to read magnetic tapes, I have a hard time believing that they wouldn't be sensitive enough to read magnetic tapes. Could you clarify what you're attempting to do?There will typically be an amplifier behind the tape head, which will be optimized for the expected frequency band of the signal (audio, video, DAT, LTO, etc.). Which frequencies do you expect?

author: petert
author_rank: Regular Contributor
timestamp: August 04, 2019, 03:14:35 am
content: I attached the tape-head directly to an oscilloscope, without any amplifier, besides the highest scaling/amplification available on the oscilloscope.For the hall sensor I was using an amplifier, but it was not enough. The noise was amplified as well, yet still no signal to be seen (for the cards I tried). And I wanted a good alternative.I'll make a simple op-amp amplifier for the tape-head and report back. A suggestion for a suitable hall sensor would still be nice.Regarding frequencies, I have no specific requirements yet (exploratory phase), definitely nothing higher than audio-frequency, probably well below that. It might be however that the magnetization on the cards/stripes is not that strong.Edit: the tape head is from an old cassette recorder, so it can read and write onto audio tapes.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 04, 2019, 08:07:44 am
content: From here:https://en.wikipedia.org/wiki/Orders_of_magnitude_(magnetic_field)240 mG Strength of magnetic tape near tape headYour Hall sensor:1.3 mV/G typical for the A1302Do the math...*Edit: the link to the Wiki page

author: petert
author_rank: Regular Contributor
timestamp: August 04, 2019, 08:53:36 am
content: Quote from: PA0PBZ on August 04, 2019, 08:07:44 amIt would give me a 312mV reading, which I could easily see on the scope (which goes down to about 10 mV/div). But since I don't see it (without an amplifier so far at least) something doesn't add up. Yet the magnet test works.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 04, 2019, 09:10:29 am
content: Not sure how your math works but the field strength is about 1/4 G so I'd expect 1/4 of 1.3mV = 325uV

author: petert
author_rank: Regular Contributor
timestamp: August 04, 2019, 09:55:32 am
content: Quote from: PA0PBZ on August 04, 2019, 09:10:29 amRight, I missed the milli in the mG!So it would be 1.3mv/G*0.24G = 0.312 mVI would need to amplify by a factor of 100 at least, to see anything on the scope. (I'll test it, again.)I still wonder if the hall sensor could even pick up such small signals, though... And if there are more sensitive ones that people actually made to work, with such small magnetic fields.

author: StillTrying
author_rank: Super Contributor
timestamp: August 04, 2019, 10:05:32 am
content: How are you going to get the hall effect gap any were near narrow enough, close enough, and parallel enough to the tape.

author: petert
author_rank: Regular Contributor
timestamp: August 04, 2019, 10:10:46 am
content: Quote from: StillTrying on August 04, 2019, 10:05:32 amGood question. I assumed that swiping it along the tape, having it touch it directly would be enough.Do you have any information on what the reasonable limits are?

author: MagicSmoker
author_rank: Super Contributor
timestamp: August 04, 2019, 10:34:23 am
content: Quote from: petert on August 04, 2019, 09:55:32 amYou might find a Flux Gate or Giant MagnetoResistance effect sensor that is sensitive enough to use as a tape read head, but definitely not one using Hall effect - you're going to need a sensitivity of at least 100mV/G, after all.That said, there is no good reason not to use a conventional tape read head and its associated preamplifier circuitry given that it was refined over decades of use in *billions* of tape decks, credit card readers, etc.

author: StillTrying
author_rank: Super Contributor
timestamp: August 04, 2019, 10:46:15 am
content: "having it touch it directly would be enough."Yes hall should work down to DC - a stopped tape. But the +/- peaks of a recorded 1kHz would be only around 0.02mm apart, I don't think you could get the hall sensor to detect such small areas, it would be looking at many +/- peaks at the same time."Do you have any information on what the reasonable limits are?"Not really, it's years since I last played with them. I always think these 3M magnetic tape viewers are interesting.http://www.tapeheads.net/showthread.php?t=39958