Electronics > Projects, Designs, and Technical Stuff
Hall sensor for reading magnetic tape
petert:
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?
guenthert:
--- Quote from: petert on August 03, 2019, 05:29:19 pm ---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.
--- End quote ---
Given 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?
petert:
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.
PA0PBZ:
From here: https://en.wikipedia.org/wiki/Orders_of_magnitude_(magnetic_field)
240 mG Strength of magnetic tape near tape head
Your Hall sensor:
1.3 mV/G typical for the A1302
Do the math...
*Edit: the link to the Wiki page
petert:
--- Quote from: PA0PBZ on August 04, 2019, 08:07:44 am ---240 mG Strength of magnetic tape near tape head
Your Hall sensor:
1.3 mV/G typical for the A1302
--- End quote ---
It 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.
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