 <Hack> Extremely cheap mains detector and magnetic viewer
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Conundrum
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Sun May 02 2010, 02:13PM
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Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062
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Hi all.
It occurs to me that combining an off the shelf 4 pin Hall sensor (the bare kind with the 2.5V output) and two MN1381C 2.2V ultra low power voltage sensors would enable a magnetic sensor to be built that senses mains reliably and allows diagnostics of annoying faults such as failing CCFL inverters without opening the device.
the basic idea here is to run the sensor midpoint (between 2.05 and 2.25V) and rely on hysteresis to switch the output when a field is detected.
Take the output from the MN1381's and connect to either side of a bicolour LED, the flashing speed of which determines the polarity, and field strength of the field being detected.
By adding an iron nail to one side of the Hall sensor the field can be amplified so mains can be sensed from up to 3" away (far better than those live test screwdrivers)
conveniently it only needs 4.75V which is well within the range of three button cells in series.
An even cheaper version uses an LM311 micropower comparator with one of the same SMD sensors (harvested from a very dead CDROM drive) and then carefully glued and soldered to the top of the comparator.
This requires nerves of steel and a very fine soldering iron but the sensor is ridiculously compact compared with even a standard Hall logic sensor.
the + and - output pins are soldered directly to the inverting and non inverting inputs and the output goes to a R/B SMD LED (cheap as chips) with the red anode to +V and blue cathode to Gnd via 1uF tantalum or ceramic capacitors.
is also insensitive to voltage fluctuations and senses reliably down to 2.2V with reduced sensitivity.
in this case the new chip is what makes it work, to do this with even an LM339 would draw too much quiescent power for it to be useful and the input current on the '339 is far too high to get any meaningful output.
even better it only responds to *changes* in the field rather than + or - Gauss so leaving it next to a toolbox magnet for weeks won't kill it.
(scuttles off to make an entire PCB full of these things for a POV magnetic viewer...)
-A
"Bother!" said Pooh, as his 3G dongle failed for the n+googolth time...
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