Hall Circuit Operational Setup
Last update 10/15/98
With your DMM or voltmeter, make sure that your
dual polarity power supply is putting out the correct voltage of no less
than 10 volts on the plus terminal with reference to the common ground.
12 volts to 15 volts should work without circuit modification. Adjust
if necessary.
With your DMM or voltmeter, make sure that your
dual polarity power supply is putting out the same voltage on the minus
terminal with reference to the common ground.
Make sure that all the connections and parts are
connected right, and, that all the polaritys are correctly observed, before
connecting the power supply to the circuit board. Hook up the Hall
circuit device to the board; remove the magnet from the seismo; or simply
connect to the circuit board by itself. Switch to the "AC".
Hook up the turned off power supply to the circuit
board connections, again making sure the connections are correct.
Hook up your DMM or voltmeter to the circuit, plus on the output,
and minus to ground.
Turn on the power supply. Observe for any
smoke (ha), or overheating of any component, by smell. If after afew
minutes, things seem to be alright; proceed.
Your DMM or voltmeter at this time should read
zero or very close to it.
Remove the plus DMM from the output; and put it
on the terminal 3 location. You should see a + 5 volts, or very close
to it, like... 4.97 or 5.02v. If it is something other like .4 volts;
you likely have a wrong connection somewhere, shut off the power and recheck
the circuit Hall connections first, then other parts if that isn't the
problem.
Remove the plus DMM from terminal 3, and put it
on the junction of the 20K pot, LM236-5.0 reference and the 4.7K resistor.
You should see a - 5 volts, or a voltage very close to that. It it
is something other, you may have a wrong connection somewhere. (Make
sure the Hall device is connected right.)
Doing the above is a check of the reference voltages.
They must be working right for your bridge circuit to function properly,
and to enable a zeroing voltage.
Remove the DMM plus cable from the junction and
put back on the output.
Switch to the "DC". You will likely see a
plus or minus voltage of up to 4 volts.
Turn the knob on the 20K ten turn pot in either
direction. Observe in which direction the voltage begins to fall.
Keep turning the knob to attain a zero voltage. Once you attain a
zero voltage, leave it there, and remember the zeroing procedure.
Turn the switch to the "AC" position. You
should see a zero, or, the meter voltage should be drifting to zero fairly
rapidly. The film or mylar capacitor for your "period" can account
for this action, by virtue of its activation, and with no signal it is
discharging. With time, the meter should zero.
Remember the above as it is the "zeroing" or bridge
balance procedure you need to do with the seismometer setup.
Now; check the amplifier. Use a magnet and
bring it close to the Hall sensor. If you see little or no signal
output, increase the gain on the other single turn 20K pot. If all
things are working right, while you are moving the magnet and adjusting
the pot you should see various gains on your DMM or voltmeter. You
will need to remember this, for when you do have your seismometer setup,
and its working, and you wish to increase or decrease the gain.
At this stage, with all things working, now you
can place the Hall on the seismometer, and begin the centering (zeroing)
of the sensor to your magnets. First shut off the circuit of course.
With either the physical movement/adjust of the
neodymium magnets; and/or the use of your seismometer adjusting screws,
you need to position the Hall in the center of the magnets. I use
a junction block along the line to the seismometer separate from the seismometer
itself, which carrys the 3 Hall signals, and hook my DMM up to ground and
the signal (terminal 2); with the DMM scale on at least 5 volts.
Now, the meter should read some voltage. The idea now is attain a
voltage of 2.49 volts or very close to that. Move your magnets/adjusting
screws to attain the voltage. If after locking down the adjustments
screws with their nuts, the voltage changes; you will have to compensate
the zero or the mechanical adjustments to get the right voltage; unless
it doesn't drift off that much, say to 2.48 volts or so.
Gently cover up your seismometer, with whatever
you use. If the voltage goes off too much you may have to re-do,
or compensate again via the DC switch & 10 turn 20K pot on the circuit
procedure as mentioned before.
Now with all things working right, remove the DMM
from the line junction box or wires.
Go back up to your circuit. Now, assuming
you already have a channel for the seismometer output on your computer,
and with the SDR running, you can connect up the output of the circuit
to your input of the computer. Here you should immediately see a
signal. If the signal is way too large, you can reduce the circuit
gain. If the gain is still way too large, you may have to put a resistor
between the output and the input of the computer....start with 10K, and
observe the results, while further adjusting the 20K single turn gain pot
on your circuit. The idea here now, is too adjust the gain to where
you can see the ever present microseisms, and perhaps back off on the gain
to where they are present but not overwhelming the monitor. Be sure
to know the SDR operation and its adjustments, they are important.
With all things working right, your seismometer
and computer are now a seismograph! Open a bottle of saspurilla,
and enjoy the wait for your first quake to show up.
Invariably with time, your seismometer adjustments
will drift off. This can be seen in 3 different ways. Number
one, the monitor line goes flat, or only occasional wiggles. Number
two; you can check the Hall signal coming into terminal two for the reading.
Number three, if you have the meter circuit on you Hall circuit, their
will be a small offset from its normal zero (discounting mass movement).
Sometimes, it maybe necessary to wait for local "noise" to abate to zero.
Rarely do I need to wait for anything, with seismometer adjustments; but
once or twice now, I've had quakes coming in, while I was doing it!
The linear Hall device has a dynamic range (where
its still linear) of roughly a reading of 1.5 volts up to 3.5 volts.
So....if your seismometer output drifts off per the last paragraph...to
say 2 volts; you can use the DC switch and the 20K ten turn pot to "zero"
it once again without having to adjust your seismometer adjustment screws.
Remember to switch back to the "AC" afterwards. You may sometime
get to a voltage where you can not zero; and this means going to the adjustment
screws on the seismometer.
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