PSN-L Email List Message

Subject: Thanks for tiltmeter help/info
From: George Bush ke6pxp@.......
Date: Sun, 13 Mar 2005 12:56:32 -0800

	I was not planning on mentioning the tiltmeter to the list until I got
some construction pictures on my website. But when John Popelish requested
additions to his list to study I submitted the INA121 used on the tiltmeter
to add to the list for for evaluation. Thank all of the rest of you for
your comments.

	John requested more information on how the INA121 was used in the
tiltmeter, so I sent a schematic and the two references. Thank you who
offered info on getting these references. I have the original pages I tore
out of the Scientific American back when I subscribed, and same for the
Nuts n Volts article. I also have the Ametuer Scientist CD and have used it
to "mine" many articles on seismometers and wierd pendulums. I would
recommend it as a resource to anyone interested in constructing seismometers.

Chris Chapman wrote:

>Hi George,
>    The circuit in seems
to be flawed. It appears to depend on the reverse leakage current of the
rectifier diodes to provide the DC level stability and this is both
variable and highly temperature dependant. A resistor needs to be connected
across both capacitors to provide a fixed impedance to earth. You do need a
true differential opamp for this type of circuit to get the very high CMRR
>    The inherent weakness of this type of circuit is that the properties
of the tuned circuits and the diodes are temperature sensitive and this
limits both the accuracy and the drift. The temperature drift on a Si diode
is about 2.5 milli V /C Deg. I note that there is no low pass filter at the
amplifier output! This will allow greatly increased output noise. An INA118
amplifier could give better performance. This type of circuit was used on
seismometers for a short period during the 1970s before being replaced by
fet switch technology.
	Chris, I appreciate your comments about the toxicity of mercury and I will
be taking appropriate steps to minimize exposure (instrument will be
located in the garage, ventalated and away from human habitation). 

	I also appreciate your comments about the circuit as I have not been able
to get it to work. I have experienced high frequency "ringing" on the two
cables from the antennas to the detectors, very high output noise (.5v.),
grounding problems, noise on power busses, sensitivity to light (I suspect
the diodes), and capacitive sensitivity if I bring a hand near the cables
that connect to the cup antennas. I was planning on changing the topology
of the circuit and separating the osc into a metal box, same for the amp,
and put the detectors into the antennas on top of the cups. I may be better
off in investigating another circuit. I will now start to look for circuit
information on capacitance-based seismometer position sensors.

	I am delighted to see the tiltmeter interest and will report my progress
to the list.

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