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Subject: log display of seismic signals
From: sean@...........
Date: Fri, 18 Aug 2000 13:45:10 -0500 (CDT)

Re: Log displays to indicate seismic signal intensity:

I have made several log incremented LED displays to monitor the output 
of an STM-8 demo seis at the Univ. It is relatively easy to make on
perforated/solder pads board. It uses the LM3915 IC by National
(they have an LM3914 for a linear display and a LM3116 for a VU
(semi-log) display). They are set up for center scale as in the 
literature, and extended another decade also per the literature.
The set up requires separate + and - 12V supplies to drive all
the LEDs. I use multi-color, 10-segment DIP mounted bargraph LEDs
(from Newark).

The LM3915 is 3db/step, with a 30 db range, and three can be cascaded
for a 90 db display. A simple inverting amp is used to make negative
signals drive the negative indicating display, as in the "precision
null meter" in the literature. The LED outputs of the IC are along 
one side and can connect directly to the LED display chip. I mount them 
vertically, with zero in the center. You can also install linear
LM3914s on one side of center (zero) to show the difference between
a linear and a log response.

With a 60 db log display at the lab, the highest LED has been termed
the "fat" indicator, because it takes a heavyweight person to turn it on.
I haven't figured the magnitude-distance value for it.

Pre-fab kits for VU displays are available (Tech-America) with all
the components, discrete LEDs, and a circuit board. A bipolar display
requires adding the inverting amp, and true log display requires
the LM3915 IC, although the VU scale still makes a nice wide-ranging


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Larry Cochrane <cochrane@..............>