  Gravimeters are devices which measure changes in the value of the
gravitational acceleration, \textit{g}. This information is used to infer
changes in density under the ground allowing the detection of subsurface voids;
mineral, oil and gas reserves; and even the detection of the precursors of
volcanic eruptions. A micro-electro mechanical system (MEMS) gravimeter has
been fabricated completely in silicon allowing the possibility of cost
e-effective, lightweight and small gravimeters. To obtain a measurement of
gravity, a highly stable displacement measurement of the MEMS is required. This
requires the development of a portable electronics system that has a
displacement sensitivity of $\leq 2.5$ nm over a period of a day or more. The
portable electronics system presented here has a displacement sensitivity $\leq
10$ nm$/\sqrt{\textrm{Hz}}$ ($\leq 0.6$ nm at $1000$ s). The battery power
system used a modulated LED for measurements and required temperature control
of the system to $\pm$ 2 mK, monitoring of the tilt to $\pm$ 2 $\mu$radians,
the storage of measured data and the transmission of the data to an external
server.
