  Optically detected magnetic resonance (ODMR) spectroscopy of defect-rich
semiconductors is being increasingly exploited for realising a variety of
practical quantum sensing devices. A prime example is the on-going development
of compact magnetometers based on the nitrogen-vacancy (NV) defect in diamond
for the remote sensing of magnetic signals with high accuracy and sensitivity.
In these applications, the ODMR-active material is integrated into the overall
apparatus to form a self-contained sensor. However, some emerging applications
require the sensing material to be in physical contact with an external object
of interest, thus requiring an independent readout device. Here we present an
ODMR-meter, a compact device specially designed to allow convenient,
contactless monitoring of ODMR in a target object, and demonstrate its
application to temperature monitoring with NV defects. Our prototype is
composed of a handheld readout head (integrating all the necessary optical
components and a microwave antenna) and a control box connected to a laptop
computer, all made primarily from commercial off-the-shelf components. We test
our device using an NV-rich bulk diamond as the object, demonstrate a
temperature sensitivity of $10~{\rm mK}/\sqrt{\rm Hz}$ in static conditions,
and demonstrate the feasibility of handheld operation. The limitations to
measurement speed, sensitivity and accuracy are discussed. The presented device
may find immediate use in medical and industrial applications where accurate
thermometry is required, and can be extended to magnetic field measurements.
