An inexpensive, low power, disposable instrument for cloud droplet detection is described for meteorological balloon applications. It utilises the optical reflection method, implemented by a low power, high brightness LED as the optical source and semiconductor photodiode as the detector. Laboratory tests of the cloud droplet sensor show a strong response to pulses of ultrasonically generated cloud droplets. During a test flight through a layer of stratocumulus cloud, during night time conditions, the instrument showed a clear response to the base of the cloud, providing additional detail about the fine structure of cloud edges, from that currently available from conventional radiosonde measurements.