  As online trigger for events containing $\Lambda$ hyperons in p+p collisions
at $\mathrm{3.1\,GeV}$ a silicon-based device has been designed and built. This
system has been integrated close to the target region within the FOPI
spectrometer at GSI and was also employed as a tracking device to improve the
vertex reconstruction of secondary decays. The design of the detector
components, read-out, the trigger capability as well as the tracking
performance are presented. An enrichment factor of about 14 was achieved for
events containing a $\Lambda$-hyperon candidate.
