  We present the design and measured performance of a light emitting diode
(LED) module for spatially mapping kinetic inductance detector (KID) arrays in
the laboratory. Our novel approach uses a multiplexing scheme that only
requires seven wires to control 480 red LEDs, and the number of LEDs can be
scaled up without adding any additional wires. This multiplexing approach
relies on active surface mount components that can operate at cryogenic
temperatures down to 10 K. Cryogenic tests in liquid nitrogen and inside our
cryostat demonstrate that the multiplexer circuit works at 77 and 10 K,
respectively. The LED module presented here is tailored for our millimeter-wave
detector modules, but the approach could be adapted for use with other
KID-based detector systems.
