If a N.O. reed switch is used, then the non-inverting input (+) of the op-amp wilt be placed at a logical HIGH level, that is, about +12 V with the configuration is shown. This voltage [eve[ is reflected as a HIGH at the output V,. But when the reed switch closes, the noninverting input is grounded, so the op-amp sees an input voltage of zero. This LOW condition is reflected as a LOW output.