Magnetic Proximity Switches
The reed switch is connected in series with a light emitting diode (LED) and a current limiting resistor R1. When the switch closes, the DC power supply V1 is connected to the LED/R1 circuit, causing the LED to light up. The lighted LED indicates that the object being detected is present and in position.
Simplest circuit
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.
Operational electronic form