Abstract: There is described an electro-optical monitoring switch circuit design typically used for fire protection, sprinkler system valves. It comprises a light generating device responsive to a repetitive, pulse signal. Light emitting from the light generating device is directed towards the valve stem portion of the valve. Light emitted by the light generating device is reflected off of the valve stem of the valve in a first position and received by an electro-optical device such as a phototransistor. If the valve stem is moved to close the valve, there is no reflected light. Assuming the first position condition, the phototransistor produces an electrical signal substantially identical to the light generating signal. A phase comparator circuit monitors the output of the phototransistor, comparing it to the signal which excites the light generating device. When the two are synchronous, the comparator circuit produces a similar signal at its output. This obviates false indications due to noise.