The Model 685 is a supervisory detector module. The module is designed to detect the presence of a preselected frequency. The audio input section of the Model 685 is balanced and isolated, using a high impedance audio transformer. This allows the unit to detect across a wide variety of audio circuits, without loading or unbalancing the lines. There are adjustments available to tune the detector's center frequency the threshold of detection, and the relay dropout delay. The center frequency adjustment allows the unit to be trimmed for operation with a particular oscillator (Models 546B & 562B). The threshold adjustment can be used to set the card for a minimum signal level. If the level falls below the desired setting, the detector will trigger a change in the onboard relay closures. The adjustable delay prevents the unit from triggering during switching functions. The default setting of the Model 685 is tone present = relay activated.