A supervisory control and self-powered indicating system is provided for controlling the operation of a remote electrically operated device from a control station and for indicating at the control station either the operability of the remote device or the absence of an alarm condition, in which a pair of face-to-face diodes are connected in series across the transmission lines at the remote location, with the mid-point between the diodes connected through a third diode and through a condition responsive switch to earth ground. The return connection through earth ground at the control station is made through a relay coil and back-to-back diodes connected in series across the transmission lines at the control station. Telemetry is provided through the opening and closing of the condition responsive switch, with the time that this switch remains in a given condition being monitored by the length of time the switch associated with the relay coil at the control station remains in this given condition. Alternatively, the condition responsive switch changes state in response to a predetermined alarm condition which is indicated by a change of state of the switch associated with the relay at the control station.

A medium to high viscosity liquid flow detector and alarm sub-system for use with apparatus for automatic liquid dispensing machines for mastics, sealants, adhesives, and the like, comprising a liquid pressure sensing device disposed in the liquid flow path of the liquid dispensing gun of the machine. The liquid pressure sensing device derives an output electrical signal indicative of the instantaneous liquid pressure in the liquid flow path. A differentiating circuit differentiates the liquid pressure indicating signal to derive a rate of change in fluid pressure signal. The rate-of-change signal is supplied to an indicator for signalling the existence of an undesired rate-of-change in liquid pressure condition and is indicative of a bubble or void in the liquid mastic, sealant, or the like being dispensed. The rate-of-change signal is supplied to a first comparator which also has supplied thereto an input bubble size reference signal that establishes a predetermined size bubble or void below which no alarm signal will be produced. Upon the occurrence of a bubble or void which is greater than that established by the adjustable bubble size reference signal, an output alarm signal is produced.