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How Does an Open Collector Output Work?

An open collector output is a solid state switching circuit used with many Gems level sensors. The following level sensors use the open collector type output: 


The open collector works with DC power and basically toggles the current flow from the output signal wire to ground. In the symbol below, the sensor signal is the base leg, the positive DC power source (also powering the load in this sinking NPN configuration) is the collector leg, and the Emitter is the Ground (or negative) leg. 

When the internal sensing signal is positive (liquid present), it allows current to flow from the collector to the Emitter: 


When the internal sensing signal is negative (liquid absent), it prevents current from flowing to the Emitter: 



Some Gems flow sensors also have open collector outputs. These include the following Turbine Flow Sensor models: 

All of these flow sensors use a frequency output. The open collector output has fast and accurate on-to-off switching capability that produces a square sign wave. Flow rate is measured in hertz (Hz) which is the amount of these on/off pulses within a given time frame. 

A pull-up resistor is required for the pulsed frequency outputs on Gems Turbine Flow Sensors.