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Speed Sentry (RPM) Transmitter 


factory programming available - $25



in stock

Current Loop output

Works with NPN or PNP prox sensors


                Three wire current loop (4-20 mA) output
  10 - 5000  RPM range
  Low cost speed measurement for PLCs and control systems
  Works with PNP or NPN proximity sensors
  Programmable set 4-20mA output for desired speed range 
  Reverse polarity protected
  3.5mA output for Zero Speed condition (no pulses for over 10 seconds)


The Speed Monitor requires a source of electronic pulses from a sensor. The timing of the pulses is calculated by a microprocessor. The microprocessor takes previously programmed information (such as the number of pulses per revolution) and scales the output to drive a 4-20mA output signal. The Speed Sentry allows the output to be programmed for a range that is significant to the user. For example if a user is only interested in a range of 100-500 RPM, the unit can be scaled so that 4ma = 100RPM and 20mA =500RPM




    The use of an NPN or PNP proximity sensor is a common way to measure rotational speed using gear teeth.
  A laser ranging sensor can be used to detect irregular surfaces such as a bevel gear teeth.
  A hose clamp can be used with a proximity sensor to produce pulses from shaft rotation.
  Many liquid flow sensors produce pulses proportional to the volume of fluid such that a flow rate can be calculated for process control.
  While the major use of the Speed Sentry is rotational (RPM) speed measurement, even a linear source of pulses can be used to provide a measurement system an indication of process speed and rate.
  Weather measurement devices such as an anemometer also often provide a pulse stream that is proportional to speed. 






RANGE 10 to 5000 RPM
POWER +24VDC @ 100mA


Current loop (4-20mA) three wire 

Part Number SPD101

Design considerations

The Sentry Speed Monitor is a low cost way to convert speed and rate information into an analog 4-20mA output for a PLC, process control meter, distributed control system (DCS), or other industrial control system. The SS1 can be mounted through any 1/2" hole and the two wires connected to an analog input card or other two wire 4-20 mA current loop circuit.

The SS1 is designed for low cost applications where standard accuracy is acceptable.

65db was selected for the lower limit to avoid detecting ambient background noise.

Because the SS1 has a PVC housing,  care should be taken to insure that the operating environment does not get so hot that the PVC housing will soften. This may happen at temperatures around 180 to 190 degrees F.

The SS1 should not be used as the sole sensing element in critical equipment or safety applications. 



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