GE Scout KPH External Trigger Prototype

The Scout 100 and 140 can provide multiple Manual KPH (Keyphasor) Thresholds.  These are nominally configured as: +3V, -3V, -8V, -13V, and -18V.  It is possible that a valid small amplitude KPH signal within these limits will not cause the Scout to trigger.

The purpose of this device is to convert a KPH signal to a signal that is compatible with the 3V manual threshold of the Scout 100 or 140.  This device will be provided as a signal conditioner in-line with a Tachometer to KPH cable.  This circuit is powered directly off of the Scout’s five volt power rail.  It accepts a -24V Keyphasor as an input signal either directly from a Proximitor or from the monitor’s buffered transducer output.  The output signal is 0-5 volt pulse that can be used to drive the Scout input directly.

The prototype was built using a two layer, bare copper PWB fabricated with a CNC PWB router.

Features & Design Techniques

Converts a negative voltage pulse signal with unknown bias to a positive voltage, TTL compatible signal

  • Low current device with a single +5Vdc source available
  • Uses opto-isolator to setup a trigger/threshold and convert negative voltage to positive
  • Uses Zener diodes to set threshold level

Hysteresis was implemented via hardware

  • Implemented via a feedback loop on the opto-isolator
  • Logic chip drives a CMOS switch to bypass or engage an additional diode which shifts the threshold level

Diode resistance was modeled to determine effects on frequency response.

Circuit Block Diagram


Amplitude0 to -24 V
Minimum trigger amplitude1.5 Vpp
Minimum Pulse Width100 µS
Minimum slew rate6 V/µs
Duty cycle≥ 50%
Input Impedance20 kΩ ± 10%

Responsibilities & Scope

  • Circuit design and schematic capture in Altium
  • Altium Designer PWB layout
  • Tolerance analysis
  • Assembly and testing
  • Entire project from start to finish

Project Photos




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