- November
Posted By : Isaac Mazel
Comments Off on GE Scout KPH External Trigger Prototype
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

Circuitry block diagram


Amplitude 0 to -24 V
Minimum trigger amplitude 1.5 Vpp
Minimum Pulse Width 100 µS
Minimum slew rate 6 V/µs
Input Notch
Duty cycle ≥ 50%
Input Impedance 20 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

Altium Designer PWB Layout
External Trigger Prototype. Shown are indicator LEDs showing if triggering is taking place or which direction the threshold voltage is off. A threshold level selection knob is included as well as a bypass switch.