(Note: this write up is taken verbatim from T. Ricketts' original HIRES manual)

When driving the individual stages manually, some means must be provided for controlling the speed of the motor. This card is mounted in the small control box located on each stage, and produces a pulse width modulated signal for the manual control buttons. The width of the output pulse may be adjusted by means of a pot to set the speed of the stage to any desired level. Adjusting the speed in this way provides more torque for the motor then would normally be available if the voltage was just turned down.

On the left side of the drawing is a 555 timer (G1) with a pot (R2) for adjusting the width of the pulse. The timer produces a frequency of about 2KHz. The pulses are buffered by the FET (Q1) and level shifted by the FET (Q2) to provide a drive signal for the output switch (Q3). The output switch will provide the full +28V to the motor when turned on and is capable of providing all the current needed by the motor. The length of time that the switch is on will determine the speed of the motor. The output of the switch has a resistor to provide a return to ground (R7) and a diode (D1) to prevent inductive spikes from the motors from driving this signal below ground.

In the case of the ADC instrument, the circuit performs the same. The differences are that the board lives on the Interlock Panel instead of a box at the stage and that the ADC uses +24V instead of +28V.

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