Original Submission by T:
The simplest example of how this circuit generates Spark Advance is the pushing of a wedge type doorstop under a door.
The higher the door is from the floor, the longer it will take for the wedge to contact the door and begin to take effect.
The lower the door is from the floor, the sooner the wedge contacts the door and begins to take effect.
Auto-Retard works the same way.
The Wedge is electronically made by a circuit called a Sawtooth Generator.
The height of the door is determined by the Sensor inputs from the Engine.
When the Engine Speed is low, the door height is made highest.
When the Engine Speed is high, the door height is made the lowest.
As a consequence it takes less time for the coil to be triggered when the Engine Speed is highest.
A Comparator is an electronic device that produces output when its input leads are equal.
When the A and B voltages at the Output Comparator are equal, the Comparator activates the Ignition Module. The Ignition Module pulses the Coil and the Spark is directed to the appropriate Cylinder by the Rotor Button inside the Distributor.
The Magnetic Trigger, mounted at the back of the Engine, produces a Pulse every time one of three equally spaced Flywheel Mounted magnets passes it. The pulse occurs well in advance of the Piston's TDC position.
The Sawtooth Voltage, named from its shape and fed to the Output Comparator's A input, increases in frequency with Engine Speed but has its maximum amplitude and slope held constant.
The B Voltage will rise with the increase in Engine Speed but will also have its level determined by Engine Sensor voltages. The B voltage determines the delay by matching a point on the Sawtooth waveform A.
The higher the B Voltage (higher RPM) the shorter the delay till the Comparator activates the Ignition Module because the Output Comparator has to wait less time until the falling sawtooth voltage crosses the B voltage higher up.
The lower the B Voltage (lower RPM) the longer the delay till the Comparator activates the Ignition Module because the Output Comparator has to wait more time until the falling sawtooth voltage crosses the B voltage lower down.