MSD Trigger Pickups: Magnetic vs. Optical Tech Bulletin - Manual (Page 2)

AUTOTRONIC CONTROLS CORPORATION • 1490 Henry Brennan Dr., El Paso, Texas 79936 • (915) 857-5200 Fax (915) 857-3344 • Website: www.msdignition.com
FRM20987 Revised 08/99 PRINTED IN U.S.A.
An optical trigger is very susceptible to debris or contaminants that may enter the distributor cap. A layer of dust, moisture or
grime on the lens of the LED or receiver will affect the signal strength resulting in the possibility of misfire or timing fluctuations.
If the rise/fall (On/Off) time of the signal voltage changes there will be a shift in the reference trigger point resulting in a timing
shift. Also, the LED’s light output degrades over time, especially in a high rpm environment with higher temperatures. A lower
LED output may affect the saturation time of the photo receptor causing a weak output signal.
These reasons alone express the strengths of a mag pickup’s reliability and endurance in racing conditions. There are also
facts to consider concerning the signals that they create.
Signal Strength and Flexibility
By design, a magnetic pickup creates a positive and negative trigger pattern. As the reluctor paddle nears the magnet, a
negative voltage is created. As it lines up with the magnetic pickup the signal begins swinging up and becomes a positive signal
until the reluctor paddle passes through the field. When the signal becomes a positive voltage, it creates an easily recognizable
point, the Zero Crossing. This point is extremely important because it is always the same trigger point.
As rpm increases, the signal that the mag pickup produces becomes stronger, however this Crossing is always constant in
regards to distributor shaft position. With the predictability of this Zero Crossing , a mag pickup’s inherent retard can be taken
into consideration and adjusted within the ignition. Also, due to the very fast rise and fall time of the trigger signal it is much less
susceptible to electronic noise which can affect triggering.
MAGNETIC PICKUP TRIGGER SIGNAL
Conversely, an optical trigger creates only a positive signal so a circuit must be added to set a fixed reference point for
triggering. Usually this reference point is about half the amplitude of the optical signal. If the saturation time of the receptor
changes due to a weak LED, debris or heat, this reference point is going to be affected which will cause a shift in the timing.
As the window of the interrupter begins to enter the beam of the LED a signal is created. During this process, the receiving
sensor begins to be uncovered and starts to turn “on”. Next, it reaches its maximum (saturation) point, then begins to be
covered followed by being completely turned off.
OPTICAL TRIGGER SIGNAL
MSD’s Magnetic Pickups have proven their endurance and precision on race tracks around the world. Don’t settle for an
unproven design - use the name and product you trust; MSD Ignition. If it wasn’t the best, MSD wouldn’t use them.