MSefi.com read only forum (Jan.15,2018)

My first post here, I'm in learning mode, this seemed like the most appropriate thread.

I found this circuit (on-line) with a reference to the MegaSquirt project. I'm not sure which version of MegaSquirt this circuit was used in, as the posted schematics appear slightly different.
My question relates to the 1n4733A zener.

During the positive portion of the input signal, the diode within the opto will limit input voltage to about 1V
During the negative portion of the input signal, the 1n4003 diode will limit input voltage to about -1V

It seems like the zener has no purpose, can someone explain?

I also noticed that this version of the circuit (http://www.msextra.com/doc/general/ms1v3schems.html) has provision for either an ignition coil signal or a VR signal, however the opto based circuit (similar to above circuit) appears to be for the ignition coil signal not the VR signal. Perhaps the first opto based circuit posted above was used for either signal in an earlier version?

Jester,

This circuit isn't used on any of the MegaSquirt main boards, so it's hard to comment. Perry hasn't posted here for years. You can try the email pedward@dainst.com (from the schematic), or you might try Googling his name ("Perry Harrington" from the schematic), to see if you can contact him and ask him about the circuit.

Lance.

An LED will be destroyed by too much current flow through it. Since a VR sensor produces a varying voltage output with speed - as well as fequency - the zener will clip the voltage across the LED to the safe level.