Ok, let me get this straight;

normally the trigger is low (off) and the SMBT... does not conduct, making the gate -ve via the ground link.
then when trigger'd the SMBT conducts and the makes the gate +ve which stops the flow through the MOSFET.

is that correct?

i am trying to modify this arrangement so that the MOSFET is off when the trigger is off, then when there is a +5v signal the mosfet is turned on. but am encountering problems (probably in my choice of Mosfet (STD30PF03L) and transistor (FMMT491TA)

any help would be great,

Cheers,
David

David,

The MJLJ rev-limiter output is as follows:

Below rev limit: high impedance (open circuit)
At or above rev limit: connected to ground

The SMBT2907A is there to invert the output so the P-Channel mosfet is on when the rev limit output is de-activated.

To have the mosfet on when the rev-limit output is enabled would involve eliminating the SMTB2907A and having the rev limit output drive the mosfet gate directly. Of course, you would need a pull-up resistor on the gate when the rev limit output is inactive.

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Brent Picasso
Founder, Autosport Labs