The Tachometer Output.

The tachometer on MotoGuzzi 1100i is an electronic unit that requires a 10V square wave signal to drive it. The frequency of this square wave determines the reading the tachometer will give. The ECU determines required frequency from the engine speed.

The following circuit is used to drive tachometer. The microcontroller directly drives the BC549 transistor. The resistors on the collector of the transistor are to give a 10V signal. This is what was observed from the Weber Marelli unit and it is not known if it is a requirement.