Now, when I press the tremolo effect pedal, it goes on; than I have to press the pedal again to turn it off.
I want that it only triggers the tremolo effect for as long as I press down the pedal.
**broken link removed**
The lower right two transistors are a bistable, this is what latches and unlatches.
As it switches two different FET's you would need to remove the bistable and replace it by a single transistor inverter, to provide the different signals.