variable resistor tied to base of npn transistor

Hello. I am working on a circuit that is based off of the circuit on the following website: http://www.electroschematics.com/6899/12v-ldo-solar-charge-control/ . Near the bottom right of the circuit on the link, there is a variable resistor, the arrow of which comes from the base of a npn BJT. When trying to model this in systemvision, I cannot connect the BJT base to the variable resistor; it says it cannot connect different technologies and to add a converter block. When I add a rotational converter, it gives me the same message. I need the voltage across the resistor controlled per the link. The design I'm working on is called 'Charge Controller'.