A light detector circuit using an ideal photo-diode is shown in the figure. The sensitivity of the photo-diode is $0.5\:\mu A/\mu W$. With $V_r=6\:V$, the output voltage $V_\circ=-1.0V$ for $10 \mu W$ of incident light. If $V_r$ is changed to $3\:V$, keeping all other parameters the same, the value of $V_\circ$ in volts is ____________ $V$.
