How do I add a circuit to disable this system during the night?
Since Q2 has been changed into a NPN, I am less confident about this, but theoretically this should work.







simulate this circuit &ndash; Schematic created using CircuitLab

NPN Q1 should, when Pin 2/3 are high and the led is lit (Meaning Pin 7 is high/floating and the Solar Cell is not producing voltage), be turned on, creating a small short circuit (limited by R6). This should bring Q1's collector and Q2's base close to ground potential, which should deprive Q2 of current, preventing it from turning on.