Ive made up a laser security sytem that uses a pnp and npn transistor so that even when the laser is tripped the LED will stay on . Everything is going well except for the LDR. When there is no light the led will stay off. And once there is light it will turn on. How do i reverse it? here is my diagram. Please help.

 

Use an inverter.

 

Use a transistor to reverse the output's phase.

 

Your question is missing a LOT of details. Also, there is no LDR or laser in your schematic. What is the function of the switch? Also, what does the circuit do? Does it sound an alarm or some other kind of indicator that the system has been tripped? What is it you are trying to do?

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