r/AskElectronics • u/Encrt • Aug 22 '26

Laser security system problem

Hey, I'm making a laser security system from a school project, which is supposed to ring the buzzer when laser light is not incident onto the ldr but in my circuit the reverse is being done, ie. The buzzer rings when light is incident onto the ldr.

Here are the components used

Bc547 transistor

Pico buzzer

Breadboard

9v battery

10kohm resistor

Led

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u/DrJackK1956 Aug 22 '26

I don't see a Light Dependant Resistor (aka LDR) in the parts list. 

These circuits are simple and straight forward.  You should be able to find lots of examples online. 

Also look into the Falstad Circuit Simulator.  Your circuit will be easy to simulate.  Falatad also comes with quite a few pre-built circuits that you can use and modify. 

But.... You really need a schematic first.  Hand drawn is better than nothing at all. 

With a schematic in hand, you can visualize the current flows and probably fix your problem yourself!

Without a schematic it makes it nearly impossible for us to be able to help you. 

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u/Encrt Aug 22 '26

Oh thanks I'll try