My alarm system circuit consists of:

  • BC-547 transistor
  • 2.2 kΩ resistor
  • An LDR
  • 3.7 V lithium-ion battery connected with a switch
  • LED light
  • Buzzer
  • Laser light

The resistor is attached to the collector and base of the transistor. One wire of the LDR to the base of the transistor Then the positive wires of the LED and buzzer are attached to the emitter of the BC-547 transistor and the negative wires to the second wire of the LDR then finally the positive wire of the 3.7 V battery is attached to the point where both the resistor and transistor join and the negative wire to the second wire of the transistor where the buzzer is connected.

enter image description here

It works, but the problem is that when the light from the laser falls on the LDR, the buzzer just makes a bit feeble noise but doesn't completely stop.

I have already spent a lot on this project (while creating the other crafts.) Please tell me a cheap and easy way to make it work.

  • \$\begingroup\$ Your third photo is an attempt at a schematic. Please click on edit and the schematic symbol. A schematic editor will open and you can draw a clear and concise schematic with values on all components. Before we have that, nobody can help you. \$\endgroup\$
    – winny
    Jun 5, 2023 at 13:20
  • 1
    \$\begingroup\$ The schematic editor does not support touch devices. And I don't have a PC. \$\endgroup\$
    – Shaurya
    Jun 5, 2023 at 13:51
  • \$\begingroup\$ Replace the LDR with a photo transistor. This will draw more current to bring the base voltage down. \$\endgroup\$
    – Jens
    Jun 7, 2023 at 17:49

1 Answer 1


Make sure you've connected the 3 leads of the transistor the right way round. Different transistors have different pinouts, and one connected wrongly can still sort-of work.

LDRs have quite a large area to collect light, and lasers produce a small spot of light. It may be that you aren't illuminating enough of the LDR to make its resistance drop enough.

Try changing the resistor. If it's too low resistance, then the LDR will never be able to pull the base of teh transistor close enough to 0V.

  • \$\begingroup\$ Thanks sir. My resistor is a 2.2kΩ one. Which one should I try changing to? Should I try a 4.7kΩ one? \$\endgroup\$
    – Shaurya
    Jun 5, 2023 at 16:19
  • \$\begingroup\$ @Shaurya resistors are cheap. Keep trying different ones to see what works best. \$\endgroup\$
    – Simon B
    Jun 5, 2023 at 20:22
  • \$\begingroup\$ Okay sir. Thanks a lot! \$\endgroup\$
    – Shaurya
    Jun 6, 2023 at 3:24

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