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Im begginer and currently Im trying to make DIY laser alarm (while laser is pointing at LDR sirene should remain silent and when laser is turned off, sirene should make sound). I found online following schema

enter image description here

As sirene Im using KPE212A for transistor I have bc337-40 and LDR is VT43N1

Problem is that instead of sirene going on/off based on laser pointer, I get only change in pitch of sound (Sirene is constantly on, but only increases its volume while laser is pointing at LDR). Could you please advice what is wrong and how I could fix it ?

W5VO
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Pavel Hornak
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1 Answers1

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The resistance of the LDR does not drop low enough to turn off the buzzer.

So you can try something like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Spehro Pefhany
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  • Thanks :-) I will try that, could you please explain a bit why do I need D1 and why are there two transistors ? – Pavel Hornak Jul 29 '18 at 22:23
  • The diode adds a bit more voltage drop, it's possible it works without D1&R2, you can try that first. The two transistors add some voltage drop and allow R1 to be several times higher and still drive the buzzer okay (because of additional gain). The two transistors are a Darlington pair. – Spehro Pefhany Jul 29 '18 at 22:33
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    BTW, I don't think the laser is ideal on an LDR. If the spot does not illuminate the width of the element it won't got down in resistance as much. Diffusing the spot may help too. – Spehro Pefhany Jul 29 '18 at 22:34