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I'm trying to build a simple burglar alarm circuit and I've found one online that uses a knife switch, 9 V power source and an SCR. I'm trying to change it over to a PIR motion sensor but am unsure whether the mechanics are the same. Attached is a diagram of the original diagram and the one I'm attempting to make with the PIR below it.

If I'm using the 9 V power source and the same SCR that's recommended in the first diagram, would it be reasonable to assume that the 10k resistor is still needed on the output of the PIR? I know very little about SCRs but I'm guessing it would still be needed because the SCR gate can't be activated with the full 9 V from the power source? (too much?)

I would then just have a kill switch somewhere on the positive side that we could activate while we are in the vicinity then just reverse it so the system is live when we leave (not in diagrams)

Any advice or thoughts would be helpful as I only know enough to be dangerous and not nearly enough to make any of it work :)

enter image description here

Null
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cwhitehouse
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  • You show a loudspeaker that will go "click" when the SCR conducts then nothing turns it off. Don't you want a power siren instead? – Audioguru Dec 09 '21 at 00:12
  • It depends on the PIR. Some have a relay output, which would replace the switch. Some don't. – Simon B Dec 09 '21 at 09:07
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    My suggestion, forget about these overly simple alarm circuits and study some hobby-friendly alarm circuits from yesteryear. A great example is #56 from the Elektor bumper annual of hobby circuits (https://worldradiohistory.com/UK/Elektor/80s/Elektor-301Circuits-1-to-78.pdf). You should be able to simulate them in falstad (https://falstad.com/circuit/). But, before you do any of this I recommend you get a piece of paper and draw a block diagram (no electronics required) of how you think the alarm should operate. – Buck8pe Dec 09 '21 at 09:22

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