@▷ Alarm when the supply to rack with 1N4001 | Diagram for Schematic

Alarm when the supply to rack with 1N4001

To use such circuits.If you have identified the polarity of the power supply or power supply connector on the wafer is not clear.The wrong polarity can cause damage and range.But when using this circuit with active circuit, it can be Protect And show a correct polarity.
Operation of the circuit is not complicated. When the supply terminals to make LED1, D1 has been bias direct. It makes LED1 light, and The piezo speaker PZ1 emit noise.The lamp LED1,it would be R1, assist current limiting and voltage.To prevent LED1, no damage.But if the supply polarity is LED1, D1 will be actuated bias. D1 is not bias the LED1 is not lit.And piezo speaker PZ1 not emit noise. The D2 works, The D2 works, because it was direct bias.Make a power supply circuit as usual.

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protection circuit