When reed switch is opened, pin 2 of Arduino goes high with the help of the pull-up resistor R2, and the alert signal is available through the piezo-speaker PZ1 (not an active piezo-buzzer). At the ...

If our example resistor ended in gold (orange, white, black, gold), its 39 Ohms value could be five percent higher or lower. For a possible minimum of 37.05 or a maximum of 40.95.

The security flaws on this common hotel keycard lock are nothing short of face-palmingly stupid. Look closely at the picture above. This is a hotel room door swinging open. The device he holds in h… ...