Project 6: polarity tester.
Check the polarity of a battery
Required parts: 9V battery*, 1K resistor (brown black red gold), red LED, green LED, wirejumper
How it works: When the 9V battery is connected to the circuit with the right polarity the green LED (good) will light up. Current can flow from the "+" of the battery through the green LED and via the resistor back to the "-" of the battery. The red LED (wrong) will not light up as it is polarised in the opposite direction. When swapping the connection of the battery (switch the red and the black wires) the red LED will light up. This way we can détermine whether a battery is connected the right way or not.