Logic Probe

As part of the electrical course, we were to construct a logic probe. After constructing it, we were to answer a set of questions.

Q1: Why do both the red LED and the green LED go when you connect it to the battery?

A: The current flows through the green LED, through the brass rod, through the red LED and back to negative to complete the circuit.

Q2: Why does the green LED go out when the probe contacts the battery positive?

A: The green LED goes out because when the probe touches the positive, that makes the brass rod positve and the green LED shorts out. This is because the current coming from the positive alligator clip meets the (now) positive brass rod and has no where to earth.

Q3: Why does the red LED get brighter when the probe contacts the battery positive?

A: The red LED gets brighter because all of the current flows through the one LED and it gets all of the available voltage. The current flows from positive, through the probe, through the red LED and back to negative, not through the green LED.