Transistors are the electrical companion to the relay. They take the function of a switch which only activates when current is applied to the base wire. The first advanced project for the transistor is detailed in schematic below. The current applied to the base wire is variable based on the setting of the potentiometer (P1). If the resistance is high, the current to the base is low, and the transistor passes a low current. If the resistance is low, the current is high, and the transistor passes a high current.
Points A1 and A2 are measurement points for the current in the circuit. My multimeter has a broken fuse, so I was unable to measure the current in my circuit, but I was able to measure voltage changes across the potentiometer and transistor. A picture of my version of the circuit is detailed below.
The logic probe was the major project for the day. It is the first project which is intended to be a permanent addition to the electrical tool kit, and is assembled from parts which cost less than $5.
The basic model, with one LED, has a default condition of a dim LED when it is connected to power and ground. When it probes a high voltage location the light gets brighter, when it probes a ground location it turns off. 
The more advanced model contains 2 LEDs, and has a default condition of a dim green LED. When power is probed, the red LED lights, when ground is probed, the green LED gets brighter.
Points A1 and A2 are measurement points for the current in the circuit. My multimeter has a broken fuse, so I was unable to measure the current in my circuit, but I was able to measure voltage changes across the potentiometer and transistor. A picture of my version of the circuit is detailed below.
The logic probe was the major project for the day. It is the first project which is intended to be a permanent addition to the electrical tool kit, and is assembled from parts which cost less than $5.
The basic model, with one LED, has a default condition of a dim LED when it is connected to power and ground. When it probes a high voltage location the light gets brighter, when it probes a ground location it turns off. 
The more advanced model contains 2 LEDs, and has a default condition of a dim green LED. When power is probed, the red LED lights, when ground is probed, the green LED gets brighter.
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