Npn Transistor Wiring Diagram. Transistor q 1 has 2 inputs on the emitter side. The circuit diagram of the npn transistor is shown in the figure below.
Most sensors use only 3 wires by having one wire do double duty carrying both power and output signals. When the base voltage of the transistor is zero the transistor will be in cut off region and acts as an open switch. When there is light near the ldr its resistances gets low and the input voltage at base terminal is below 07v which is not enough to turn on the transistor.
To test the polarity of an npn sensor remove the load wire from the input module.
You can see below fig 2 shows the common emitter connection of pnp transistor. The following is a wiring diagram of an open collector npn sensor. The uppermost circuit diagram illustrating the function of the npn transistor shows the load positioned before the transistor. So now that weve gone.