This email was taken from a recent email interaction with our technical department:

Question:

    I was trying to select some prox switches for a home project (PR08-***).  I have worked with these in industry as a maintenance electrician but this is the first time I've designed a project from scratch at home.  I don't understand the terms "sinking" vs "sourcing". 

    I'm using these essentially to turn on a light when a cabinet door is opened.  I don't want any moving parts.  These will trigger a SSR which will close the circuit for a small set of incandescent lights.  I want the lights to work as if by magic such that even under close inspection the layman will not understand how they work. 

    Please explain, "sinking" vs "sourcing" and I will order the appropriate items.

Answer:

Sinking and sourcing refer to the type of digital inputs and outputs used. A sinking digital I/O (input/output) provides a ground. A sourcing digital I/O provides a voltage source.

Consider a simple circuit that consists of one digital input connected to a digital output. The circuit needs a voltage source, a ground, and a load. A sourcing digital I/O provides the voltage needed for the circuit. A sinking digital I/O provides the ground needed in the circuit. The digital input provides the load required for the circuit to work.

Figure 1 below shows a sinking digital output that is connected to a sourcing digital input. In this circuit, the sourcing digital input provides the voltage and the load. The sinking digital output controls the line by using a transistor to leave the line high (at +V) or to ground the line to 0 V.

Figure 2 shows a sourcing digital output that is connected to a sinking digital input. In this circuit, the sourcing digital output provides the voltage and the sinking digital input provides the load and the ground. The digital output controls the line by using a transistor to leave the line at 0 V or to raise the line to +V.

Because you need both a voltage source and a ground in order to create a complete circuit, you need to have a sourcing input or output connected to a sinking output or input. If you wish to connect a sourcing input to a sourcing output or a sinking input to a sinking output, you will need to add an additional resistor.