Hardware Reference
In-Depth Information
There are different types of transistors, which allow current to flow in different ways. For
simplicity this chapter just uses examples of the type NPN.
TIP
Open-collector outputs can just sink current. That is, they allow current to flow to ground;
they are not a source for current. This means that circuits have to be wired up from a power
source, through the component being controlled, through the transistor and then to ground.
Figure 1-7 shows a typical setup. Remember, current has to flow for electricity to do work, so
until the transistor turns on and allows current to flow to ground the LED will not come on.
The transistors on PiFace already have the connection to ground wired up.
Figure 1-7:
How to wire up
an open-
collector output.
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