Test Your Reactions - Raspberry Pi Hardware Projects - page 16

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In describing circuits, there are a few terms that you may come across:
❍ Voltage - this is, in electrical terms, how “strongly” the electrons are pulled
home - that is, how much work they can do while they flow through the circuit.
Think of it a bit like a water wheel and a reservoir. The greater the distance the
water falls, the more work it can do turning the water wheel as it flows past.
Voltage describes the work that can be done and is measured in volts - for exam-
ple, an AA battery has a voltage of 1.5V between its terminals. If another one is
connected end to end, then there is greater potential to do work, a voltage of 3V.
❍ Ground, or 0V (or sometimes referred to as negative ) - a reference point to
measure voltage from. If a point in a circuit is at ground, then it is at 0V, and no
work can be done. With the water example, if the water is on the ground, it
can't fall any further so can't be harnessed to do any work.
❍ Resistance - how easily the electrons can flow. Different substances allow
electrons to flow with different degrees of ease. Conductors, such as metals,
have a low resistance and make it easy for the electrons to low. Insulators,
such as plastics, have high resistances, which make it hard for electrons to
flow. Different materials resist the flow of electrons by different amounts. Even
water has a fairly low resistance, so it will allow electricity to low through it,
which is why you shouldn't use your Raspberry Pi in the bath!
You can think of these like this: Voltage describes the potential to do work (analogous to the
height of water), and current describes the rate electricity follows (the rate of flow of water
passing a point). Resistance describes how easily electricity flows through a material - volt-
age, resistance and current are interrelated - without a voltage existing between two points,
no current will flow. The resistance between the two points affects how much current will
flow. If you want to know more, look up Ohm's Law online.
Outputs
As well as controlling the on-board LEDs and relays, PiFace Digital has “open-collector” out-
puts that can be used to control circuits. You can connect to these outputs with the screw
terminals next to the LEDs.
Open-collector outputs describe the circuit that drives the output and offers a number of advan-
tages. he term open collector describes how the output transistor is connected. Transistors are the
switches at the heart of computers - there are tens of millions of transistors in the processor at
the heart of the Raspberry Pi. Luckily they're only tens of nanometres (a nanometre is a thousand
millionth of a metre - you could fit 2000 transistors across the width of a human hair) in size.
Although transistors behave in a similar way to switches and relays, the direction current flows
through them affects how they behave, which needs to be considered when connecting to them.