This article was first published in 2008.
Last week in
How to Electronically Modify Your
Car, Part 1 we backgrounded how DIY electronic modification of cars can result
in brilliantly effective and often amazingly cheap modification. This week we
start on the knowledge that you need to have to be able to perform your own electronic modifications.
All electrical systems that we will deal with in
cars comprise circuits. Rather than attempt to define an electrical circuit,
it’s easiest to show one.
Here’s a 12V car battery. It has two terminals,
one marked positive (+) and the other negative (-).
If we connect a light bulb across the charged
battery, power can flow from the positive terminal, through the light bulb
filament and then back to the battery negative terminal. That completes a circle
(ie a circuit) and so the light bulb glows.
If we break the circuit anywhere, the light goes
off. That break could be a gap in the wiring (arrowed), it could be a broken
filament in the bulb, or it could be an open circuit inside the battery. It
could also be an open switch.
So, for a circuit to work, the electricity
has to be able to find itself from the battery positive terminal all the way
back around to the negative terminal. (In this circuit it’s very easy to see at
a glance where the electricity flows. In more complex circuits, it often is good
to carefully trace out the path.)
In cars, the negative terminal of the battery is
connected to the car’s bodywork. This connection is called a ‘ground
connection’. Because the car body is made of metal, it acts as a single very
large wire. Therefore, as shown here, instead of one side of the light bulb
being connected directly back to the negative terminal of the battery, it can
instead be ‘grounded’ – that is, connected somewhere to the car’s metal body.
The metal body acts as the imaginary green wire shown on this diagram that
connects the ground back to the negative terminal of the battery.
Parallel and Series Circuits
When multiple devices like the light bulbs are
connected as shown here, they are said to be wired in parallel. This
means that each light sees 12V. It also means that if one light bulb fails, the other
keeps on going. However, a break at the arrowed point will switch off both
(So where would you put a switch to turn off both lights? Or a switch to
turn off just the top light?)
All power-consuming devices in a car are wired in
parallel. Often they’re wired as shown here, with the negative side of the loads
connected to ground. Remember, all the ‘grounds’ connect to the negative
terminal of the battery, so you can mentally picture them all running back to
this side of the battery.
When multiple devices like the light bulbs are
connected as shown here, they are said to be wired in series. This means
that each light sees half of 12V (ie 6V) and that if one light fails, the
circuit is broken and so both lights go out.
(And the same questions again:
Where would you put a switch to turn off both lights? Or a switch to turn off
just one light? Last one is hard isn’t it?!)
When the electricity can take a short-cut
(arrowed) straight from the positive to the negative terminals, and so not have
to pass through any electrical component that normally takes power, there is
said to be a short circuit. In this case, not only would the light bulb
go out, but so much electricity would flow in the circuit that a fire would
To avoid the risk of fire, a fuse is used to
protect circuits. A fuse is just a thin piece of wire that literally burns out
if too much electricity flows through it. Here a fuse has been placed as
physically close to the positive terminal as possible - as most of the circuit is then protected, that's a good way of doing things.
Car Modification - Miniature Halogen Reversing Light
take the example of adding a brighter reversing light to your car. One approach
is to make your own compact halogen light. This can often be mounted so
unobtrusively that it’s hard to spot, even from directly behind. But it will
certainly make a difference to how much light you have behind you when you’re
going backwards at night.
tow-ball cover, glass lens and a domestic-type 20W, 12V halogen light...
quickly become a compact and powerful reversing light.
how do we apply the electrical ideas covered above? We need the original
reversing light and the new lamp to both ‘see’ 12V, so that means a parallel
wiring approach. So:
1: By looking at the wiring at the rear light cluster, you should easily be
able to identify the two wires that feed one reversing light.
2: Unless the car is in reverse gear and the ignition is on, no power is
being fed to this light, so you can safely bare some insulation from each of the
3: Solder the wires from the new light to these wires, one going to each.
You’ve now wired the new light in parallel with the old.
4: You don’t want the bared wires touching each other (otherwise the
electricity will be able to take a short cut past both the original light and
our new one – remember that’s a short circuit) and so you can now insulate the
soldered connections with good quality insulation tape.
5: Test the system. If the original light works but the new one doesn’t,
there must be a break in the circuit of the wiring you have added – a break in
the wire, a broken filament in the bulb or a break at your soldered connections.
There cannot be a break anywhere else, because the original light works. If
neither of the lights now work, perhaps you have created a short circuit and
blown the fuse for the reversing lights.
More Parallel and Series Circuits
In car modification, the ideas of parallel and
series circuits are applied all the time. So you can see how important these
ideas are, let’s take a look at some more examples:
When using a multimeter, some measurements are
taken with the meter in parallel with the circuit, and some are taken
with the meter in series with the circuit.
For example, the output voltage of this vane
airflow meter is being measured with the meter in parallel with the
...while the current flow of this headlight
circuit is being measured with the meter in series.
(We will cover how to use a multimeter later in
this series – the important point to realise is that multimeter measurement
techniques use both series and parallel circuits.)
Some engine management sensors change in
resistance with temperature. To make the engine management system think that the
temperature is actually different to what it really is (this can be a quite
advantageous modification), you can add a very cheap electronic component called
a resistor. But depending on the system, you may need to add that resistor in
series or in parallel with the existing circuit.
High voltage battery packs used in electric and
hybrid cars use lots of low voltage cells wired in series. For example,
this NHW10 Toyota Prius pack uses 240 nickel metal hydride cells (each 1.2
volts) wired in series to give an output voltage of 288 volts. If they were
wired in parallel, the voltage output would be only 1.2 volts. That’s a helluva
The concepts of complete circuits, breaks in
circuits, series circuits, parallel circuits and short circuits are used all the
time in every single level of electrical and electronics work.
So don’t underestimate the power of these concepts
– they’re ones to absolutely have nailed down.
The parts in this series:
Part 1 - background and tools
Part 2 - understanding electrical circuits.
Part 3 - volts, amps and ohms
Part 4 - using a multimeter
Part 5 - modifying car systems with resistors and pots
Part 6 - shifting input signals using pots
Part 7 - using relays
Part 8 - using pre-built electronic modules
Part 9 - building electronic kits
Part 10 - understanding analog and digital signals
Part 11 - measuring analog and digital signals
Part 12 - intercepting analog and digital signals
Part 13 - the best approaches to modifying car electronics and the series conclusion