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How to Electronically Modify Your Car, Part 2

Understanding circuits

by Julian Edgar

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At a glance...

  • Circuits
  • Breaks in circuits
  • Common grounds
  • Short circuits
  • Fuses
  • Parallel circuits
  • Series circuits
  • Example car modification - additional reversing light
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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.

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Here’s a 12V car battery. It has two terminals, one marked positive (+) and the other negative (-).

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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.

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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.)

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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.

Key points:

  • A complete circuit, from one side of the battery through the electrical device and back to the other terminal of the battery, is needed for the device to work.

  • Any break in this circuit will cause the device to stop working.

  • Use of a common ‘ground’ means that the negative connection of devices can be to the car’s body, rather than directly to the negative terminal of the battery.

Parallel and Series Circuits

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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 lights.

(So where would you put a switch to turn off both lights? Or a switch to turn off just the top light?)

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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.

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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?!)

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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 probably start.

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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.

Key Points:

  • Parallel circuits and series circuits behave in different ways: it’s vital to be able to recognise the different types of wiring.

  • A short circuit is where the electricity takes a ‘short cut’ straight from positive to negative terminals of the battery

  • Fuses will protect in the case of a short circuit.

Example Car Modification - Miniature Halogen Reversing Light

Let’s 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.

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A tow-ball cover, glass lens and a domestic-type 20W, 12V halogen light...

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...can quickly become a compact and powerful reversing light.

Now, 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:

Step 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.

Step 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 two wires.

Step 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.

Step 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.

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Step 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:

  • Using a Multimeter

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.

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For example, the output voltage of this vane airflow meter is being measured with the meter in parallel with the circuit...

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...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.)

  • Modifying Engine Management Sensors

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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.

  • Hybrid and Electric Car Batteries

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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 difference!


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

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