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Building Electronic Car Kits

Very cheap and effective modification

by Julian Edgar

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Car modifiers have never had it so good in terms of cheap, DIY electronic kits to make their car go harder. You can buy and build electronic boost controls, airflow meter interceptors, rev switches, voltage switches, intercooler water spray controllers, electronic ignition and a heap of others. But success requires one very important skill – the ability to (successfully!) build the kits.

If you are well experienced with electronic kit building (that’s where you solder the components to a printed circuit board with all the parts in the right places and around the right way around and the outcome is an always-working project), that’s great. But we bet that for every kit that’s started, maybe a third never get finished or never work. And not only is that disheartening for the builder, it’s also likely to drain confidence to the extent that no more kits are tackled.

We’re not trying to put you off – even if you’ve never soldered before, with care and attention to detail, you’ll still be able to successfully build electronic car kits. But think of it as being a bit like model making – you’ll need steady fingers, to check twice before committing, and be able to follow diagrams very accurately.

Over the years I have built many electronic kits and have been involved in the design of a dozen. However, I’ve also tackled kits that didn’t initially work (ie some fault-finding was needed) and in two cases, have built kits that never worked at all. So you could say that I know both sides of the story!

Here’s how to improve your chances of success.

Building a Kit

  • Buy a kit not components

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We strongly suggest that all beginners buy a commercially available kit. (The alternative is to source the printed circuit board [PCB] and then buy the components individually.) The kit will contain all of the components, the PCB, solder and instructions. The instructions will normally be a B&W photocopy of the relevant article from an electronics magazine that designed the project and published an article on it. The kit shown here is for a Keypad Car Alarm.

  • Organise the parts

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When you open up the packet you’ll find something like this inside: the components grouped into their categories (eg all the resistors together), the PCB and the photocopy of the article. Don’t cut open the plastic and scatter all of the components everywhere: chances are that you’ll lose some. Do examine the PCB carefully, looking for any bridges that may have been formed between tracks and making sure that all the component holes have been drilled. In nearly all kits, you’ll have no problems in these areas.

  • Inspect the overlay

Every kit has a PCB overlay diagram. It shows where each component goes on the PCB and is one of the most important parts of the instructions. Not only does it show part locations, when the component is polarised (ie it must be soldered in only one way around if it is to work) the overlay diagram shows the correct orientation. The orientation of diodes, integrated circuits (IC), transistors, LEDs and electrolytic capacitors are all shown – either by the band at one end (diodes), dot at pin #1 (IC), shape of the component (transistors and LEDs) or a ‘+’ mark (electro capacitors). Resistors and somecapacitors don’t have a specific orientation – they are non-polarised.

  • Parts list

The parts list is more than just a list of the parts. Not only are the parts shown, but for some components, the specific names that they are given on the PCB overlay are also nominated. For example, a parts listing might show a BD681 NPN Darlington transistor. But in addition it may be listed as ‘Q3’ – and on the overlay diagram the BD681 transistor is shown as ‘Q3’. This numbering of the different transistors is important, as in some cases it can be primary clue as to where the transistor goes on the PCB. Similar numbering applies to diodes, voltage regulators and other components. Additionally, the components listing is useful as you can cross off each one as you place it on the PCB.

  • Constructing – resistors and links

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The first components to be placed on the PCB are always the resistors. These don’t have a polarity, so that’s certainly one thing that makes them an easier starting point! However, they do have differing values, as indicated by the colour codes. However, don’t worry about the codes; instead, use a multimeter to measure their resistance. Note that a 2.2 kilo-ohm resistor won’t necessarily have a value of exactly 2200 ohms – but it will be close. Use the multimeter to sort out each resistor’s value and place them on a piece of paper that is marked to show which is which. Note also that if the circuit contains two 2.2 kilo-ohm resistors you should find a pair of resistors of this value – sorting them out this way makes it a bit easier. Some kits (like this one) also have some plain wire links to be placed on the PCB – do these along with the resistors.

  • Constructing – diodes

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Next up are the diodes. These can come in different shapes and forms, but their band shows which way around they should be placed on the PCB – yes, these components are polarised and so must be placed on the board with the correct orientation. Use the photos as well as the overlay to help you sort out which diode is which. To help you get things in the right places, always orientate the PCB as it is shown in the overlay diagram. Make a final check before turning the board over and doing the soldering, and install only one component at a time. This not only allows close-packed adjacent components a chance to cool but means that more checking is being done of each step. Some diodes have very small writing on them showing which type they are. Use a magnifying glass to read it.

  • Constructing - transistors

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The transistors are the next to go on. These are three-legged polarised components, often with the legs arrange in a triangular pattern which makes getting their orientation right a bit easier. However, some transistors have their legs all in a line, so in this case other clues need to be used. For this kit the overlay clearly specifies which way the metal back of the transistor needs to face; this is also clear in the pics. Sorting out which transistor is which can be done by reading the codes printed on them and matching those up with the parts list, which will tell you which is called Q1, which Q2 and so on. Then you need to look at the overlay and see where each of these goes. Note that a voltage regulator often looks pretty much like a transistor – three legs and so on – and should be orientated and positioned using the same basic approach.

  • Constructing - capacitors

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Next up are the capacitors. The polarised ones are cylinders marked with a line of negative (-) symbols next to one leg. Logically, the other leg is the positive – and that’s important, because it is the positive (+) side which is always marked on circuit and overlay diagrams. It’s really easy to get these around the wrong way – the mind sees the negative symbols and somehow ties that to the positive symbol seen on the diagram... Other capacitors are non-polarised (ie they can go in ether way around) but they often have confusing markings. In identifying these caps look at the kit’s Capacitor Codes description and also check the pics of the completed kits.

  • Constructing - ICs

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Last to be soldered into place will be any integrated circuits – called ICs or chips. In this case a socket has been used – the IC then plugs into the socket. ICs must be orientated correctly to work and in this case you can see a cut-out at one end (the right-hand end in this case) of the socket. This cut-out represents the end where the Number 1 pin of the IC must be placed – Number 1 pin is represented by a dot on the IC. Don’t orientate it just by the way the writing on the chip looks in the pics – this can change! If the kit uses a socket, make very sure that all of the pins go into the socket – ie that none are folded up under the body of the IC or pushed down outside of the socket.

  • Constructing – final components

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Here is the nearly finished kit – LEDs (their polarisation shown by a flat on the body) and the terminal block and ribbon cable (which goes to the keypad) have been added. Oh yes, and the IC has been plugged into its socket.


No matter how strong the urge is, before you apply power check each component against the overlay diagram. Is the orientation correct? Is it in the right place? Then turn over the PCB and check your soldering. Have you bridged any close tracks? Are any solder joints looking dull and suspicious or are they all shiny bright with the solder formed really well around the lead and track? Plenty – and we mean plenty - of people have torched their project through not making a last minute check of their work. At this stage it’s also often a good idea to have someone else look your project over.

Making Changes

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This alarm kit uses a remote keypad, with the keypad and the PCB connected by 7-way ribbon cable. In the original instructions ribbon cable isn’t used - instead the two parts plug into one another. But in this case we wanted to mount the two parts separately, thus the use of the ribbon cable. In many cases when building a kit you may want to make some minor changes like this.


  • Soldering Iron

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If you intend to build only one or two kits, a general purpose soldering iron complete with stand and a reel of solder will suffice. The price is right (about AUD$35) and the iron will also be useful for making the soldered connections to car wiring. But while a small and cheap soldering iron can be successfully used to build electronic kits, a variable temperature soldering station like the one shown here will allow you to get better results, primarily because the cord connecting the iron to the base station is supple and so the iron is easy to manipulate.

  • Multimeter

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A multimeter is essential when building automotive electronic kits. You’ll use it to measure voltages and current, resistance and (in a car) often other parameters like temperature as well.

  • Power Supply

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A variable output power supply allows you to easily test kits. A design like this one with variable current limiting will also instantly show you if you have a made a big mistake and have a short circuit or something equally catastrophic. But if you’re on a tight budget, a car battery is fine as a source of power.

In addition, a small selection of basic tools will make building kits a lot easier. These tools include side-cutters, needle-nose pliers, a heatsink (that can be clipped onto components that would otherwise get to hot when being soldered), and a pair of pointy-nosed tweezers.


So what are good kits to start with? (All the following kits are sourced from Jaycar Electronics and can be purchased on-line.)

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The Nitrous Fuel Mixture / Motor Speed Controller (Jaycar cat no KC-5382) is a very useful kit, able to do far more than its name suggests. Uses include running an injector, dimming lights, motor speed control (eg of a water/air intercooler pump), pulsing a light or horn, and pulsing a solenoid to control flow. We covered the kit in detail at The Nitrous Fuel Controller - That's Also a Lot More!. This is an excellent beginners’ kit and costs AUD$25.

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The Courtesy Interior Light Delay kit (Jaycar cat no KC-5392) is another simple to build kit. As its name suggests, it keeps the interior light illuminated for a little while after you get in and shut he door. It’s most suited to older cars that don’t have a body computer or remote central locking. Cost is $19 and we covered the kit at Delaying the Dim

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Going up a step in complexity is the Universal Voltage Switch (Jaycar cat no KC-5377). This kit is a great building block as it can be used to switch things on the basis of varying voltages, eg the voltage output signals from existing engine management sensors. Examples of uses include triggering radiator fans (using the output of the existing engine temp sensor), switching on an intercooler fan at high loads (triggered from the airflow meter), and sounding a low oil pressure alarm. Cost is just $30 – cheaper than a pressure switch – and the kit is adjustable both for set-point and hysteresis (ie the difference between switch-on and switch-off points). We covered the kit at The Simple Voltage Switch and then ran another feature on Adding a Delay to the Voltage Switch.

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A further step up in complexity of construction and use is the Digital Fuel Adjuster. This is a one-dimensional interceptor that can be used to adjust the fuelling on cars equipped with a voltage outputting airflow meter. It’s programmed by a hand-held digital controller - another kit. The Digital Fuel Adjuster (Jaycar cat no KC-5385) costs AUD$80 and the Hand Controller (cat no KC-5386) costs $60. That’s the cheapest way you can fully map your out-of-closed loop mixtures ... Our series on the DFA starts at The Digital Fuel Adjuster, Part 1 .


Start off with simple kits that you build with care and concentration. Then, when you’ve achieved success with those, go on to the more complex ones. Even if you’ve never previously thought of taking this approach, you’ll be glad you did.

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