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How to Electronically Modify Your Car, Part 9

Building electronic kits

by Julian Edgar

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At a glance...

  • DIY electronic kits
  • Step-by-step of kit building
  • A warning!
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This article was first published in 2009. Some of the kits mentioned in this story are no longer available from AutoSpeed but are available from Jaycar Electronics.

Last week in How to Electronically Modify Your Car, Part 8 we looked at using a prebuilt electronic module – the eLabtronics Voltage Switch. As we saw, this allowed a range of interesting modifications, especially through utilising the car’s standard voltage-outputting sensors.

That module was prebuilt – but how about using an electronic kit, something that’s usually cheaper to buy than a prebuilt module?

Building Kits

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I have been involved in building electronic kits for most of my adult life. I’ve built plenty of kits, and have helped design and develop them as well. But, I have to say: I think that kits are the reason that many people abandon the DIY electronic modification of cars.

Huh? Why so?

Well, the trouble with electronic kits is that it is so easy to make a mistake in their construction, resulting in a product that simply doesn’t work. The skills needed to fault-find a kit are vastly higher than the skills needed to build a kit, so in nearly all cases, if the kit doesn’t work when you’ve finished building it, then you’re stuck with a pile of useless junk. And junk that’s cost you money, time and enthusiasm.

I don’t have any figures to back this up, but it wouldn’t surprise me if as many as one-third of all electronic kits that get sold never end up working.

I think it’s vastly better to pay more and (1) buy a prebuilt module, or (2) buy a kit that has been constructed and tested. Now I might be accused of bias – the AutoSpeed shop sells prebuilt modules and prebuilt kits, and the latter are a lot more expensive than just the bare kit. But it’s simply so easy to spend say $50 buying a kit, work really hard for three or four hours making it – and then find it doesn’t work.

So if you’re a beginner to electronics – and this series is aimed at just such people – don’t embrace the buying and building of electronic kits as a natural extension of your DIY electronic car modification. To build a kit, not only do you need to develop a whole swag of component recognition skills, you also need to be meticulous in construction and be very good at soldering. Furthermore, you need to be able to interpret instructions that assume quite a lot of background knowledge.

However, kits allow you to do modifications that would otherwise be difficult, and certainly to do them at a low price. So let’s look at what is involved in building a kit, starting with what’s probably the simplest kit you can buy.

Example Car Modification - Voltage Monitor kit

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This kit comprises just a tiny printed circuit board (PCB), a dual-colour LED, a trim-pot (ie a small pot designed only for screwdriver adjustment) and a handful of other components. What it does is monitor to the voltage of its power supply, lighting the LED green when the voltage is above the set level, and changing the LED to red when it falls below the set level.

So what use is this?

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One of the best uses is as a battery monitor. That might be the car battery, or in portable battery-powered gear, it might be the voltage of the replaceable batteries. You can even use it to monitor the battery of an electric bike. It can be used with voltages ranging from 6 – 30V. (Note that because the device is powered by the voltage it is measuring, it’s not suitable for use with sensor outputs.)

It’s available from the Autospeed store for AUD$8.95

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The first step is to solder into place the five resistors. To solder components to the board, you poke the wire leads through the right holes and then solder the leads to the pads on the back of the board. You then snip off the excess lead length.

Use a multimeter to sort out which resistor is which, and then use both the overlay diagram in the instructions and this diagram to put them each into their right places.

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The installed resistors should look like this.

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Once you have done that, place the trimpot on the board and solder it into place.

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The capacitor is next. It is a polarised component, which means that it must be soldered into place with the right orientation. (The resistors, on the other hand, can go either way around.)

The capacitor's polarity is indicated by a 'minus' symbol near one lead. This lead goes closest to the edge of the PCB, ie the positive lead is nearer the integrated circuit (IC).

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The next step is to solder the LED onto the board. Note that again the instructions are slightly deceptive - better to follow this diagram. As can be seen here, the LED has three leads, each of different lengths. Orientate the LED and PCB so that the shortest lead goes into the top hole, the longest into the middle and the other lead into the bottom hole.

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Note that if you get this around the wrong way all that will happen is that the LED will start off red and then turn green as the voltage falls, rather than the other way around.

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Finally in the component assembly there's the three-legged voltage reference and the 8-legged IC. The voltage ref is assembled with the orientation shown here - the flat facing away from the edge of the board, while the 8-legged IC goes with the notch (sometimes shown on the IC as a dot) facing in the direction shown here.

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Then solder into place the two wiring pins and the finished PCB should then look like this.

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The best way to set up the monitor is on the actual car. Grab your multimeter and connect the module to power and earth, making sure that you don't get these connections reversed. Depending on where the trimpot is set, the LED will then glow red or green. Turn the pot until the LED is green, and then back the other way until it just turns red. Then start the car. With the increase in battery voltage from the running alternator, the LED should immediately go green.

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Now that you've shown it works, use your multimeter to measure the voltage feeding the module. On my car, the running-engine battery voltage is no less than 14.4 volts, while when the engine is switched off, the battery voltage immediately drops to about 12.4V. If I have the lights and ignition on (but the engine still off) the battery voltage plummets, dropping to the high elevens in a minute or so. However, if I leave the lights off and just play the sound system loudly, the battery voltage drops only very slowly.

So looking at all these voltages, I made the decision that I wanted the LED to change from green to red at about 12.0V.

By turning the adjustment trimpot very carefully, it's possible to set the module so that the LED changes to red very close to 12V - just what's wanted in this case. And of course, if you want the LED to change colour at, say, 11.5V, just adjust the pot a little further.

Other Kits

A couple of other simple kits are:

  • Mixture Meter

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This kit allows you to display the raw voltage output of a traditional narrow band oxygen sensor, as fitted to most cars of the last 25 or so years. This allows you to see when the air/fuel ratio is rich, lean or stoichiometric (“normal”). It also allows you to see when the engine management system is in open and closed loop. See Cheaply Monitoring Air/Fuel Ratios for more. The kit costs AUD$14.95.

  • Nitrous Fuel Mixture / Motor Speed Controller

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This 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!. The kit costs AUD$24.95


Don’t get me wrong – successfully constructing kits can be a fun and rewarding part of electronically modifying cars. However, if you are a beginner to electronics, don’t assume that making a kit must be easy and that success will inevitably follow. In many cases, that’s not the case.

However, on the other side of the ledger, starting with simple kits will develop skills, and then more complex kits can follow.

Next week we’ll change pace and look at the different types of signals you’ll find in car electronic systems.

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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