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DIY Detonation Detector - Mk II

A new and brilliant DIY detonation detector.

by Julian Edgar

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Way back in October 1999, AutoSpeed ran an article on building your own detonation detector. For reasons which we'd given in a previous article ["DIY Detonation Detection - Part 1"] - reasons which are still just as valid today - the best DIY detonation detector is an audio system that allows the engine tuner to listen to the noises that the engine is making. The detector that we then gave instructions on building was based on a commercial product available from Dick Smith Electronics called the Whisper 2000. This unit - pictured here after modification - was a type of hearing aid dressed up to look like a Walkman. With some minor mods (involving relocating its microphone to a clip clamped to the engine) it worked very well as a detonation detector - however, not long after that article was written, Dick Smith dropped the product from their shelves.

And ever since, we've been receiving requests for an alternative.

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So here it is - our new DIY Detonation Detector, Mark II.

Unfortunately it is fiddlier to make than the old design, because no off-the-shelf product is used as its basis. However, if you're even half familiar with soldering and electronic components, you shouldn't have any difficulty building it.

And this design does have several advantages over the old one. Firstly, it uses decent quality 'bud' style earphones, rather than lightweight headphones. Most professional tuners prefer bud earphones in this application because they usually combine them with noise-reducing earmuffs worn over the top of the earphones. That way, pretty well all the noise reaching the ears is through the earphones and so is from the engine-mounted microphone.

The other advantage is that the new design has both less distortion and better sensitivity.

In fact, while you can turn down the volume so that even loud noises are only a whisper, at the quiet end of things you can hear sounds that you simply never normally would be able to. We'll give you an example - by attaching the clip to the sump plug and turning up the sensitivity, you can actually hear the oil flowing back to the sump after the engine has been switched off!

The Parts

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The basis of the system is the 0.5W Amplifier Module from Dick Smith Electronics (cat no K5604), which costs AUS$8.25. (Note that this same kit is also available from Jaycar Electronics - cat no KC-5152 at only AUS$6.70).

In addition to the kit, you will need these parts, which should be easily available worldwide:

  • Piezo transducer (eg Dick Smith L7021, Jaycar AB3440)
  • Good quality bud-style earphones
  • 10K potentiometer and knob
  • 3 metre length of twin-core shielded microphone cable
  • In-line stereo socket to suit the earphone plug
  • On/off switch
  • 9V battery clip and 9V battery
  • Box

This lot should come in under AUS$30, and even if you run a sturdier metal box rather than the more common plastic item, it will still be well under $40.

Making It

The first step is to assemble the PC board of the amplifier kit.

Use a multimeter to measure the resistance of the two resistors so that you know which one is which. Also, make very sure that you get the polarity of the three round capacitors the right way around (the negative side is indicated by a line of 'minus' symbols down the body), and put the chip in with the correct orientation (the notch shows which end is which).

Note that there are two changes to the supplied assembly instructions - leave out the 1 kilo-ohm resistor and replace it with a wire link. This increases the gain of the amplifier. The other change is to leave out the trimpot - we'll be using a bigger external pot instead.

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So once you have done this, the PCB should look like this.

The next step is to solder the wires from the 9V battery clip into place, with the black wire from the clip going to one of the normally open switch terminals, the other side of the switch going to the Ground PCB pin, and the red wire from the battery clip going straight to the Positive power pin. This is the power on/off switch.

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Solder some insulated wires to the three spots on the PCB where the trimpot was supposed to go, and then solder the other ends of these to the new pot that you purchased. The centre wire from the PCB spot goes to the centre terminal on the new pot, with the other wires going either way around. This pot is the volume control.

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Next you'll need to connect the earphones. There are two approaches - the more elegant way is to install a socket in the box that the earphones plug into. Or, you can use an inline plug, which is what we chose to do. Note that although the stereo earphones have three connections (right channel, left channel, ground), we use the earphones in mono. This means that the two conductors at the top of the plug (the tip and first ring down from the tip) get connected together, with the base of the plug forming the other wire.

But there's something missing, you're all saying. Where's the microphone? Well in fact with we trialled a number of different microphones before picking on one which gave the best results overall. As the parts list shows, this is a piezo transducer. Not that it's not the same as a piezo buzzer - instead as the Jaycar catalog states, "This is an electro-acoustic transducer which contains no electronics and will transform AC voltages to sound pressure waves. It will also generate an AC voltage across its terminals when stimulated with a sound pressure wave." It's the latter function that we use it for here.

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The transducer is glued facedown inside the arm of a battery clip, as shown here. The cable ties hold it firmly in place as the glue dries.

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The assembly can then be covered in heatshrink, with an extra tie used as here to give the cord even more security.

The twin-core microphone cable each has a conductor soldered to the two terminals on the transducer, while back at the PCB these are connected (either way around) to the input and the ground. The braided copper sheath can be soldered to the ground terminal at the PCB, while at the transducer end the braid can be cut off short - and not soldered to anything. The braid acts as a shield, stopping electrical noise getting into the system.

Using It

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Using the device is very simple. You simply clip to the microphone to whatever you are interested in listening to. Noises are transmitted through the metalwork directly to the clip and microphone, making the instrument extremely sensitive. To detect detonation, the clip is best placed directly on the block, in the type of place that the factory knock sensors are positioned - no surprise there! Adjust the volume control to give a comfortable loudness level, and sit back and listen. Over the clatter of pistons, valve gear, explosions and gearbox whines, detonation sounds like a sharp "splat!, splat!".

Note that you'll be hearing lots of noises other than detonation - don't expect just to hear detonation isolated from everything else. It's simply not that easy. Instead, use your brain and ears as the 'filter', discarding the normal sounds and waiting for something that's abnormal.

Depending on the location of the microphone clip, additionally gearbox noises, injector clicking, suspension clunks, turbo whistling - the whole lot can be individually identified and problems isolated.

Note that it's best to listen from the passenger seat while someone else drives the car - that way, the driver can still hear emergency vehicles and concentrate on driving, not listening to strange sounds.


Dick Smith Electronics

Jaycar Electronics


If you expect to be using this device a lot - for example, in a professional dyno shop - then there are a few minor modifications that we can suggest. Add a large flashing LED to show when the power is on will save an inadvertent flat battery, and using a LED designed to both flash and be connected straight to a 9-12V battery will make this wiring easy. Just place it in parallel with the PCB power supply, remembering to get it the right way around. Secondly, you might want to invest in four rubber feet, so the chances of the box marking a dashboard or centre console are lessened.

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