Here is an audio comparison of a high level (read very expensive) knock sensor system that costs hundreds of dollars vs a DIY knock sensor system that costs less than $5.

Key Takeaway – In my experience using Equalizer APO software massively improved my ability to audibly detect knock at all engine speeds.

DIY Knock Sensor System vs High End Knock Sensor (Audio Comparison)

The DIY knock sensor system in the video can (probably) be made by most people for less than $5.

This DIY knock sensor system outputs audio only in real time (there could be a small delay depending on your laptop, less than half a second in my experience).

You can also choose to record the audio for playback later if you wish.

In this article I will detail how you can make this DIY knock sensor.

What the DIY knock sensor can’t do

No visuals, no connection to external ECUs, no datalogging. If you need these features, an aftermarket knock sensor system could be the way to go.

Aftermarket Knock Sensor Systems

Aftermarket knock sensor systems cost from a few hundred dollars to a thousand or more dollars.

There is a reason for this.

These systems will generally seek to automate the knock sensing process which means they will give the user a visual indication of knock eg a LED and they could also send a signal to the engine’s ECU which will indicate knock, the more expensive systems may even vary the voltage going to the engine ECU to show the severity of the knock.

What the DIY knock sensor can do

It allows you to listen to engine knock in realtime, (or very close to real time) and also give you the ability to record the audio from the knock sensor for playback later, if you wish.

It does not do anything else.

If you need a visual indication of knock, if you need to send a signal to an ECU to indicate knock, if you want to datalog or anything else, the only option I know of, would be to purchase an aftermarket knock sensor system. There is no easy DIY solution for these features.

What you need to buy to make a DIY knock sensor

You only need to buy two things (okay, maybe four)

1. A bare male mic jack and

2.Three mteres (ish) of mono mic cable.

3&4 If you think your knock sensor will be outputting more than 2 volts at peak output, you should make a voltage divider to reduce the voltage going to your laptop’s mic socket. Parts 3 and 4 will be the resistors you need.

If you are not sure if you need a voltage divider, put a multimeter on the knock sensor output to measure it.

Other (free) things you need

APO Equalizer Software

VLC Media Player Software

The ability to tap into the factory knock sensor. A positap is what I recommend but if you want something cheaper, splice crimps will work also, even if they can be a bit unreliable.


-Laptop with mic and headphone sockets

How To Build Your DIY Knock Sensor

1.Wire your headphone jack to your mic cable (3 meters length should be enough) and run the cable from the laptop to your existing knock sensor (positive and negative cables) – Jack Wiring diagram is here if you need it.

Insert your voltage divider on the positive wire, if you are using one.

2.Tap the other end of the mic cable into your existing knock sensor cables (or fit a dedicated knock sensor) using the positap or whatever method you chose.

3.Download and install APO Equalizer and VLC Media Player.

4.Find the knock frequency of your engine, using a knock frequency calculator

5.Plug knock sensor cable you made above into the mic input on your laptop

6.Open APO Equaliser and customise the equalizer so it enhances the knock frequency +/- 400hz and reduces all other frequencies.

For example, if your knock frequency is 7200hz. Enhance the levels from 6800hz to 7600hz and reduce the input from 7601hz and up, and from 6799hz and down. You only want to hear noises in the 6800hz to 7600hz frequency range.

You can obviously customise these ranges, frequencies and levels if you need to.

How the APO Equaliser settings look when increasing the levels at the knock frequency and reducing the levels on all other frequencies

7.Open VLC media player and select the capture device as “Microphone” and press “Play”.

8. Connect your headphone to the laptop (and put on your headphones)

You are now listening to the knock sensor output in real time.

Adjust the gain in the APO Equalizer software to get the volume you need.

The beauty of this solution is that you can listen to the knock sensor signal in the background while you use the laptop for other things eg tuning the engine ignition timing.


There could be a slight delay in hearing the engine sounds.

You may need to adjust frequencies given by the knock frequency calculator.

You may need to reduce the voltage going into your laptop’s mic socket.

The next level

As I wrote above, if you need more features such as visual indications, knock signals for ECUs, permanent installation or datalogging then the next step would be to buy an off the shelf knock detection solution.

The DIY knock sensor is purely for hearing engine knock, nothing more.

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