Hey all, I was trolling around and found some cool info on building a diy knock microphone. I took it to the next step and did it for about $55.
Basically you need a cheap knock sensor
Knock Sensor 22060-30P00
Two or three wire to female jack
CERRXIAN 3.5mm Stereo Audio Video Female Jack to 3 Screw
Two small female spade terminals
Portable Headphone amp 3.5mm Stereo Audio Out
Mpow BH129 Bluetooth Receiver for Car
And a 3.5mm headphone extension of your choice
On the knock sensor One of the terminals is a ground the other signal. Wire the ground to the ground screw on the headphone jack and the signal to signal. In my case I bridged the signal to both left and right so I could listen over all speakers and not get a one sided effect.
Next I ran the extension to my headphone amp Input. The output then can either go straight to your stereo aux port or to the Bluetooth transmitter.
Bolt the knock sensor somewhere you can hear knock from ( I could even hear it bolted to the air pump bracket) but ideally right on the engine block would be better.
Turn the amp up and plug in to your aux port on your stereo.
I had the Bluetooth transmitter to record on my tablet via audacity. But I found it was really helpful in the tuning process to just run the aux wire to the stereo and listen as I was driving.
Knock sounds like a static electricity snap.
I'll post pics from my computer and a video of a knock event.
