Project Objective: To create a circuit that that allows the knock sensor
output of an engine to be heard through a pair of headphones. Once this is accomplished,
the knock sensor output will be analyzed and used to create a signal processing system that
can detect true knock with greater precision than the stock knock sensing circuit. This will
benefit supercharged motors and motors with forged internals the most as they are most prone
to having false knock as seen by the stock ECU.
Pictured above is the preliminary test circuit. This is a simple amplifier circuit that takes a 12-18v power supply. Since most cars operate at around 14v, the amplifier will have a swing of +/-7V about 7v. Since we expect our input signal have a swing of +/-3V on average, this should allow for sufficient amplifcation of the input signal to make it clearly audible with headphones. The knock sensor reads from 0-5V, but output of 5V is virtually never seen. The circuit does have a gain that can be varied from 1 to 50. I do not expect to need the gain except to listen to the knock sensor at cruising engine RPM when the knock sensor output is much less than 1v. In the coming weeks I will install and test this circuit on a number of cars and record the knock sensor output. This output will be utilized to differentiate the specific frequency of real knock. The stock knock sensing circuit relies on a voltage threshold that can be triggered by noise of any frequency picked up by the stock knock sensor. The stock knock sensor does have a rather narrow frequency sensitivity, but it still picks up a lot of noise that is not actually knock. This amplifier is the first step towards collecting the data needed to develop a more advanced knock sensing algorithm that distinguishes knock based on frequency and voltage level.