I am detecting detonation in a SI engine using a knock sensor (pizeo element). What I would really like is to have 3 leds that light up as the knock gets more severe. I have the knock sensor running into an op amp powered by a 9v battery. I am listening to the output with a pair of head phones, but I would like to add a visual output. The knock sensor is a pizeo element, so as I understand it the more aggressive the engine is knocking the greater the output voltage will be from the sensor. I was hoping to do something like this; First led to light when the output from the op amp is between 1-3 volts, the second to come on at 3-6 volts and the third to light up between 6-9 volts. (arbitrary values I'm not sure what the actual output voltages are, but you get the idea). The idea is that when no knock is present no lights are on, and under heavy knock all three lights are on. Any input would be greatly received.