Mysterious Noise Issue in Automotive Ignition Coil Application, Arduino
Maybe I can help you some more. Erratic noise when signal is disconnected is usually a sign of a poor ground or a ground loop that your test equipment is part of. Make sure all grounds are tight. Use 14awg wire to grnd test equipment to your main Arduino grnd (optional). Use the test probe only at location where the probes grnd wire is connected to. Lastly, check for noise on your 12 volt power feed. I do not see the required (mandatory) 470uF 16-25vdc electrolytic bypass capacitors on the main 12 volt line, which means it can pass noise to any and all circuits it connects to. If you have extra pieces of C2, you can use those if you do not have a 470uF capacitor.