In the last post, my turn signal device worked great when the car lights were running on battery. However, as soon as I turned the engine on the device was caught in a reboot loop.

I hooked up my Xprotolab micro-oscilloscope, while the 2013 VW Passat engine was on, to examine the stability of the 12V running light. To my surprise, the signal showed >6 V dropouts with <5 ms duration and >1 Hz frequency, which means that the power management in my Arduino device needs some updates. The photo below shows the dropouts with a 10 uF buffer capacitor in the circuit.

While sizing a buffer capacitor for the Arduino device power, I realized that I was using a 5V voltage regulator on VIN which has a voltage spec of 7-12V (Arduino R3). So then I switched to a 9V voltage regulator and added a 1 mF buffer capacitor on the output side. Now we are fully functional!