I am using the Arduino to modify a pulse coming from a speedometer sensor. See the CAD (camera aided design) drawing below showing how it is wired up. The basic idea is that the raw sensor signal goes into an inverting schmitt trigger with RC filter to create a clean 0 - 5vdc square pulse. The pulse goes into an interrupt (pin 3) on the Arduino and the Arduino then calculates the frequency, modifies it as needed, and recreates the frequency as modified on pin 7. It is working exactly as hoped.
Here's the curious part. I forgot to hook up the power to this circuit today (no power to Arduino -- LED's off, etc.) and noticed that the speedometer continued to work based on the original signal from the sensor. The speedometer is triggered by ground pulses (the speedometer sends out 5vdc, and each time that singal is grounded, it counts as a pulse). The only signal to the speedometer is coming from pin 7 on the unpowered Arduino. So, how does this happen? Somehow the orignal pulse is getting through the unpowered 74c14, making it's way to Pin 3 of the Arduino, and then making it's way to Pin 7 of the Arduino? Anyone care to explain how/why that is happening?
