What I am trying to do is build a digital speedometer for my car.
There is a sensor in the diff, which sends a pulse 6 times every rotation, the signal then goes into a chip in the gauge cluster which i presume converts the pulses to a signal to drive the speedo.
I tried mesuring the signal with my digital multimeter but still not sure what kind of signal it may be.
With the multimeter set to DC, and the car not moving it reads about 4.98V when i start driving the voltage decreases ( i think i got the voltage down to about 3.7v doing 130kph)
With the multimeter set to AC I get a 0V reading which increases as I speed up, but seems to jump around alot and not be very accurate.
With the multimeter set to Hz, I get 0Hz when the car is not moving and it increases with car speed although it is not linear the signal seems to be stable (136Hz~ = 100kph, 64Hz~ = 50kph, or there abouts. Actually after reading that, it could be linear?) When I come to a stop it reads approx 4.67Hz for about 2 or 3 seconds then drops to 0Hz, but this could just be my multimeter.
Any idea what kind of signal this is? I thought maybe PWM/Squareware signal. Also, what would the best way to read this signal with the arduino?
Should I use a optoisolator or somthing like that to protect the arduino?