MPGuino: Max Current for VSS Line?
 

Hi,


My question: What is the maximum current the vehicle speed sensor line (for the MPGuino) should see? 0.25 amps? more? less? I want to make sure I don't let to much current through the new vss wire and damage my MPGuino. Is there a minimum required current for the MPGuino to pick up the signal? If so, what is that minimum current?

Long story short, I am adding another VSS signal for both MPGuino and my aftermarket cruise control in my vehicle instead of using the stock VSS signal.

My stock VSS is a reed type switch. An open switch has +12v on one side and ground on the other and magnet from speedometer makes the reed switch make contact and completes the +12v circuit.

I am making a similar system using a bicycle speedometer magnet sensor, and two magnets on my cv shaft. It is basically a reed type sensor, open circuit without a magnet, and completed circuit with magnet. I am going to feed +12v to one side of the wire, and the other wire goes to ground. I'll be using a resistor to limit current flow through the wire.

My question again...: What is the maximum current the vehicle speed sensor line (for the MPGuino) should see? 0.25 amps? more? less? I want to make sure I don't let to much current through the new vss wire and damage my MPGuino. Is there a minimum required current for the MPGuino to pick up the signal? If so, what is that minimum current?

With 12 to 14 volts on the MPGuino input pin you would see about 0.5 mA current draw. About 0.3mA will go through the 50K pull down resistor and most of the rest travels through the Zener diode to ground with the current limited by 50K input resistor. The current into the actual processor pin is only about 1 µA.

Assuming you have the current pre built MPGuino that includes that pull down resistor the wiring can be very simple. You could simply connect the input pin to 12 volts through the read switch. I would add a 1K resistor or small value fuse where you connect to 12 volts to protect incase of shorts.

Mike