# Detect current in car handbrake circuit

Using Arduino, how can I detect that the switch is on? I have access to the wire within the red rectangle only. That's because the switch is closely connected to the car body (ground). When the handbrake is up, the switch is pushed by a coil to touch car body and close the circuit. And when the handbrake is down, the switch go away from the car body and the circuit is open.

• Can you insert a voltage monitor? Or other parts? Jan 19 '18 at 15:41
• If you literally only have access to that wire, you cannot. You will need access either to the ground or 12v, or the bulb to see if it is lit. Jan 19 '18 at 19:39
• Actually that's not quite true. You could do it by cutting the wire and putting a resistor in series, then measuring the voltage across the resistor. Jan 19 '18 at 20:00
• this seems like a school assignment. ... did i guess correctly? Jan 20 '18 at 5:45
• @MegamindSaiko it will monitor the voltage. Jan 20 '18 at 17:42

If you truly only have access to the red rectangle content, and not the Ground or +12V supply, you could use an opto-isolator, assuming you can use enough of the wire in the red rectangle to be able to form a voltage divider to get at least 1-2 mA through the input side of the opto.

• It gets easier if you have access to the Ground point, too... Jan 19 '18 at 19:34

Wrap the wire around a ferrite or some iron core and detect magnetic field.

• This may not be the most practical answer, but here's an upvote for creative thinking! Sep 27 '19 at 17:49

If you can modify the cabling, you can add a shunt resistor in series and measure the voltage drop between the terminals. This way you can measure the current draw that would be 0 if the switch is open and something if the switch is closed. Other way you can use an ACS712 Current Sensor.