Detect external ground on an input pin

Question

I am making a bike alarm for my bike. I haven't started yet. Basically, I'm thinking of a pin to be connected to the metal post (to which the bike is locked.) I want to detect if that connection (to the ground via the metal post) is broken. Is that possible?

• Chris Stratton: If there is another connection from there to the Arduino's ground, and if you have an internal or external pullup resistor on the pin, then yes it should be straightforward.
• Zeeshan Ahmad: I only want one 1 single wire from arduino to detect ground, not two/a loop
• Chris Stratton: That's not going to work very well. You could try something with captive sensing but it is likely to be unusably sensitive to false alarms (wind, rain, other bikes in proximity, even current leakage via tires wet with road anti-icing brine, etc). The whole project is a bit impractical anyway - how will you protect against disconnecting the power source?
• Zeeshan Ahmad: Well I was considering the fact that the thief could connect the wires directly excluding the post so preventing an alarm at all. Do you have any suggestions?
• krol: @ZeeshanAhmad do you use a chain, wire, or a bike lock to lock up your bike? If so, you could use the metal of the chain/wire/lock to form a circuit. If the circuit breaks (the thief cuts the chain or opens the lock), then we know to set off the alarm. Just a thought

Answer 1

How about fastening a wire to the post, and then the other end, on the bike, goes into a "jack" (the sort of thing you plug headphones into). With the wire inserted, it makes a connection, and the Arduino knows the wire is there. As the thief steals the bicycle the plug (or loose wire) is pulled from the jack, breaking the connection. i.e.

Bicycle protection

• Zeeshan Ahmad: I will have to try this, and see how it goes