Detect when a device has been connected to Vcc

Question

I'm using an Arduino Uno to read input from an NES controller (a simple 8-bit shift register). This is working just fine, but I'd like to be able to detect when the controller is disconnected to shut down the I/O polling, otherwise the ATmega reads a bunch of nonsense input from the floating input pins.

My main problem is that I am extremely not an EE, so I'm not quite sure how to go about doing that. The controller has 5 pins: Vcc (5V), ground, strobe (latch the 8 parallel inputs), clock (shift the bits out), and a data output pin. It looks like there's a pretty substantial voltage drop at the ground pin on the controller when no buttons are being pressed.

Essentially, I just need to detect when the Vcc-to-ground circuit is broken, and set one of the Arduino's input pins high or low accordingly. Doesn't matter which logic level represents disconnected, as the code can be easily adapted either way.

I've got a few hundred resistors of various sizes, a box of assorted NPN, PNP, and JFET transistors, and only a cursory understanding of how to use them. :)

Answer 1

I suggest using another method:

1. You do not do anything to data input pin on Arduino
2. Guess you have one more input pin available on Arduino, right? Call it "V"
3. So you change the SW to ignore all data when this V pin is Low
4. All the electronics you need is: -- common PNP transistor -- resistor 4K7 or about -- resistor 47K or about
5. Connect: -- pnp Emitter to +5 V power supply and to 47K resistor -- pnp Base to Power Input of NES controller and to another end of 47K -- pnp Collector to 4K7 resistor and to V input pin of Arduino -- other end of 4K7 goes to Ground

Current consumed by NES makes PNP transistor conducting and it holds V pin at logical High. That's it

^{simulate this circuit – Schematic created using CircuitLab}