How to execute commands on Arduino (via PC) without touching the current sketch?

Question

I am working on my first Arduino project. It's a Reverse Geocache. Basically, it's a puzzle where a battery-powered Arduino is mounted inside a locked box, and is able to unlock the box if the user brings the box to the correct GPS coordinates and presses a push-button (that activates the Arduino, that runs a sketch, that checks the GPS, and operates a servo to release the lock).

It's all working, but I am trying to build in a backdoor, so I can unlock the box when I need to.

I have made the Arduino USB port accessible, and my hope is that I could connect a PC, and execute the appropriate command to operate the servo to unlock the box. The command is:

servo.write(OPEN_ANGLE);

The sketch that drives the whole puzzle depends on these libraries, but I am guessing the only one needed to operate the servo is PWMServo.h:

#include <PWMServo.h>
#include <SoftwareSerial.h>
#include <TinyGPS.h>
#include <EEPROM.h>
#include <LiquidCrystal.h>

I would like to be able to operate the servo without altering/replacing the uploaded sketch.

Is it possible to execute code "command prompt" style? Or can I somehow detect from within the sketch code that there is a USB connection, and have that trigger the unlock?

Finally (not the main question, but I would appreciate clarification), I have been told that the power to the Arduino should be disconnected before uploading new sketches (not sure why or if this is true). If forcing an unlock via the backdoor required uploading a new sketch, I seem to have a problem... For the puzzle to work within the locked box, the battery must be permanently connected, and also connecting to a PC via the USB seems to immediately power up the Arduino too. Does this mean uploading a new sketch would be impossible?

Answer 1

The simple thing would be for your sketch to read from the serial port continuously. If it finds something it stores it and when a newline arrives it sees if that happens to be a password known only to you. So, for example, all you would have to do is plug in the USB cable, activate the Serial Monitor on your PC, type swordfish and the servo opens the box.

You would need to allow for garbage arriving, so make sure that you discard input if it exceeds the size of the buffer you allocate for this purpose.

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I have been told that the power to the Arduino should be disconnected before uploading new sketches (not sure why or if this is true).

I don't see how that can be true as you have to reapply power to upload the sketch. I never do that.

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Is it possible to execute code "command prompt" style?

No, you upload compiled code.

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Or can I somehow detect from within the sketch code that there is a USB connection, and have that trigger the unlock?

Then all the person you give the box to has to notice the USB port, plug something in, and it unlocks. I think not.

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An even simpler solution would be to have two locations that unlock the box. For example, your kitchen could be one, so to unlock it you just take the box to your kitchen. The other location would be the "real" one that you want your friend to have to find. All the stuff about "you are 200 km from your destination" would of course refer to the real destination. You just make the code check that if it happens to find itself in your kitchen it unlocks the box.