Debugging Common Issues in Robot Code

The Robot Is Usually Telling the Truth

When a robot misbehaves, our first instinct is to open the editor and start rewriting code. It is almost always the wrong place to start. Before you change a single line, treat the hardware as the prime suspect and prove it is innocent. A robot that resets the moment its motors spin up is not suffering a logic bug; it is browning out because the battery cannot supply the surge current. A sensor that reads 1023 constantly is not returning "impossible data"; it is likely sitting on a pin with nothing connected to it.

Check the boring things first. Are the grounds of your motor supply and your logic supply actually joined? Are the jumper leads seated properly, or have the breadboard contacts loosened after a few plug-and-pull cycles? Is your USB cable a data cable, or one of the charge-only ones that came free with a phone? Most "the code stopped working" evenings end with a reseated lead and a fresh set of batteries.

When Motors Twitch, Stutter, or Run Backwards

Twitching is often a signal problem rather than a code problem. A control pin left floating will pick up noise from the motors next to it, so make sure unused inputs are tied to a defined level. If the motor twitches but never turns, your pulse-width modulation duty cycle may simply be too low to overcome the motor's internal friction and gearbox resistance. Most driver boards have a minimum duty cycle below which nothing happens at all, then a sudden lurch above it.

If a motor runs the wrong way, fix it in exactly one place. Either swap the two motor output wires, or invert the direction in code. Beginners who do both end up with a robot that reverses every time they touch it.

Servo jitter has its own usual cause: the servo is drawing its current through the board's regulator, which cannot cope with the spikes. Give the servo its own supply and join the grounds.

Let Serial Output Be Your Torch

Serial printing is the cheapest debugging tool you own, and beginners use it far too sparingly. Print early, print often, and print raw values rather than your interpretation of them. If your code decides a wall is close, print the raw distance reading in the same line as the decision. You will instantly see whether the sensor is wrong or your threshold is.

• Gibberish in the monitor? Your baud rate does not match the one in your code. Check both ends.
• Nothing at all? You may have selected the wrong port, or the upload silently failed.
• Print a boot message. A single "starting up" line at the top of your setup routine confirms the board is genuinely running your latest version — not yesterday's.

Printing a marker inside each branch of a decision is also a quick way to discover that a condition you assumed was true never actually fires.

Impossible Sensor Values and Variable Ranges

When a sensor returns something that should be impossible, ask what the number would mean if the pin were simply disconnected. A constant maximum reading usually means a floating input. A constant zero usually means a short to ground or no power reaching the sensor. Values that jump wildly with no pattern suggest noise, a shared supply, or a wire running alongside a motor cable.

Then look at your variable types. A value that should be between 0 and 1023 turning up as a huge number is the classic signature of unsigned arithmetic wrapping around when it goes below zero. Integer division quietly discarding remainders catches people out too: dividing by a whole number and expecting a decimal will not give you one unless you use a floating-point type.

Overflow is the other quiet killer. Multiplying two large integers before assigning the result can wrap around long before the answer reaches your variable. Cast one operand to a longer type first. And keep your units consistent — mixing degrees with radians, or centimetres with arbitrary sensor ticks, produces numbers that look plausible and are completely wrong.

Change One Thing at a Time

The single most common debugging mistake is changing four things at once and then having no idea which one helped. Make one adjustment, test, and note the result. Keep a paper log by the bench: the time, what you changed, and what happened. It sounds fussy until the third evening on the same fault, when the log tells you which combinations you have already ruled out.

Comment code out rather than deleting it, and build a tiny test routine that does one job — blink an indicator, then drive one motor, then read one sensor. If a fault is intermittent, suspect power or wiring rather than software. Deterministic code fails deterministically.

A Short Pre-Flight Checklist

• Battery voltage checked under load, not at rest.
• All grounds joined, all leads reseated.
• Baud rate matched between code and monitor.
• Raw sensor values printed alongside computed ones.
• Only one variable changed since the last successful test.

Work through that list in order and you will solve most stubborn robot faults in ten minutes rather than an evening. And put the kettle on — the answer often arrives while you are waiting for it to boil.