Power, Safety and Building Your First Robot

Nine chapters have designed the parts of a robot one at a time: the frame (Robotics Fundamentals 2), the motors and wheels (Robotics Fundamentals 3), the drive and its kinematics (Robotics Fundamentals 4), the sensors (Robotics Fundamentals 5), position and feedback (Robotics Fundamentals 6), control (Robotics Fundamentals 7), behaviour (Robotics Fundamentals 8) and arms (Robotics Fundamentals 9). None of it runs without a power system, and none of it should be switched on near people without safety built in. This final chapter does both, then brings everything together. It covers the batteries (chemistry, capacity, voltage sag, runtime), the power budget and the choice of regulator, the low-voltage cutoff and how to charge and store a LiPo pack without starting a fire, the wiring (wire gauge, voltage drop, connectors, fuses, a main switch, a common ground, a star layout), the emergency stop and the habits of safe testing, and a short look at the ethics of building machines that act in the world. Then comes the build: a complete plan for a small two-wheel line-following, obstacle-avoiding robot, with a specification, a parts list, design calculations that reuse every earlier chapter, a block diagram, a wiring table, an assembly order, a test plan with expected readings, and a troubleshooting table. The chapter ends with a recap of the whole course and the path on to Programming for Robotics.

The same caution applies to safety. This chapter teaches habits and arithmetic for hobby-scale robots (batteries of a few watt-hours, motors of a few watts). It is not a safety certification and does not replace the manufacturer's instructions for a charger, a battery or a motor, or the law for anything you sell or use at work. Where lithium batteries are concerned, the manufacturer's instructions win over this chapter.

1. Batteries: the chemistries

A robot's battery decides how long it works, how fast it can start, how much it weighs and how dangerous it is. Basic Electronics 9 section 8 introduced the four numbers that describe any battery (nominal voltage, capacity, maximum discharge current expressed as a C-rate, and internal resistance) and the main cell types. Here is the comparison again for robot use, with lead-acid added because larger robots use it.

The energy-per-kg figures are typical orders of magnitude from memory and vary a lot between products; the pattern is what matters. Weight is the robot's enemy: every gram of battery has to be accelerated, carried up slopes and braked (Robotics Fundamentals 3 section 6), so a small mobile robot usually wants lithium, while a large slow one can afford lead-acid, and a beginner who wants forgiveness takes NiMH. The build in section 9 uses a 2S LiPo pack to show the careful handling it needs, and notes a NiMH alternative.

2. Sizing a battery: capacity, C-rate, sag and runtime

Four calculations size a battery, and they are the ones from Basic Electronics 9 and Robotics Fundamentals 1 section 5 written in one place. Energy is voltage times capacity. Runtime is usable energy divided by average power. The C-rate checks that the pack can supply the peak current. The voltage sag checks what the robot sees at the moment of a start or a stall, which is what resets the board. A simple derating then covers the real-world losses without needing the Peukert exponent (a refinement for lead-acid that this chapter leaves out).

Two more points about runtime. First, a pack that is quoted at a high C-rate is not the same as a pack that is good at that rate: look at the internal resistance and test the sag. Second, the capacity on the label is measured at a gentle current at room temperature; a heavy load, a cold day and an old pack each take a share, which is why the simple multipliers above exist. Measure the real runtime once the robot works, and keep that figure rather than the calculated one.

3. The power budget: average against peak

A power budget lists every load, with an average current (which sets the runtime) and a peak current (which sets the size of the regulator, the wire, the fuse and the pack's C-rate). Basic Electronics 11 section 2 built one for a phone-driven robot; here is the full budget for the robot of section 9. The motors use the sizing recipe of Robotics Fundamentals 3 section 6 (a 0.8 kg robot, 0.3 m/s cruise, the typical 6 V gearmotor of that chapter: 2.0 Ω, k = 0.006, I₀ = 0.15 A, 50:1 gearbox of efficiency 0.729). The currents of the sensors and the board are typical figures from memory; replace them with the data sheets of your parts.

4. Regulators: buck against linear

The motors run straight from the pack through the driver, but the board and the sensors need a steady 5 V (or 3.3 V). Basic Electronics 9 section 9 compared the two ways of making it. A linear regulator drops the surplus voltage as heat: it passes the load current unchanged, so the pack supplies the same current as the load and the wasted power is (V_in − V_out) I. A buck (switching) converter converts power, not current: it draws P_in = P_out / η with a typical efficiency η of 85 to 95 %, so from a higher voltage it draws proportionally less current. The price of the buck is switching noise and a slightly bigger part.

The rule of thumb that follows: use a buck converter for any rail that carries more than about 100 to 200 mA from a pack well above the rail voltage; use a linear (LDO) regulator where the current is tiny, where the input is only a little above the output, or where a very quiet supply matters (a sensitive analogue sensor). A common good design is a buck to a rail a little above the target, then a small LDO for the quiet part. Keep the buck's switching noise away from the line and distance sensors (shorter wires, a ground plane or a clean star, and a filter capacitor), and choose a converter rated for the peak rail current with margin: 215 mA is 11 % of a 2 A module.

5. Low-voltage cutoff, charging and storing a LiPo pack

The low-voltage cutoff

A lithium cell that is run flat is damaged (Basic Electronics 9 section 8: never below 3.0 V per cell, and 3.3 V per cell under load is a sensible cutoff). The robot must watch its own battery and stop before that. The usual circuit is the voltage divider of Basic Electronics 10 section 4 feeding an analogue input, and the usual logic is a state of the behaviour machine (Robotics Fundamentals 8 section 2): a warning threshold, a cutoff threshold, and a debounce so that the voltage dip of one start does not trigger it. Read the pack at rest or smooth the reading (Robotics Fundamentals 5 section 9), because the reading under load sags by I R.

Charging and storing a LiPo pack safely

Charging is the most dangerous thing most hobbyists do with a battery. A lithium cell charged beyond 4.20 V, charged after a deep discharge, charged while damaged, or charged without balancing can overheat, swell and burn, and a LiPo fire is hot and hard to put out. These rules are not optional.

• Never leave a charging pack unattended. Stay in the room, within sight, and stop at the first sign of swelling, a hot smell, hissing or heat. Not overnight, not while you are out, not “just for a minute” in another room.
• Use a proper lithium charger with the right cell count (a balance charger for a LiPo pack with a balance connector), set to the right chemistry and cell count, at a current of no more than 1C (1.0 A for the 1000 mAh pack; gentler is safer). Do not charge with a bench supply set to “about right” and do not use a charger meant for another chemistry.
• Charge on a non-flammable surface (a ceramic tile, a metal tray or a fire-resistant LiPo bag), away from paper, curtains, sofas and beds, with a clear space around it, and keep a suitable extinguisher or a bucket of dry sand within reach and a clear way out (ask your local fire service which extinguisher suits lithium battery fires).
• Never charge a damaged, swollen, punctured, crushed or very old pack, or one that was deeply discharged (below about 3.0 V per cell) and has sat that way. Retire it: discharge it slowly with a bulb, and take it to a battery recycling point, not the household bin.
• Let a warm pack cool before charging, and let a freshly charged pack cool before use; never charge a pack that is hot to the touch.
• Store at about 3.8 V per cell (7.6 V for a 2S pack) if it will not be used for more than a week, in a fire-resistant bag or box, in a cool place, not in the robot. Many balance chargers have a “storage” mode that does this. A pack stored full or empty ages quickly.
• Transport and handling. Protect the leads from shorting (cap the connector or tape it), do not carry loose packs with keys or coins, and do not leave one in a hot car or in direct sun.

If a LiPo pack starts to swell, hiss or smoke: stop charging if you can do so safely, move away, and do not try to carry it through the house; follow the guidance of the battery's maker and the fire service. The robot of section 9 runs from a pack that is small (7.4 Wh), which limits the energy but not the need to treat it with respect; the NiMH alternative in the parts list is the gentler choice for a first build.

6. Wiring and fusing

Wire gauge: heating and voltage drop

Wire is chosen for two things: the heat it can carry (its current rating, set by the insulation and the length of the run) and the voltage lost over its length (Basic Electronics 9 section 12). The round-trip length counts, because the current flows out and back. Voltage drop is V = I R_wire and the power lost as heat is P = I² R_wire.

Connectors, the switch and the ground

• Connectors. Choose a connector rated above the current it carries, and keyed so it cannot go in backwards. Typical maker's ratings (approximate, from memory; always read the data sheet): Dupont and breadboard contacts about 1 A (signals only); JST-XH about 3 A (balance leads, small loads); XT30 about 30 A; XT60 about 60 A. The build uses an XT30 for the pack and screw terminals or solder for the motor supply. Crimp and solder properly: a poor crimp has a high resistance, heats up, and is a classic cause of “it only fails when the motors work hard”.
• The main switch. Put a switch in the positive lead, after the fuse, rated for the stall current at the pack voltage in DC (a DC rating is lower than the AC rating printed on many rocker switches: a switch marked 6 A at 250 V AC may be rated only a fraction of that at DC). A 5 A DC-rated switch is the minimum for the 5.03 A stall of the build.
• A common ground. Every part that exchanges a signal must share a ground, or the signal has no reference. But the way the grounds are joined matters (below). The motor current's return path must not share a thin wire with a sensor's reference.

Fuses

A fuse protects the wire and the battery from a short or an overload; it does not protect the load. Two limits fix its size: it must be above the normal current (so that it never blows during ordinary work) and below the rating of the thinnest wire it protects (so that it blows before the wire overheats). The usual rule is to choose the nearest standard value at 125 to 150 % of the normal (largest sustained) current, then check it against the wire rating and against the start-up and stall transients. Place it in the positive lead as close to the battery as possible, so that only a few centimetres of wire are unprotected; use a fuse with a voltage rating at or above the pack voltage; and, for a lithium pack, a fast-acting or a slow-blow type chosen to tolerate the short start-up peaks (Basic Electronics 9 section 12 advised a fast-blow type close to the positive terminal, and warned that a resettable polyfuse is not enough on its own against a strong lithium pack).

Star distribution, strain relief and separate supplies

The motors switch large, noisy currents, and a board shares the same pack. If the motor current and the board's current flow through the same stretch of wire, the motor's voltage drops and spikes show up on the board's supply and on its ground, and the board resets or reads nonsense (the brown-out and grounding of Basic Electronics 9 sections 10 and 11). The cure is a star: every branch has its own wires from one central point, so the branches do not share resistance.

Strain relief means that a pull on a wire is taken by something other than the joint. A solder joint or a screw terminal fails when it is flexed or pulled, and the failure is intermittent and hard to find. Clamp each wire bundle to the frame with a tie or a clip a few centimetres from each connector so that the wire, not the joint, takes the strain; leave a small loop of slack; use stranded wire (not solid) anywhere that moves; and keep wires away from wheels, gears and moving joints. A tidy loom is also a safety feature: loose wires catch on wheels and short against each other.

For the logic supply, separate the motor and logic supplies whenever the motors are large enough to disturb the pack: a dedicated buck to the logic, the motors fed straight from the star, bulk capacitors at the driver (Basic Electronics 9 section 5 on servos made the same point with a separate servo supply, and Basic Electronics 10 section 8 on driving a motor from a board safely). In a small robot like the build, one pack feeds both through a star, which works; on a larger robot, or one with servos or a gripper (Robotics Fundamentals 9), give the motors their own pack or converter and join only the grounds.

7. E-stops, safe testing and fail-safe design

A robot can fail in ways the author did not imagine: a program freezes, a sensor reads nonsense, a wire comes loose, the floor is slippery, a person steps into the path. Safety is the set of habits and hardware that make the failure harmless. The central question is not “will it fail?” but “when it does, what does it do?”

Hardware kill switch against software stop

The lesson of the table is that the emergency stop must not depend on the thing that might be broken. A software button that tells the program to stop is not an emergency stop; a mushroom button that opens the motor supply is. Use all four layers: the software handles ordinary stops, the watchdog catches a frozen program, the driver and fuse limit a stall, and the hardware e-stop handles everything else.

Fail-safe design principles

• Fail to the safe state. Decide what “safe” means for each failure (Robotics Fundamentals 8 section 7): for a small wheeled robot it is motors off. For an arm holding a load, or a robot on a slope, it may be “held”, not “released” (Robotics Fundamentals 9 section 10: a brake or a non-back-drivable gearbox).
• The safe state is what the hardware does when power or signal is lost, not only what software chooses. A normally-closed e-stop opens when pressed or when its wire breaks. Pull-down resistors (10 kΩ) on the driver's standby and PWM pins keep the motors off while the board is starting up or has reset, when its pins float.
• Stop on silence. Treat the absence of a good signal as a fault: a distance sensor that reports nothing, an encoder that stops counting while the motors are driven (a stall or a broken wire), a command link that goes quiet. A stall check, “duty above 30 % and the wheel speed under 10 % of the wanted speed for 0.3 s: cut the motors”, catches a jammed wheel long before the fuse does.
• Limit everything. Cap the duty, the speed, the acceleration and the current so that no single bug can command the maximum. A lower limit costs a little performance and removes a whole class of accidents.
• Make the safe action the easy one. The e-stop is big, red, reachable without leaning over the robot, and needs a deliberate action (the start button) to run again: releasing the stop must not restart the motors by itself.
• Test the stops. A stop that has never been tested may not work. Press the e-stop, freeze the program, pull a sensor lead and flatten the battery on the bench, on purpose, before you trust them (the tests of section 9).

Safe testing, in order

1. Power off: look first. Check the polarity, the connectors, the fuse and for any loose strand or bare metal (continuity and short checks, section 9).
2. Wheels off the ground. Put the robot on a block so that the wheels spin freely. Run every motor, the encoders, the sensors and the stops with nothing to run into. A bench supply with a current limit of about 0.5 A in place of the pack makes the first power-on gentle: a short or a stall folds the supply back instead of the pack burning.
3. Speed-limit the first floor runs. Cap the duty at about 25 %: from the motor model, 25 % of 7.4 V is 1.85 V, and (1.85 − 0.17 × 2.0) / 0.006 = 252 rad/s at the motor, which is about 0.16 m/s at the wheels. Raise the cap in steps only after each step behaves.
4. Clear the area. A flat, clear floor, no people or pets in the way, a hand within reach of the e-stop, and a stopping distance (below) known and respected.
5. Add one thing at a time. Straight line, then turns, then the line, then the obstacle stop; each test passes before the next begins (Robotics Fundamentals 8 section 7: test each behaviour alone).
6. Log and review. Record the state changes and the battery voltage; a log of what the robot did is the only witness to a failure.

Stopping distance, with the numbers of the build

The stopping arithmetic of Robotics Fundamentals 1 section 2 (reaction distance plus braking distance) and Robotics Fundamentals 8 section 7 fixes how early the robot must act. For the build:

Arms and other pinch points

Anything with joints can trap a finger. If you extend the build with an arm (section 9, extensions), apply Robotics Fundamentals 9 section 10: size each joint for the payload at its worst pose, find the pinch points (the inside of an elbow, a gripper's jaws), cap speed and force, decide how each joint holds its load when power is cut, and test slowly with everyone outside the reach. A wheel is a pinch point too: keep fingers and cables away from the gap between a wheel and the chassis when the robot is powered.

8. Ethics and responsibility

A robot is a machine that acts in the world, and what it does is the builder's responsibility. Four points, briefly.

• Privacy. A camera, a microphone or a robot that moves about a home records people, often without their thinking about it. Decide what is recorded, where it is stored, who can see it, and tell the people it concerns; do not point a robot at places where people expect privacy, and treat recorded images and sound as sensitive data (in the UK, data protection law can apply to cameras that capture people outside a purely private setting; take proper advice for anything beyond your own home).
• Safety of people and pets. A small robot can trip, pinch or frighten. Keep it slow, lightweight and soft-edged where it shares a room; keep fingers, hair, pet tails and small children away from wheels and arms; fit the stops of section 7; and do not run an untested robot unattended, least of all with a lithium battery on board.
• Dual use. Most robotics ideas can be used to help or to harm: the same navigation and vision serves a delivery robot and a weapon. Think about how what you build could be misused, share it responsibly, and be wary of work whose evident purpose is to hurt people or to watch them without consent.
• Honest claims. Say what the robot actually does, and what it was tested for. “Runs about 1.5 hours” is true if you measured it; “avoids all obstacles” is false if a black chair leg beats the sensor. Overclaiming about autonomy, safety or accuracy misleads the people who rely on it, and a robot sold or used at work has legal obligations (safety marking, product liability) that a hobby project does not.

9. The build: a small line-following, obstacle-avoiding robot

The design below is a plan for the simplest robot that uses every subsystem of the course: two driven wheels and a caster (differential drive, Robotics Fundamentals 4), geared motors with encoders (Robotics Fundamentals 3 and Robotics Fundamentals 6), a line sensor array and an ultrasonic sensor (Robotics Fundamentals 5), a feedback loop for the wheels and the line (Robotics Fundamentals 7), a behaviour state machine (Robotics Fundamentals 8), and the power and safety of this chapter. It is hardware-neutral: any board with the right pins will do (Basic Electronics 11 section 1 compares them), and the code is described in pseudocode only. Real board code, in MicroPython or C, is the work of the planned course Programming for Robotics.

9.1 Requirements and specification

The maximum speed is 0.4 m/s and the line-following base speed is 0.30 m/s, slowed on bends by the rule v = v_max / (1 + c|x|) (Robotics Fundamentals 8 section 3). The build is intentionally modest: a robot that meets this table is a real, working machine, and every number in it can be checked.

9.2 Parts list (approximate prices in GBP)

Prices vary by supplier, and these are rough ranges from memory. A cheaper kit (yellow TT-type motors without encoders) will not match the calculations below: those motors are bigger and slower and have no feedback, which changes the sizing, the odometry and the test results. The calculations are for the parts listed.

9.3 Design calculations (reusing the course)

9.4 Block diagram

9.5 Wiring table, pin by pin

The table gives each connection first in hardware-neutral terms (what it needs) and then as an example for one named board, the Raspberry Pi Pico. The GP numbers are the Pico's GPIO numbers, not physical pin numbers, and are an example from memory: check every pin against your own board's pinout diagram (some pins have fixed roles, and on the Pico W the on-board LED is wired differently).

With an Arduino Uno (5 V logic) the same plan works, with changes: the Uno has only two external-interrupt pins (D2 and D3), so each encoder's channel A goes to one of them and channel B to an ordinary pin (or use pin-change interrupts); PWM comes from pins 3, 5, 6, 9, 10 or 11; the echo divider is not needed; the battery divider can use A0 (a 5 V reference gives 3.1 V at 8.4 V, still safe); and the line module can run from 5 V. Check the pin capabilities against Basic Electronics 10 section 1.

9.6 Assembly order

1. Build the frame. Fit the motors, wheels and caster to the chassis; check that the robot sits level and that the centre of mass is over the wheels and not far behind them (Robotics Fundamentals 2 section 3); measure the real track width and wheel diameter and write them down.
2. Mount the battery and the star board. Fix the pack low and central with a strap, put the fuse holder, the main switch and the e-stop where they can be reached, and lay out the star board. Do not connect the battery yet.
3. Wire the power side with the battery disconnected: the fuse, the switch, the e-stop, the buck module and the divider; strain-relief each bundle. Keep the fuse out until the continuity checks of section 9.8 pass.
4. Mount the board and the driver; wire the driver's motor outputs, its logic pins and its pull-downs; solder the motor wires (never through a breadboard).
5. Add the encoders, then the line module and the ultrasonic sensor with its echo divider. Mount the line module 3 to 8 mm above the floor and about 40 mm ahead of the axle; mount the ultrasonic sensor at the front, pointing forward and level.
6. Run the power-off tests (continuity, shorts, resistance of the motors), then the voltage tests with the fuse in and the wheels on a block (section 9.8, tests 1 and 2).
7. Test the pieces in order: encoders, then the driver, the stops, the line module and the ultrasonic sensor, each alone, with the wheels in the air, before any floor test.
8. Then the floor: speed-limited and with a clear floor, then straight line and spin calibration, then the line, then the obstacle test, raising the speed cap in steps.

9.7 The top-level control loop (hardware-neutral pseudocode)

Only the top-level loop is shown, in short Python-style pseudocode. The function names stand for what your board's library calls them; the odometry update, the line position, the PID and the state machine are the code of Robotics Fundamentals 4, Robotics Fundamentals 5, Robotics Fundamentals 7 and Robotics Fundamentals 8. The pseudocode was run with simple stand-in functions to check that the structure holds together.

9.8 Test plan with expected readings

Each test has an expected reading (a number you can compare with your meter or your log) and a pass condition. Run them in order, and do not move to the next until the last passes. The expected values come from the calculations above and are approximate; real parts differ, and a reading that is merely different from the figure is not necessarily a fault, while a reading that is wildly different (or a short) always is.

9.9 Troubleshooting

9.10 Extension ideas

• Add a gyro (Robotics Fundamentals 6 section 4) and fuse it with the wheel heading by a complementary filter, for a drift-free straight line and an accurate spin.
• Replace the ultrasonic sensor by a time-of-flight sensor (Robotics Fundamentals 5 section 3) for a narrower, faster reading, or add side sensors for wall following (Robotics Fundamentals 8 section 3).
• Sense the motor current (Robotics Fundamentals 5 section 7) to detect a stall directly and to measure the real power budget.
• Log and map: record the pose and the sensor readings, then build an occupancy grid and plan a path with A* (Robotics Fundamentals 8 section 4) as a second program.
• Redesign the frame as a 3D-printed or laser-cut part (Robotics Fundamentals 2 section 4), with a proper battery tray, a protected e-stop and a bumper.
• Add a small servo arm (Robotics Fundamentals 9) with a separate servo supply and the arm safety rules of that chapter, and re-budget the power and the centre of mass.
• Telemetry: a Pico W or ESP32 can stream the log over Wi-Fi so that you can watch the battery, the state and the PID terms live.

10. Course complete: a recap of Chapters 1 to 10

You can now reason about a small robot from the battery to the behaviour: why a motor was chosen, what the wheels do, what a sensor can and cannot tell you, why a loop oscillates, and what makes a machine safe to run. The calculations are all things you can repeat with a pencil, and the build above is a way to find out how much of it is true on your own bench.

The path on: Programming for Robotics, and possible later courses

Programming for Robotics (planned; not yet written) is the next course on this site, and it is the other half of this one. Robotics Fundamentals has kept the code hardware-neutral and short on purpose. Programming for Robotics will turn the same ideas into real code on real boards: reading sensors and driving motors, timing and interrupts for encoders, the PID loop and the state machine as programs, odometry and logging, and the top-level loop of section 9.7 written in MicroPython or C. If you build the robot of this chapter, that course is where its control loop becomes running code.

Possible later robotics courses, mentioned only as ideas and not planned: project-style courses whose topic is building a robot, perhaps around a commercial kit, in the manner of this chapter's build plan but carried through to a finished and tested machine. If those appear, they will build on the habits of this chapter: size it, budget it, fuse it, test it in order, and never trust a stop that you have not tested.

11. Challenges

That completes Robotics Fundamentals. The next step on this site is Programming for Robotics (planned), where the loops, the filters, the PID and the state machine of these chapters become code that runs on a board.