Why plan a stop before building the rest
Any machine that moves can cause harm if something goes wrong: a wheel can run over a foot, an arm can pinch a hand, a spinning part can catch hair or clothing. Software bugs, sensor failures, radio dropouts and simple mistakes are all normal events in hobby projects. An emergency stop is a way to bring the machine to a safe state quickly when something does go wrong, and the time to design it is at the start, not after the first scare.
This article is general guidance for hobby-scale machines. It is not a substitute for the requirements that apply to commercial equipment, workplaces or public use, which are set by standards and local rules. If a machine could seriously injure someone, take professional advice.
Do not rely on software alone
The most common mistake is making the stop button just another input that the program reads. If the program has crashed, locked up or been given a bad command, it may never notice the button. A dependable emergency stop acts on the power path, so it still works when the software does not.
A good design principle is to treat the controller as untrusted for the purpose of stopping. The stop should:
- Remove power from the motion hardware, not merely ask the program to stop.
- Work without any battery-powered logic being healthy.
- Be simple enough to understand and test by inspection.
Software-based stops still have a place as a first, gentler layer, for example a command that slows the machine to a halt, but they should sit alongside a hardware cut-off, not replace it.
Cutting power: common approaches
The details depend on the machine’s voltage and current, and this is where care is needed.
- A main switch in series with the motor supply. On a small, low-voltage battery robot, a robust switch or a contactor on the motor supply line may be all that is needed. The switch must be rated for the voltage and for the current it will interrupt, including stall currents, because switching heavy DC current can arc and wear contacts.
- A relay or contactor controlled by a stop circuit. The stop button opens the coil circuit of a relay that supplies the motors. Because the relay drops out when the coil loses power, a broken wire or a flat control battery also results in a stop.
- Disabling the motor driver. Many drivers have an enable input, which is a useful additional layer, but it is typically still a logic signal and should not be the only barrier.
- A removable link or plug. For a large machine, physically pulling a connector provides a clear, visible disconnection.
Fusing is a separate protection against overcurrent and does not replace a stop. Selecting the right wire gauge and fuse rating for the supply is part of hobby power planning.
Machines powered from the mains need particular care. Mains voltage can kill, and mains voltage safety is a job for a qualified electrician, not a hobbyist. For lithium-based packs, remember that lithium polymer cells can deliver very large fault currents, so protective measures need to be matched to the battery type and the manufacturer’s guidance.
Fail-safe thinking
A fail-safe design reaches the safe state when something breaks, not only when someone presses the button.
- Use normally closed contacts for the stop circuit, wired so that the machine runs only while the circuit is complete. Then a cut cable, a loose terminal or a failed contact stops the machine instead of leaving the button dead.
- Make the stop latching. Once pressed, the stop should stay engaged until someone deliberately resets it, so the machine does not restart by itself when the button is released.
- Keep the reset separate from the start. Resetting the stop circuit should not by itself make the machine move. A separate, deliberate action should be required to start motion.
- Think about startup states. Make sure the machine powers up with outputs off, so a reboot or brown-out does not cause sudden motion. Modelling machine behaviour as clear modes, as in state machines, helps define a safe default state.
- Consider a watchdog. A circuit or timer that removes power if the controller stops sending a regular “I’m alive” signal catches freezes that a button cannot.
Wireless and remote operation
If you drive the machine over radio or Wi-Fi, assume the link will drop at some point. The sensible default is that loss of signal stops the machine. A wireless stop button is convenient but should not be the only stop, since the radio itself can fail. A wired or local hardware stop remains important. Radio equipment is also subject to local rules and licensing, which vary.
Stored energy and coasting
Cutting power does not always mean immediate stillness.
- Motors and wheels coast, and a heavy machine can travel some distance before stopping. Brakes, or shorting motor terminals through a suitable driver arrangement, can shorten this, but the behaviour depends on the driver, so check its documentation.
- An arm can fall under its own weight when power is removed. Think about where it will end up, and whether a mechanical support, brake or counterweight is needed.
- Gearing that resists backdriving can hold loads, but should not be treated as a safety brake without testing.
- Springs, flywheels and charged capacitors can store energy after the switch is off.
Design the stop so that the unpowered state is the safe state.
Placement and keeping people clear
- Put the button where operators can reach it quickly, without reaching across the moving part.
- Use an obvious, large button. The widely used convention is a red mushroom-head button on a yellow background, which people recognise.
- Do not hide it behind guards or cables.
- Keep the machine’s reach clear. Mark the area where it can move, and keep bystanders, children and pets out while it is powered.
- Add guards over pinch points and spinning parts where practical.
- Never reach into a machine that has power connected, even if it looks idle. A machine that is waiting for a sensor trigger can move without warning.
A tidy, well-lit space helps, as in any home workshop setup, and protective gear is sensible when testing, in line with workshop protective gear guidance.
Test it, and keep testing
An untested emergency stop is only a hope.
- Test with the machine lifted or restrained before trying it on the floor.
- Test it under load, and at the highest speed the machine can reach.
- Deliberately break things: disconnect the control battery, unplug a wire, crash the program, and see whether the machine still stops.
- Check the reset behaviour: confirm it does not restart on its own.
- Repeat periodically. Contacts wear, connections loosen and modifications change behaviour.
Write the test steps down. A short checklist you actually follow beats a good intention.
A realistic mindset
No stop system removes all risk, and a hobbyist cannot match the engineering behind certified safety equipment. The aim is to make the common failures end in a stop, to keep people out of harm’s way, and to be honest about what a project can safely do. If you are unsure whether a machine is safe to run, keep it small, slow and lightweight until you are.