A safe workspace

This course only uses low-voltage DC. "Low voltage" means a few volts: the 5 volts from a USB port, the 3.3 and 5 volt pins on the boards, and battery packs of AA cells. Three fresh AA cells give about 4.5 volts. Later you may add a bench power supply turned down low. DC, direct current, means the electricity always pushes the same way. A wall socket gives AC, alternating current, which flips direction many times every second.

Mains electricity is off limits, with no exceptions. Wall sockets give 100 to 240 volts, depending on your country. That is enough to kill. In this course you never wire anything to a wall socket. You never open up a phone charger or an appliance. Some parts you can buy, such as relay boards, are labelled for mains voltages. Here they only ever switch low-voltage things. Wiring them to mains is a job for a qualified electrician.

Low voltage can still hurt you, just in a different way. Your dry skin resists electricity strongly. A few volts can't push enough current through it for you to even feel. That is why touching a 5 volt wire is safe. Chapter 2 explains this with numbers. The real danger is a short circuit. That is a path with almost nothing in the way, such as a wire laid straight across a battery's two ends. A battery can push a huge current through it. The wire gets hot, the plastic melts, and it can burn your fingers or start a fire. At low voltage, worry about heat, not shocks.

This course is limited to low-voltage DC: steady, one-direction voltages of a few volts, from USB (5 V), the boards' own 3.3 V and 5 V pins, AA battery packs (about 4.5 V for three fresh cells), and later an optional bench supply set to a low voltage. DC (direct current) means the voltage keeps one direction; the wall socket's AC (alternating current) reverses many times a second.

Mains is out of scope, without exception. Wall sockets supply 100 to 240 V AC, depending on the country, and can drive a lethal current through a body. Nothing in this course connects to mains, opens a power adapter or appliance, or switches a mains load. Some parts are sold for exactly that job, such as relay modules and solid-state relays labelled for mains voltages. In this course they switch low-voltage loads only; wiring them to mains is a qualified electrician's work.

Low voltage is not the same as harmless. The skin's resistance keeps the current from a few volts through a dry hand far too small to feel, which is why touching a 5 V rail is safe. Chapter 2 §5 shows why with numbers. But a battery or supply can push a very large current through a short circuit, a path with almost no resistance, such as a wire straight across its terminals. That current heats wires, melts insulation, and can burn fingers or start a fire. The danger at low voltage is heat, not shock.

When you walk across a carpet or pull off a jumper, your body picks up an electric charge. Touch something metal and it jumps off as a tiny spark. That spark is called an electrostatic discharge, or ESD for short. Its voltage can reach thousands of volts. But there is so little charge behind it that you usually can't feel it at all.

Chips are fragile inside. Their pins lead to parts with insulating layers far thinner than a hair. A spark too small for you to feel can still punch through one. The board might die at once, or start acting oddly weeks later.

Three habits protect your boards:

1. Discharge yourself first. Touch a grounded metal object, or an anti-static mat that is connected to ground, before you pick up a board.
2. Hold boards by the edges. Grab the sides and the plugs, not the chips or the pins.
3. Store boards in anti-static bags. They usually arrive in one. Put them back in it when you are done.

A wrist strap that clips to a grounded mat is a good upgrade. A proper strap has a resistor hidden in its cord, so any charge drains away slowly and gently. Never make your own out of plain wire.

Walking across a carpet or pulling off a jumper builds an electric charge on your body. Touching a conductor lets it out as a tiny spark, an electrostatic discharge (ESD). The voltage can be in the thousands of volts, but the charge is so small that most sparks are too weak to feel. A chip's pins connect to transistors with very thin insulating layers, and a spark you never noticed can weaken or destroy one. The board may fail at once, or work unreliably later.

Three habits cover a learner's bench:

1. Discharge first. Touch a grounded metal object, or an ESD mat connected to ground, before handling a board.
2. Handle by the edges. Hold boards by their sides and connectors, not by chips or pins.
3. Store in anti-static bags. Boards and sensors ship in them; keep them there when not in use.

A wrist strap clipped to a grounded ESD mat is the next step up. A proper strap has a resistor built into its cord so that any discharge is slow and gentle; never improvise one with a bare wire.

Lithium cells pack a lot of energy into a small space. Examples are the round 18650 cell and the flat, soft LiPo pack. If one is shorted, overcharged, punctured or crushed, it can heat up fast, leak burning gas and catch fire. You don't need one for this course. If you add one later, follow three rules:

• Use a protected cell or pack. It has a small circuit that cuts the power if the cell is shorted or run too low.
• Charge it only with a charger board made for that kind of cell. Never wire it straight to a power supply.
• Never leave it charging alone. Never use one that is puffed up, dented or warm.

Loose cells with no brand, or cells pulled out of an old laptop, are off limits. Take damaged cells to a battery recycling point. Never put them in the bin.

Soldering joins metal parts with melted metal. The tip of the iron is hotter than a frying pan, by hundreds of degrees. So it goes back in its stand every time you put it down. The smoke comes mostly from flux, a cleaning chemical inside the solder wire. Open a window or use a fan to pull the smoke away from your face. Wash your hands afterwards, especially if your solder contains lead. Wear safety glasses when you cut wires, because the clipped ends fly off. Most labs in this course need no soldering. The one likely job is fixing pins onto a small sensor board.

Lithium cells (round 18650 cells, flat LiPo pouches) store a lot of energy in a small space. Shorted, overcharged, punctured or crushed, they can overheat rapidly, vent flammable gas, and catch fire. They appear in this course only as an optional later part, and only on these terms:

• a protected cell, or a battery pack with a protection circuit, which cuts the cell off if it is shorted or drained too far;
• charged only by a charger board made for that cell type, never directly from a supply;
• never left charging unattended, and never used if it is swollen, dented or hot.

Loose, unbranded cells and salvaged laptop cells are out of scope. Recycle damaged cells through a battery collection point, not the bin.

Soldering joins parts with molten metal. An iron's tip runs at several hundred degrees Celsius, so it lives in its stand whenever it is not in your hand. The smoke is mostly from flux, the cleaning chemical inside the solder wire; work with a window open or a fan pulling fumes away from your face. Wash your hands after soldering, and always if the solder contains lead. Wear safety glasses while trimming leads, because clipped wire ends fly. Most labs in this course need no soldering at all, except perhaps to fit header pins to a sensor board.