Every machine we ever made began the same way: something had to push. We spent ages taming that push into small, labeled boxes. Read the labels. The push is yours now.
⟨ transmission ends ⟩Every circuit so far ran on one 9 V battery. By the end of this page you will pick the right power source for anything you build: batteries, a USB socket, a wall adapter, or the bench supply, read each one's label, and know the safety rules that come with them. One rule runs through it all. A power source promises two numbers: a voltage it holds, and the most current it can give. The load decides how much actually flows.
The bench below has a wall adapter, the black box that plugs into an outlet and makes a small steady voltage, labeled 9 V DC, 2 A MAX. Beside it sit 4 devices that each take a different amount: the LED loop from First Light, a desk fan, a heater, and a blower motor. The adapter never pushes its 2 A into anything. Each device draws what it draws, and the label's 2 A is only the most the adapter can honestly supply. Plug it into each device and watch the numbers.
A regulated adapter: it holds 9.00 V from no load up to its ceiling. The devices are modeled as fixed resistances: the LED loop is First Light's 470 Ω and red LED (15 mA), the fan 45 Ω (0.20 A), the heater 6.9 Ω (1.30 A), the blower 3 Ω, which asks for 3 A. This adapter answers an overload by limiting at 2.2 A and letting its voltage sag to 6.6 V while it heats; many real bricks cut out and retry instead. Either way, past the ceiling nothing good happens.
The heater drew amps where the LED loop drew thousandths of one, from the same adapter, and it held 9 V for both. Only the blower, asking past the ceiling, broke the promise. That is the whole contract, and it is the same for every source on this page. Now the oldest sources on the shelf: batteries. A single cell's voltage is fixed by the chemistry inside it, and cells stacked nose to tail add their voltages. The holder below feeds the propeller motor from The Three Numbers: a spring presses the row of cells from one end, and a sliding contact rides the other, so any number of cells closes the loop.
Alkaline cells give 1.5 V each, NiMH rechargeables 1.2 V, the lithium coin cell 3 V: the chemistry sets the step, and a series stack adds the steps. Four alkalines make 6.0 V, four NiMH 4.8 V, and mixing them adds honestly too. The 9 V block is exactly such a stack, sealed at the factory: 6 tiny cells of 1.5 V. The motor is modeled ideal, its speed proportional to voltage. The holder's spring presses one end of the row and its sliding contact rides the other, so any count from 1 cell up closes the loop.
The most common source in the world now is the USB socket, the flat socket every computer and charger carries. It holds 5 V, with a budget: a freshly plugged device may take 100 mA, and 500 mA once it asks for more over the socket's two data pins. The port below is live. Plug things into it.
Classic USB numbers: 5 V, 100 mA by default, 500 mA granted after negotiation over the data pins; the lamp's 14 mA needs no asking. The cup warmer wants 900 mA, past this port's budget, and the port protects itself by cutting the power; unplug it and the port recovers. The small in-line USB meter is a real bench tool, a few dollars, worth owning. Newer sockets go further: USB-C negotiates over its own pins, and under Power Delivery the two ends can agree to raise the rail itself to 9, 15 or 20 V. The cutoff is modeled clean; real ports vary in how gracefully they refuse.
Back to wall adapters, because choosing one is a skill you will use for life. The label carries the voltage and the current ceiling. The third fact lives at the barrel plug, the round plug on the cord: a metal sleeve wrapped around a center hole, and either contact can be the + one. On an old adapter that marking is often worn or missing, and guessing wrong kills the device. The meter from The Meter settles it: red probe on the center contact, black on the sleeve, and the sign of the reading is the answer. Two adapters hang below wearing the same label, 9 V DC 1 A, their polarity marks worn away. The label cannot tell them apart. Read both.
The meter is parked on DC volts. Red on center, black on sleeve: +9.00 V means the center is the + contact, −9.00 V means the sleeve is. Both adapters are regulated, so each plug reads its label voltage with no load; the label is identical on purpose, because a label never promises the wiring. The tag under each adapter keeps what you measured.
Same label, opposite wiring: only the meter could tell. Now the choice. The radio below needs 9 V DC, 500 mA, center +: voltage exact, polarity right, and a current ceiling at or above the need. Four fresh adapters hang on the rack, polarity printed and legible this time. Only one matches all three facts, and the wrong ones are not harmless.
The radio takes 9 V and draws up to 500 mA. 12 V is a third too much and cooks it; reversed polarity kills it at once; the 300 mA adapter starts it, then sags and overheats when the radio draws its full 500 mA. The 1 A rating above the 500 mA need is not waste, it is headroom. A damaged radio stays damaged until the reset.
The best source on any bench is none of these. It is the adjustable supply, and it has two knobs, not one. The first sets the voltage it holds. The second sets a current limit: the most it will let flow, no matter what goes wrong downstream. Below the limit the supply holds its voltage steady and the CV light is on, constant voltage. The moment a fault asks for more than the limit, the supply switches to holding the current steady instead, the CC light, constant current, and lets its voltage fall as far as it must. Set 9 V with the limit at 30 mA, power the loop, and then make the classic mistake on purpose: drop the stray jumper across the resistor.
The loop is First Light's 470 Ω and red LED, on the road's mini board at about 5× life; the supply is drawn hand-sized beside it, not to the board's scale. 15 mA at 9 V. The jumper across the resistor leaves only the LED, which would take about 3.5 A and die, the number from First Light. At a 30 mA limit the supply crosses to CC and holds 30 mA at about 1.9 V: over the LED's comfortable 15 mA but survivable. At a 2 A limit the same slip delivers 2 A and the LED dies in a flash, slowed here so it can be watched. The supply is modeled ideal; real ones behave the same way slightly less crisply.
Now the rules that keep you safe, not just the parts. A shock is not voltage passing through you; it is current, and the same V = IR from The Three Numbers decides it, with your body as the resistance. The scale of danger is measured in thousandths of an ampere:
What guards you at the bench is your own skin. Dry, it is a high resistance, and the bench’s small voltages cannot push a feelable current through it. Wet or broken skin loses most of that resistance, and the same voltage drives far more current. Below, the three sources stand beside the ladder, each read through a body model, a resistance standing in for skin. Slide the skin from dry toward wet and watch where each one’s current lands.
I = V ÷ R body, computed on the model and never tried by hand. Dry skin near 100 kΩ keeps all three sources under the feeling rung; soaked skin near 1 kΩ takes the car battery to 12 mA, across the danger line, which is why wet hands and electricity never mix even at the bench. Real bodies vary hugely from person to person and contact to contact, so the rungs are read with margin, never tested.
Every rung above the danger line is a place a bench never needs to go. That is this road's hard line: everything you build at this level runs on the low, sealed sources of this page. Batteries, USB, adapters, the supply: each exists exactly so the high-voltage half of the world stays inside a sealed box. In Level 3, with an isolated supply and the training to use it, that door opens; not before.
When you are ready, test yourself:
A speaker needs 12 V DC at 300 mA. Which adapter do you buy?
Your circuit draws 50 mA at 5 V. Is a 5 V 2 A brick too strong for it?
3 fresh alkaline AA cells sit nose to tail in a holder. What do the end terminals give?
A bench supply is set to 12 V, limit 100 mA, and its output is accidentally shorted. What do its meters read?
Your hands are wet from washing up. Why dry them before picking up a wired 12 V project?
You can now power anything this road will ask you to build, and you know where the hard line is. The next tool is not a source but a trick played with two resistors: how to turn any voltage into any smaller voltage you want. Next, The Divider.
Lesson 7 done.
Lesson 8 is in the full course.
Every lesson that is ready opens today. New lessons open the day they land.
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