Your First Circuit

components-and-schematics ended on paper: a battery, a resistor, an LED, and a loop drawn between them with a few lines and symbols. A schematic is a promise. It says this circuit would work if someone built it. This lesson builds it, on a real board, with real parts, and gets a red LED to actually light up. No solder, no finished product, nothing fragile. It takes a breadboard, one resistor, one LED, and a 5V supply, plus current going the right way in the right amount.

That last part is the whole game. An LED that gets too much current dies in a flash. An LED that gets too little looks dead when it is fine. One resistor, chosen on purpose, is the difference, and choosing it is a piece of Ohm's law arithmetic that takes about thirty seconds once the setup is clear.

The board that holds a circuit together

A breadboard is a plastic block full of small holes that grip a component's legs, wired together underneath in a fixed pattern, so a circuit gets built and rebuilt by pushing parts in and pulling them back out. No soldering, no commitment. It is the fastest way to try a circuit before anything is permanent.

The holes are not independent, and that is the one thing worth learning cold. Underneath the plastic, metal clips join the holes into groups. Two legs pushed into the same group are connected to each other whether that was the intent or not. A group of connected holes is a node: a single electrical spot where every hole in the group is the same point in the circuit.

The main field of the board is split down the middle by a channel into two banks. Each bank is organized into short strips of five holes. The five holes in one strip are one node. A leg in any of the five touches a leg in the other four, and touches nothing outside that strip. Strips do not connect across the center channel, and neighboring strips do not connect to each other. That channel is there so a chip can straddle it with its two rows of pins landing in separate strips, which matters later, not now.

Running along the two long edges are the power rails: single strips that run the whole length of the board, usually marked with a red line for plus and a blue line for minus. Put the supply's positive lead on the red rail and its negative lead on the blue rail, and power is available anywhere along the board by jumping a short wire from a rail to a strip.

One path or many

Two words describe how parts sit relative to each other, and the LED circuit depends on getting one of them right.

Parts in series sit in a single line, end to end, with one path for the current. The same current flows through every part in turn, because there is nowhere else for it to go. If 20 milliamps leaves the resistor, 20 milliamps enters the LED. The voltage, on the other hand, splits across the parts in the line.

Parts in parallel sit side by side, both ends tied to the same two nodes. They share the same voltage, and the current splits among them. A wide road that forks into lanes: same start, same end, traffic divided.

For a single LED, the resistor and the LED go in series. That is the arrangement that protects the LED, because the one current flowing through the resistor is the exact same current flowing through the LED, and the resistor is the part that sets how big it is. Control the current in the resistor and the LED's current is controlled by the same stroke.

Parallel has a trap worth flagging now. Two LEDs wired in parallel on a single shared resistor do not split the current evenly. The one with the slightly lower turn-on voltage hogs most of it and burns brighter and hotter, while the other sits dim. Each LED gets its own resistor. There are no shortcuts around that, and the reason is the same current-hungry behavior that makes the resistor necessary in the first place.

Sizing the resistor

An LED is not a plain resistor, and that is why it needs a keeper. A plain resistor lets current rise smoothly as voltage rises. An LED barely conducts at all until its voltage reaches a turn-on point, and then its current climbs almost straight up for the tiniest further rise in voltage. Wired straight across a 5V supply, an LED pulls whatever current the supply can deliver, which is far more than it survives. The resistor is the brake. It turns "whatever the supply can give" into a number picked on purpose.

The turn-on point has a name: the forward voltage, meaning the voltage an LED drops across itself once it is conducting. A red LED sits around 2 volts. Orange, yellow, and most reds land near 1.8 to 2.2 volts. Green, blue, and white sit higher, near 3 to 3.4 volts. These are real ballparks, not exact figures, and a specific part's datasheet gives its own number.

The math uses Ohm's law from what-electricity-is, in the form R equals V divided by I. The trick is knowing which voltage and which current to feed it. Work it in two steps.

First, find the voltage across the resistor. The supply is 5 volts. The LED takes its forward voltage, about 2 volts, and the resistor gets whatever is left. So the resistor's voltage is 5 minus 2, which is 3 volts.

Second, decide the current and solve. A red LED runs happily around 20 milliamps, which is 0.02 amps. Ohm's law gives R equals 3 volts divided by 0.02 amps, which is 150 ohms.

A 150 ohm resistor in series with that red LED sets the current to 20 milliamps. That is the whole calculation. Change the LED color and the forward voltage changes: a blue LED at 3.2 volts on the same 5V supply leaves 1.8 volts for the resistor, so R equals 1.8 divided by 0.02, which is 90 ohms, rounded up to the nearest common 100 ohms.

Building the loop

An LED cares which way current runs through it. It has two legs that are not interchangeable. The longer leg is the anode, the plus side. The shorter leg, next to the flat spot ground into the rim, is the cathode, the minus side. Current flows from anode to cathode and only that direction lights it. Wired backwards, the LED simply stays dark. At 5 volts it takes no harm from being reversed, it just does nothing, which is its own kind of confusing the first time it happens.

The loop on the board runs like this. The supply's positive lead goes to the red rail. A jumper wire carries plus from the rail to a strip. The resistor bridges that strip to a second strip, its two legs in two different strips so it is not shorted. The LED's anode goes into that second strip, sharing the node with the resistor's far leg. The LED's cathode goes into a third, fresh strip. A jumper carries that strip to the blue rail, and the supply's negative lead is on the blue rail. Close the loop and the LED lights.

The resistor can sit on either side of the LED. In a series line the current is the same everywhere, so whether the resistor comes before the LED or after it, the LED still sees 20 milliamps. Put it wherever the layout is tidiest.

When the resistor is missing or wrong

Skip the resistor and the LED goes straight across 5 volts. Current shoots far past 20 milliamps, limited only by the supply and the LED's own tiny internal resistance. Best case, the LED flares and dies in a visible flash. Worse case, it survives dim and degraded and fails hours later. The resistor is not a suggestion.

A resistor that is too small does a milder version of the same thing. A 47 ohm resistor gives 3 divided by 47, about 64 milliamps, roughly three times the rating. The LED runs bright, hot, and short-lived.

A resistor that is too large is harmless and frustrating. A 10,000 ohm resistor gives 3 divided by 10,000, about 0.3 milliamps. The LED is technically on and looks stone dead in daylight. Plenty of "why won't it light" circuits are working exactly as built, just far too faintly to see.

Checking it with a multimeter

A multimeter is a handheld meter that measures electrical quantities, selected with a dial. Two of its modes settle almost every question about a small circuit: voltage and continuity.

Voltage gets measured across a part, with the circuit powered. The dial goes to DC volts, and one probe touches each end of the part. Across the resistor the meter should read about 3 volts. Across the LED it should read about 2 volts. The two add up to the 5 volt supply, because the voltage in a series line divides among the parts and has to total the source. That sum is the fastest sanity check there is.

The readings also name the fault when something is wrong. If the resistor reads close to 5 volts and the LED reads near 0, the whole supply is dropping across the resistor and none across the LED, which means the LED is backwards or dead. If the resistor reads near 0 volts, no current is flowing at all, so the loop is broken somewhere and the meter has just pointed at which half to inspect.

Continuity checks connections with the power off. The dial goes to the continuity setting, usually marked with a small speaker or sound-wave symbol. The meter pushes a tiny current between the probes and beeps when they are joined by near-zero resistance. Touch the two ends of a wire and a beep means the wire actually connects. Touch two points that should be one node and a beep confirms it. Silence where a beep was expected means the connection is not there, a leg is not seated, or a jumper reaches the wrong strip. Continuity also catches a short: the plus rail and the minus rail beeping against each other is a dead giveaway that something is bridging them that should not be.

So far a battery has done all the deciding, and its only decision is on. It cannot blink the LED, cannot fade it up, cannot switch it off the instant a sensor trips. Swapping the battery for a small computer changes that. A microcontroller like an Arduino carries a row of pins, metal connection points along its edge, and it can drive one pin high, meaning it puts a controlled voltage on that pin, or low, meaning 0 volts, all from code. Wire the LED's resistor to a pin instead of the plus rail and a program decides when current flows. The next lesson, microcontrollers-and-arduino, puts that small computer in charge of the exact circuit built here.