Look at a board with a few parts on it, and it is not easy to see what joins what. Below, board A is our first circuit, with a battery, a 470 Ω resistor and an LED. Boards B and C are two tries at copying it. Which one joins every part the same way as A?
Each board is the top half of a 17-column mini board, and the batteries are drawn smaller than real. On A and C, 9 V and 470 Ω give about 15 mA through the LED.
C was the same circuit. Its parts sit in other places, with an extra jumper wire, but every leg meets the same others as on A. B looked the same as A, but its jumper wire starts one column over, so the LED’s short leg is joined to nothing. Checking a board by eye means following every leg to its clip, and a mistake of one column is easy to miss. In the drawings beside the boards, the difference was plain to see.
A drawing like that is called a schematic. A schematic shows each part as a simple symbol, and each line between two symbols is a connection. Most circuits start as a schematic, and we build from that drawing.
Each part has its own symbol, and our first circuit needs three of them. For the LED and the battery, it also matters which way round the symbol is drawn. On the next bench the three symbols are on the paper, and the three parts are next to them. Which symbol stands for which part, and which of the LED’s legs goes on the triangle’s flat side?
The parts are not drawn to one scale, and the battery is drawn much smaller than real. The LED’s long leg is called its anode and its short leg its cathode, and some schematics mark them A and K.
Our first circuit needs only these three symbols. Every new part in the lessons ahead comes with its symbol, drawn beside the part the first time we meet it.
Here is our first circuit drawn as a schematic, with the battery, the 470 Ω resistor and the LED joined by three lines. What do you think happens to the circuit if we move the symbols to other places on the page?
The circuit is a 9 V battery, a 470 Ω resistor and a red LED. About 15 mA flows, so the LED symbol is drawn lit.
However far the symbols moved, they stayed joined the same way, and the LED stayed lit. On a schematic, where a part is drawn does not matter. What matters is only what it is joined to. That is why board C, with its parts spread out, had the same drawing as A.
So how does a drawing become a board? A set of lines joined together, with no part between them, is called a net. Our first circuit has three nets, and on the next schematic each one has its own colour. Below it is the finished board. Where on the board do you think each net is?
The board is the top half of a 17-column mini board with a 9 V battery, a 470 Ω resistor and a red LED, and about 15 mA flows. The battery is drawn smaller than real.
Each net was one clip, or two clips joined by a jumper wire. The same schematic could also be soldered onto a circuit board or wired by hand. However it is built, each net becomes one electrical connection.
Now let’s build a circuit from its schematic. Schematics are usually drawn with + at the top and − at the bottom, and this one is too. The parts are on the bench below the board. Which clip will each net use?
The battery gives 9 V and has about 2 Ω inside it, and the red LED has 1.9 V across it. With the 1.0 kΩ resistor the schematic shows, about 7.1 mA flows.
It also works the other way round, from a board to its schematic. This board has a resistor, an LED and a slide switch, which turns the circuit on and off. Below it are the symbols for the battery, the resistor, the LED and the switch, not yet joined. Each end of a symbol has a small round pad to tap. The switch is drawn as a lever lying across two contacts, which means it is closed. A closed switch joins its two contacts, so the switch is on. Can you draw the lines so the schematic matches the board?
The board has a 470 Ω resistor, a red LED and a slide switch in one circuit. About 15 mA flows while the switch is on. Drawn open, the switch’s lever lifts off one contact, and the schematic shows a circuit with no current.
Any order of the parts around the circuit lights the LED, because the current is the same all the way round. To be the board’s circuit, the schematic must join the same pairs of parts that the board does.
On bigger schematics some lines have to cross, so we need a way to tell a crossing from a join. A voltmeter can be drawn on a schematic too, as a circle with a V in it. On the next schematic it measures across the LED. Its line up to the LED’s long-leg side crosses the − line on the way. The moving green dots show where the current flows. Do you think the two lines are joined where they cross?
The battery gives 9 V and has about 2 Ω inside it, and the red LED has 1.9 V across it. With the 1.0 kΩ resistor, about 7.1 mA flows. The voltmeter takes almost no current. With the dot, the resistor alone is left in the path and about 9.0 mA flows, which it can take. Old schematics drew a crossing with a small bump in one line, where it passed over the other. Careful schematics never put a dot where four lines meet, because a dot lost in printing would change the circuit, so they draw two T junctions instead.
They are not. With no dot, the current went straight past the crossing along the − line, and the meter read the LED’s 1.9 V. A filled dot where lines meet means they are joined, and the point where they join is called a junction. The meter’s two lines join the circuit at two dotted junctions, each drawn as a T. When you put a dot on the crossing, the meter’s line joined the LED’s long-leg side to −. Both ends of the LED were now joined. The current went round the LED instead of through it, so the LED went dark and the meter read 0 V. The resistor still limits the current, so joining the LED’s two ends on a real board does no harm.
Big schematics would be full of lines if every part had its own line back to the battery’s −. Instead, those lines end in a small symbol called ground, drawn as three short lines, each shorter than the one above. On the next schematic, the battery’s − and the LED’s short leg each end in a ground symbol, with no line between them. On the board below it, the LED’s wire is still loose. Where should it go?
Only the top half of the mini board is shown, because the bottom half is not used. The battery’s + lead is in column 1 and its − lead in column 17. With 470 Ω, about 15 mA flows through the LED.
Every ground symbol stands for the same point, the battery’s − side. So every line that ends in one goes to that same place on the board, however far apart the symbols are drawn.
Real schematics do not all draw parts the same way. Many draw a resistor as a small rectangle instead of a zigzag. They also write 4.7 kΩ as 4K7, with the letter where the decimal point would be.
Four questions, if you want to check yourself.
Two schematics show the same three parts in different places on the page. The lines are drawn differently, but they join the same things. Do the schematics show the same circuit?
Two lines cross on a schematic, with no dot where they cross. How much current passes from one line to the other?
A schematic shows four ground symbols. How many places do they stand for on the board?
You build a circuit from a schematic, and the LED stays dark. In the drawing, the LED’s triangle points toward ground, but you put the LED’s long leg on the ground side. What is wrong?
Every circuit so far has run on a 9 V battery. In The Power we will look at the other ways to power a circuit, and how to choose one safely.
Lesson 8 done.
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