Demo 2.2: Before You Wire Anything

Every other demonstration here is about what a circuit means. This one is about the first twenty minutes of a laboratory session, in which a perfectly correct design sits on the bench doing nothing at all.

The reasons are almost never interesting. The chip is upside down. Two component legs are in the same five-hole row. The button has nothing holding its input anywhere when it is not pressed. The display is the wrong sort. None of these is a failure of understanding, and all of them are expensive, because a fault you cannot see is a fault you cannot reason about.

So this page is about the physical layer: which holes are already joined to which, which way round the package goes, and the handful of assembly mistakes that account for most of the lost time.

The one genuinely difficult thing

A breadboard is a grid of holes with metal clips underneath joining some of them together. Which ones, exactly:

Above and below the central channel, each column of five holes is one connection. Two legs in the same five are joined whether you meant it or not.
The long strips down the edges are the supply rails, and each runs the whole length of the board.
Nothing else is joined to anything. Two holes in different columns are as unrelated as two holes in different rooms, and nothing runs along a row.

The channel is not decoration either. A dual-in-line package is three tenths of an inch across its two rows of pins, and the channel is three tenths of an inch wide. A chip that straddles it puts every pin in a column of its own, with four spare holes beside each one. A chip that does not straddle it has every pin shorted to the one opposite, including the supply pins.

How to use it

The board lets you click any hole and see the whole connection light up. That is the entire lesson, and you can put a chip on it the right way and the wrong way to see the difference.

The chip draws a 74LS00, a 74LS04 or a 74LS76 with its real pinout, and lets you turn it round to see where the supply lands when you do.

LEDs, resistors and buttons introduces the three faults one at a time and says what each actually does.

A button, a gate and an LED is the first three tabs turned into a circuit that works, on the board rather than on a schematic. Choose whether the button asserts high or low, choose the pull resistor, choose which way round the LED goes, and press it.

A clock from a 555 is the oscillator the counter laboratory asks you to build in advance, with the arithmetic on the page.

Seven-segment displays is the difference between the common anode part one laboratory needs and the common cathode part the next one needs.

Before You Wire Anything: the Breadboard and the Chip

Which holes are joined to which, which way round the chip goes, and the handful of assembly mistakes that account for most of a laboratory session.

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Only one thing about a breadboard is difficult, and you cannot see it.

Some of the holes are already joined to each other by metal clips underneath, and which ones is the entire subject.
Above and below the channel, each column of five holes is one connection. Push two component legs into the same five, and you have joined them whether you meant to or not.
The long strips down the edges are the supply rails, and each one runs the whole length of the board.
Nothing else is connected to anything. Two holes in different columns are as unrelated as two holes in different rooms.
Click any hole below and the connection it belongs to lights up. That is all there is to know, and almost every wiring fault in the laboratory comes from getting it wrong.

Show me

A chip on the board

Click any hole. Every hole joined to it lights up.

5 holes in this connection

Reading the board

A five-hole column above the channel and the five directly below it are two separate connections, not one.
That is the single most common misreading, and it is what the channel is there to make obvious.
The rails are usually marked with a red line and a blue line, and they carry the supply to wherever it is needed.
On many full-size boards each rail is broken in the middle, so a wire is needed to join the two halves. Check yours with a multimeter on continuity before assuming.
Nothing is joined along a row. Hole A1 and hole A2 have nothing to do with each other.

How to lay a circuit out so it can be debugged

Put the supply rails in first, and put them in the same way every time: positive at the top, ground at the bottom.
Run a wire from the rails to the chip's supply pins before wiring anything else, and check them with a meter.
Keep wires flat to the board and cut them to length. A nest of long loops is impossible to trace and easy to pull out.
Use colour deliberately: red for the supply, black for ground, and something else for signals. It costs nothing and saves the whole session.
Leave a clear column between separate parts of the circuit. Crowding is what causes two legs to end up in the same five holes.

What to check before applying power

That every chip straddles the channel and none is upside down.
That the supply rails are the right way round, with a meter rather than by eye.
That no component leg shares a five-hole column with something it should not touch.
That there is no wire bridging the positive rail directly to the ground rail, which is the one fault that can damage the supply.
Then apply power and feel the chips. Anything warm is wrong, and should be disconnected before you look for why.

Walkthrough

Step 1: Find out what is already connected

Open The board and click a hole in the middle of the upper half.

Five holes light up: the column you clicked, and nothing else. Now click the hole immediately below it, on the other side of the channel. A different five light up. Those two groups are not connected, and misreading that is the single most common wiring fault there is.

Now click one of the rail holes along the edge. Thirty light up, the whole length of the board.

Press Hide the clips underneath and click around again. Without the shading it is much harder to predict what will light, which is exactly the situation you are in with a real board in front of you.

Step 2: See why the channel is the width it is

Press Across the channel and look at where the pins land: each one in a column of its own, with spare holes beside it for wires.

Now press All on one side.

Both rows of pins are now in the same five-hole columns. Pin 1 is joined to pin 14, pin 2 to pin 13, and so on all the way along. That includes the supply pins, so Vcc is connected straight to ground through the chip. It gets hot, the supply complains or the fuse goes, and nothing works.

It is a surprisingly easy mistake on a crowded board, and it is worth a glance along the row of chips before power goes on.

Step 3: Learn to read a package

Open The chip.

There is a notch at one end and often a dot beside pin 1. That is the whole of the labelling, and nothing physically stops you inserting it backwards.

Pin 1 is immediately anticlockwise of the notch, so with the notch at the top it is the top left. The numbers then run down the left side and back up the right. They do not run left to right, and assuming they do is the second commonest fault after inserting the chip backwards.

Click through the pins on the 74LS00 and note that Vcc is pin 14 and GND is pin 7: the top right and the middle left. Then switch to the 74LS76, the dual JK flip-flop the counter laboratory is built on, and look again:

Vcc is pin 5 and GND is pin 12. Not the corners at all.

That is not a misprint, it is what the part actually does, and it is exactly why the rule is to read the data sheet rather than to assume. A 74LS76 wired as though it were a 74LS00 gets its supply across two signal pins.

Now press Put in backwards and read what happens to the supply.

Step 4: Introduce the three faults deliberately

Open LEDs, resistors and buttons and step through the four settings.

No resistor. Nothing limits the current except the supply and the LED’s own few ohms of bulk resistance. That is hundreds of milliamps through a part rated for twenty; it lights brilliantly once. On a logic output it can take the driving chip with it, turning a two-pence mistake into a dead 74LS part.

LED backwards. Nothing is damaged and nothing happens, which is precisely why it wastes so much time: it looks like the rest of the circuit is broken. The longer leg is the anode and goes towards the positive side; the flat spot on the rim is the cathode. If you cannot tell, most meters have a diode test that will light it faintly the right way round.

Button with no pull resistor. With the button open the input is connected to nothing at all: not to 0 V, to nothing. A TTL input floating like that reads as a 1, so the circuit behaves as though the button were permanently pressed. A CMOS input reads whatever the room decides and changes when a hand moves near the board. The circuit works on the bench and fails during the demonstration, and the button gets replaced for no reason.

That last one is not a new idea: it is the whole subject of the previous demonstration, and your counter laboratory sheet asks for exactly this resistor by name.

Step 5: Wire the whole thing up, and choose a sense

Open A button, a gate and an LED.

Everything so far has been a piece of a circuit. This is the circuit: a 74LS00 straddling the channel, one gate with both its inputs tied together so that it behaves as an inverter, a button on the input, and an LED on the output. Press the button on the board and watch the pin voltages.

Notice first what column 8 is doing. The chip’s pin 1, the wire from the button, the link across to pin 2 and the pull resistor are four separate things pushed into four holes of one five-hole column, and they are all the same connection. No jumper joins them. That is Step 1 in use.

The button, held properly and held badly

Leave the sense on Active high and the resistor on 470 Ω, then press and release.

Released, the two tied inputs push 0.8 mA out of the pins and into the pull-down, which sits at 0.38 V, comfortably below the 0.8 V a 74LS part is guaranteed to read as a 0. Pressed, the switch takes the pin to 5 V. The gate inverts, the LED changes. It works.

Now change the resistor to 10 kΩ and press again. Nothing happens at all.

This is the number worth carrying out of the whole page. A 74LS input held low pushes current out of itself, about 0.4 mA per input, and both tied inputs do it. So the pull-down has to carry 0.8 mA and still sit below 0.8 V:

R_max = 0.8 V ÷ 0.8 mA = 1 kΩ

Ten kilohms cannot do it. The node stalls around 1.5 V, the gate reads a 1, and the circuit behaves as though the button were permanently pressed however hard you press it.

Now set the resistor to none at all.

The reading is the same 1.5 V, and this is the case to look at hardest. The pin is connected to nothing. It is not at 0 V, it is at no defined voltage, and 1.5 V is neither a valid 0 (0.8 V or less) nor a valid 1 (2.0 V or more). It is inside the band the data sheet refuses to define, and the part reads it as a 1 because a TTL input pulls itself up when nothing else is driving it.

Active low, and why 74XX parts are full of it

Switch the sense to Active low and put the resistor back to 10 kΩ.

Now the button is wired to 0 V and the resistor holds the pin at 5 V. Pressing gives a 0. The same 10 kΩ that was useless as a pull-down works perfectly as a pull-up, and the reason is on the readout: an input held high takes only 20 µA, twenty times less than the 0.4 mA it pushes out when held low.

That asymmetry is not a convention, it is the transistors. It is why 74XX circuits are full of pull-ups and short of pull-downs, and it is half of why so many 74XX inputs are active low.

Try 470 Ω as a pull-up too. It works just as well and wastes 10.6 mA every time the button is held, against 0.5 mA for the 10 kΩ version. Both work; one is thoughtless.

Finally, set the sense to Active low with no resistor at all.

Released gives a 1, pressed gives a 0. The circuit appears to work. It works because a floating TTL input happens to read as a 1, not because anything is holding it there, and the pin sits at 1.5 V with very little distance to travel before it is a 0. Anything that couples charge onto that node moves it: a hand, a long lead, another wire switching alongside.

That is the worst fault on this page. It passes on the bench, it fails during the demonstration, and there is nothing to see.

The LED, and the other half of active low

Leave the input working and switch the LED between LED to +5 V (sinking) and LED to 0 V (sourcing).

Sourcing is what almost everyone draws first: the LED hangs off the output down to 0 V, so it lights when the output is high. Look at what the readout says about it. A 74LS output guarantees only 0.4 mA while staying at a valid high, and even at the minimum valid 2.7 V the 330 Ω resistor is asking for 2.1 mA, five times more. The output cannot hold a valid high while driving it, so the LED is dim and the pin is no longer at a level anything else could trust.

Sinking turns the LED over and hangs it from the +5 V rail, so it lights when the output is low. Now the arithmetic works: (5 − 2.0 − 0.4)/330 = 7.9 mA, against the 8 mA a 74LS output is specified to sink.

	Sinking	Sourcing
LED connects to	+5 V	0 V
Lights when the output is	LOW	HIGH
The part will give you	8 mA	0.4 mA
Result at 330 Ω	7.9 mA, bright	dim, and the level is invalid

Two things follow. The first is that 220 Ω is the wrong resistor here: it would ask for 11.8 mA, outside what the output can sink. Use 330 Ω on a 74LS output and keep 220 Ω for the microcontroller laboratory, where the pin can supply it.

The second is the general point:

An active high signal asserts by going to a 1; an active low signal asserts by going to a 0. A bar drawn over a name, such as the CLR input on a flip-flop, or a bubble on a symbol, means active low, and there is nothing more to the notation than that.

What is worth understanding is why so much 74XX logic is built that way, and it is nothing to do with tidiness. A 74LS input costs twenty times less to hold high than low, and a 74LS output sinks twenty times better than it sources. Both asymmetries point the same way: put the interesting event at the low end.

So buttons go to ground with a pull-up, indicator LEDs light when a signal goes low, and reset and clear inputs are held high to do nothing. Once you have measured either of those twenty-to-one figures, none of it looks arbitrary again.

The cost is that the logic downstream now has to invert, and forgetting that is the commonest bug in the whole arrangement.

Step 6: Build the clock the counter needs

Open A clock from a 555.

A counter needs something to count, and a push button will not do it. A mechanical switch bounces: its contacts make and break for a few milliseconds, giving a burst of edges. A flip-flop is fast enough to see every one of them, so a single press can clock a counter several times and the count jumps. That is exactly what your laboratory sheet is warning about when it says the toggle case behaves erratically with a button input.

A 555 wired as an astable gives one clean edge per period, for ever. It charges a capacitor through both resistors and discharges it through R₂ alone, flipping at two thirds and one third of the supply. Watch the capacitor trace climb between those two dashed lines.

That asymmetry is the one thing to remember:

t_high = ln2 × (R₁ + R₂) × C
t_low = ln2 × R₂ × C

The high time contains R₁ and the low time does not, so the duty cycle is always above a half. It is (R₁+R₂)/(R₁+2R₂), which approaches 50% as R₁ becomes small beside R₂ and never quite arrives. Push R₁ up and watch the output become a narrow dip rather than a square wave.

Press About 1 Hz, for the counter: R₁ = 10 kΩ, R₂ = 68 kΩ, C = 10 µF gives 0.99 Hz, which is what you want for watching a count advance.

Step 7: Work out which display you have

Open Seven-segment displays.

A seven-segment display is seven LEDs arranged in a figure eight. Wiring fourteen legs out of a small package is impractical, so one leg of every LED is joined inside and brought out as a common pin, and there are two ways to do that:

	Common pin goes to	A segment lights when its pin is	The driver must
Common anode	+5 V	LOW	sink the current
Common cathode	0 V	HIGH	source the current

They look identical from outside. Fitting the wrong one gives a display that stays dark, and no amount of checking your logic will help.

This is not hypothetical: your counter laboratory needs a common anode part and your Arduino laboratory needs a common cathode one. That is not an arbitrary difference. A 74LS47 BCD-to-seven-segment decoder has active-low outputs, which is what a common anode display wants; a microcontroller pin driving a segment high suits a common cathode display.

To tell which you have without powering anything: set a multimeter to its diode test range, put the red probe on a common pin and the black on any segment pin. If a segment glows, the common is the anode. Swap the probes; if it glows that way instead, the common is the cathode.

Note also that each segment needs its own series resistor. One resistor on the common pin will not do, because the current through it depends on how many segments are lit, so the brightness would change with the digit.

Check your understanding

Quiz

Select 0/1

You push two component legs into holes A5 and E5 on a breadboard. What have you done?

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Quiz

Select 0/1

You are wiring a 74LS76 dual JK flip-flop and you connect the positive supply to pin 14 and ground to pin 7, as you did for the 74LS00. What happens?

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Quiz

Select 0/1

A push button connects a 74LS00 input to +5 V when pressed, and a 10 kΩ resistor is fitted from that input to 0 V. The circuit behaves as though the button is permanently pressed. Why?

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Quiz

Select 0/1

In a 555 astable, why can the duty cycle never quite reach 50%?

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Concept Match

Match each item to what it is

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Quiz

Select 0/1

Your counter circuit does nothing at all when you switch on, and one chip is warm to the touch. What should you check, in what order?

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Wrap-up

Seven things to take away.

A column of five holes is one connection, above and below the channel separately. Nothing runs along a row, and the rails are separate strips that may be broken in the middle.
The channel is 0.3 inch because a DIP package is. A chip that does not straddle it has every pin shorted to the one opposite, supply pins included.
Pin 1 is anticlockwise of the notch, and the numbers run down one side and back up the other. Check the supply pins on the data sheet every time: the 74LS76 puts Vcc on pin 5 and GND on pin 12.
Fit a 100 nF capacitor across the supply pins of every chip. It prevents a class of intermittent fault that is otherwise very hard to find.
Three assembly faults cost most of the session: an LED with no resistor, an LED the wrong way round, and a button with no pull resistor. All three are visible on the board.
An input has to be told what to do. A 74LS input pushes 0.4 mA out of itself when held low and takes 20 µA when held high, so a pull-up can be 10 kΩ and a pull-down cannot exceed about 1 kΩ. Left unconnected it is not a 0, it is undefined, and TTL guesses high.
Read the display before you wire it. Common anode and common cathode look identical and are not interchangeable, and your two laboratories need different ones.

Point 5 is where active low comes from, and it is worth stating once more because it explains a notation you will meet on every data sheet from here on. A 74LS output sinks twenty times better than it sources, and a 74LS input costs twenty times less to hold high than low. Both point the same way, so the interesting event goes at the low end: buttons to ground, indicators lit by a low, and clear inputs held high to do nothing. The bar over a name is reporting that decision, not making it.

Two of the rest connect forward. The 555 gives the counter laboratory the clean clock its flip-flops need, and the seven-segment display is the same device driven two completely different ways: by a hardware decoder designed from a truth table in one laboratory, and by seven output pins and a table in software in the next. Building both is the clearest illustration of the hardware-versus-software trade you will get.