Most electronic circuit diagrams leave off some of the most important connections. This is partly because electronic engineers are lazy. Partly because they don't want to clutter up complicated diagrams with even more lines which tell them things which are ‘obvious’.
The circuit diagram shown, above, in the red box is typical of what you'll find in general use. (In this case it is a pair of ‘op-amps’ used to amplify a signal, but that doesn't really matter here.) Most of the ‘missing’ information is obvious to an experienced engineer, but not to anyone else.
The problem, of course, is that when you're trying to learn electronics these missing bits on a diagram are NOT obvious. This leads to mistakes when you build circuits. It can also fool you into thinking things should work when they WON'T!
The most common missing links are the wires showing where the power driving the circuit is applied and the wires which connect the appropriate places to ‘zero volts’ or the earth. These connections are absolutely vital for most electronic circuits to work, but they're often not shown on the diagram...
The green box shows a diagram of the same circuit as before, but it tells you a lot more about how it is put together. The lines highlighted in blue and red are the Power Lines (sometimes called ‘rails’). These provide the input power required to drive the circuit and make it work. The amplifiers used here (741's) are usually driven by connecting them to +15V and -15V.
The diagram now also tells you the pin numbers of each op-amp to connect to which wires. For example, pin 7 is this op-amp's input for the +15V power, and pin 6 is where its output emerges. The standard ‘package’ and pin numbering for a 741 is shown opposite.
When you get used to electronics you'll be happy to save time and not waste effort putting ‘obvious’ details on diagrams. Until then, beware. Integrated circuit, transistors, etc, all require POWER.
Remember also that, in electronics, the electrons have to flow around closed loops. This means when you send them off into a circuit (along a power line or a signal wire) they require another link by which they can return. For the above example, the electrons powering the circuit enter by the -15V connection and are pulled out again via the +15V wires.
When dealing with signals the ‘return’ part of the loop is normally an earth or ‘zero volts’ connection. For example, when using co-axial cable the current flows one way along the inner wire and returns via the outer screen.
One of the most common mistakes people make when building their first electronic circuits is to forget, or not bother to connect, the earth/zero volts wires. Remember that, so far as electronic signals are concerned, “Zero volts is home sweet home”. Unless they can see a way home they won't bother travelling out and about through your new circuit! You must therefore give them a link to get into the circuit AND another one to get them back home again. Despite this, sometimes circuits do work without you having to connect an earth or zero volts wire as a signal or power ‘return’.
The reason for this is that many pieces of mains-powered electronic equipment already have a wire linking them to the ‘mains earth’. Two boxes of electronics which are both connected to the mains earth can pass electrons back to each other via this mains wiring. So you can sometimes get away without a return wire of your own. However, this is bad practice. You're assuming things will be connected to the mains earth. Also, mains earth is a fairly noisy and dangerous place for important signals. Much better to give your signals their own, private, return wire!
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