The first few Labs are something of a "warmup" in terms of utilizing the breadboard. We will use only one or two of the interface components at a time and will built circuits consisting of 2 or 3 components which are disassembled at the end of lab. As we get closer to the final project, we will be constructing more complicated circuits having more external connections. Since some of these will perform functions needed in subsequent Labs, we will want to leave them in place on the breadboard for future use. To keep this from degenerating into chaos, we need a plan for organizing the layout and wiring on the breadboard and interface board.
There are several aspects to this organization ranging from small details to the big picture.
One set of signals you should color code are power and ground. In particular, you should have a different color for +15 V, -15 V, and ground and you should not use any of these colors for other signals. It would be nice to use "standard" colors for these, but unfortunately there are several competing standards. One color that nearly everyone agrees on is that red should be the positive power supply voltage. Most automotive and electronic wiring uses black to denote ground. Electronic wiring which uses black for ground often uses blue for the negative supply. However, our power supply and breadboard use green to denote ground (the convention used in house wiring) and use black to denote the negative supply.
So we have two possible "standard" color codes for power: (1) red = +, green = gnd, black = -, and (2) red = +, black = gnd, blue = -. You could use either of these, or make up your own.
Another useful thing to color code would be the wires or "probes" from the interface board connector. That way you can tell which wire is CH1 of the scope or which is the function generator output without having to trace from one end of the wire to the other.
In the instructions and photographs, we will be using the following assignment:
Connector | Socket Pin | Signal |
J1-1 | 1 | Oscilloscope channel 1 |
J1-2 | 2 | Oscilloscope channel 2 |
J1-3 | 3 | Function generator output |
This final project will be a fairly complex circuit, involving at least 4 op-amps and a dozen or so resistors, capacitors, and diodes. Good engineering practice is to break down a complex system into a number of simple subsystems, each of which performs a single, well-defined function.
In the unlikely event that your circuit does not work perfectly the first time you turn it on, it will make it easier for you (and your labbie) to debug it if each of these subcircuits is laid out (neatly, of course) on its own little patch of breadboard and is connected to the other subcircuits in such a way that it may be easilly disconnected from them for testing in isolation.
One additional advantage of a tidy layout with short wires is that it is more rugged. Remember that you will have to transport your circuit from your lab bench to the testing station without parts falling off or loops of wire getting caught and mysteriously pulled loose.
On paper all our components are ideal and no components exist where we don't draw them. In an actual circuit, things are not quite so tidy: wires have non zero resistance and inductance, sources have output resistance, parasitic capacitances and mutual inductances exist between wires, and a host of other gremlins. In addition, although we are only applying input signals up to a few 10s of kHz, the active devices (op amps and transistors) we use have gain at frequencies up to a few MHz (op amps) or 100s of MHz (transistors). This means that all of the "little" pieces of wire can become fairly effective antennas, radiating energy to (and receiving from) the rest of the world, and more significantly, to other parts of the circuit.
What all this means is that a circuit which is wired correctly topologically may fail to function as expected. Although we can't eliminate all of these effects, we can do some things to minimize them. Here are a few.
To make a twisted pair, select two wires having an aesthetically pleasing combination of colors and twist them together.
The basic idea of wiring on a solderless breadboard is simple: just stick the ends of the component leads or wires into the holes. But like any seemingly simple process, there are a few subtlties that can make the difference between success and failure.
First a note of caution. The material that the clips inside the breadboard are made of (so called "nickel silver") is a compromise between good conductivity, corrosion resistance, and springiness. In particular the elastic limit is considerably less than of a good steel spring and if spread too far, can be permanently distorted. To avoid deforming the connector clips:
With the health of our breadboard assured, there are a few more things we can do to make sure that our connections are good ones.