A well and accurately-drawn schematic diagram makes it quite easy and simple to understand and infer how a circuit operates & helps in troubleshooting process; while a schematic being poor creates confusion only. By keeping in mind and following a few rules & suggestions, you can obviously draw a good & clean schematic in less time than it takes to draw a poor schematic. In this appendix we will provide advice of 3 types: general-principles, rules & regulations & hints. We have also drawn some real knee slappers for illustrating customs to avoid.
General – Principles
- Schematic diagrams have to be clear and precise. That is why, numbers of pins, values of parts, polarities, etc. must be clearly tagged and labeled to remain safe from confusion.
- Good schematics make circuit functions clear. That’s why, functional areas should be kept distinct; do not fear to leave blank areas on the page, and do not try to fill the schematic’s page. There are traditional ways to draw useful sub-units; for example, do not draw a differential-amplifier as shown in Figure, because the function will not be easily recognized. Just like flip flops, they are commonly drawn along-with clocks & inputs on the left-side, clear & set on top & bottom & outputs on the right-side.
- Connecting wires are indicated by a ‘heavy’ black dot(.); wires that are crossing, but not connecting, have no dot (do not try a little half circular’jog”. It went out in the 1950’s era).
- ‘4’ wires can’t connect at one point; means, wires shouldn’t cross & connect.
- Always use same symbols for the same device; for example, do not draw flip flops in 2 different ways (exception: “assertion level logic symbols show each of the gates in 2 possible ways”).
- Wires & components have to be aligned either horizontally or vertically, unless there is a solid reason to do otherwise.
- Pin numbers should be labeled on the outside of a symbol and signal names on the inside.
- Values or types of all parts should be indicated; it is the best way to give all parts a label, too, e.g., R6 or IC2.
- Immediately identify the parts that are adjacent to the symbol, making a distinct group that gives symbol, label, and type or value.
- Generally, signals move from left-right; do not be emphatic about this thing, though, if clarity is immolated.
- Put +ve supply voltage at the top of the page, -ve at the bottom of the page. This is how, npn-transistors will commonly have their emitters at the bottom, whereas pnp-transistors will have their emitters top-most.
- Do not try to bring all wires around the supply rails or a common ground-wire. Instead of it, use the ground symbol & label like ”+Vcc” to indicate the voltage where & when needed.
- It is useful to label the signals & functional blocks & show wave-forms; in logical diagrams it’s particularly important to label these signal-lines, for example ‘RESET’ or ‘CLK’.
- It is useful to take leads away from components to a ‘short’ distance before making jogs or connections e.g., draw transistors as in Figure.
- Let some space be left around the circuit symbols; for example, do not draw components or wires too close to an op amp symbol. This keeps the drawing un-cluttered & clean & leaves room for labels, pin numbers etc.
- Label all those boxes that are not obvious: comparator vs op amp, shift register vs counter, etc. Do not be afraid of inventing a new symbol.
- Use small circles, rectangles or ovals to indicate connector pins, card-edge connections etc. And Be consistent.
- The signal path should be clear through switches. Do not force the your reader to follow wires all over the page to find out in what way a signal is switched.
- Power supply connections are simply supposed to be for op amps and logical devices. However, show any uncommon connections (e.g., an op amp run from a single supply, where V- = ground) & the disposition of unused inputs.
- It is very helpful to include just a small table of IC numbers, types, and power-supply connections (pin numbers for Vcc and ground, for instance).
- Include a title area at or near the bottom of the page, with name of circuit, name of instrument, by whom drawn, by whom designed or checked, the date & assembly number. Must also include a revision area, with columns for revision number, date, and subject.
- We advise drawing schematics freehand on coarse graph paper (non-reproducing blue, 4 to 8 lines per inch) or on plain paper on top of the graph paper. This is really fast, and it gives very pleasing results. Use dark pencil or ink; avoid ball-point pen.