Tutorial 4: Optimizing your circuit schematic diagram

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

1. 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.
2. 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.
   

   

Rules

1. 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).
2. ‘4’ wires can’t connect at one point; means, wires shouldn’t cross & connect.
3. 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”).
4. Wires & components have to be aligned either horizontally or vertically, unless there is a solid reason to do otherwise.
5. Pin numbers should be labeled on the outside of a symbol and signal names on the inside.
6. 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.

Hints

1. Immediately identify the parts that are adjacent to the symbol, making a distinct group that gives symbol, label, and type or value.
2. Generally, signals move from left-right; do not be emphatic about this thing, though, if clarity is immolated.
3. 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.
4. 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.
5. 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’.
6. 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.
7. 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.
8. 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.
9. Use small circles, rectangles or ovals to indicate connector pins, card-edge connections etc. And Be consistent.
10. 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.
11. 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.
12. 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).
13. 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.
14. 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.

Tutorial 1: Pre-requisites to draw schematic diagram
Tutorial 2: Components connectivity and polarity
Tutorial 3: Drawing Schematic symbol
Tutorial 4: Optimizing your circuit schematic diagram