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15 August 2003

How To Draw Schematic Diagrams

By Paul Horowitz and Winfield Hill
Appendix E from The Art of Electronics 2nd Edition

A well-drawn schematic makes it easy to understand how a circuit works and aids in troubleshooting; a poor schematic only creates confusion. By keeping a few rules and suggestions in mind, you can draw a good schematic in no more time than it takes to draw a poor one. In this appendix we dispense advice of three varieties: general principles, rules, and hints. We have also drawn some real knee-slappers to illustrate habits to avoid.

General Principles

1. Schematics should be unambiguous. Therefore, pin numbers, parts values, polarities, etc., should be clearly labeled to avoid confusion.
   
2. A good schematic makes circuit functions clear. Therefore, keep functional areas distinct; don't be afraid to leave blank areas on the page, and don't try to fill the page. There are conventional ways to draw functional subunits; for instance, don't draw a differential amplifier as in Figure E1, because the function won't be easily recognized. Likewise, flip-flops are usually drawn with clock and inputs on the left, set and clear on top and bottom, and outputs on the right.

Rules

1. Wires connecting are indicated by a heavy black dot; wires crossing, but not connecting, have no dot (don't use a little half-circular ``jog''; it went out in the 1950s).
2. Four wires must not connect at a point; i.e., wires must not cross and connect.
3. Always use the same symbol for the same device; e.g., don't draw flip-flops in two different ways (exception: assertion-level logic symbols show each gate in two possible ways).
4. Wires and components are aligned horizontally or vertically, unless there's a good reason to do otherwise.
5. Label pin numbers on the outside of a symbol, signal names on the inside.
6. All parts should have values or types indicated; it's best to give all parts a label, too, e.g., R7 or IC3.

Hints

1. Identify parts immediately adjacent to the symbol, forming a distinct group giving symbol, label, and type or value.
2. In general, signals go from left to right; don't be dogmatic about this, though, if clarity is sacrificed.
3. Put positive supply voltages at the top of the page, negative at the bottom. Thus, npn transistors will usually have their emitter at the bottom, whereas pnp's will have their emitter topmost.
4. Don't attempt to bring all wires around to the supply rails, or to a common ground wire. Instead, use the ground symbol(s) and labels like +Vcc to indicate those voltages where needed.
5. It is helpful to label signals and functional blocks and show waveforms; in logic diagrams it is especially important to label signal lines, e.g., RESET' or CLK.
6. It is helpful to bring leads away from components a short distance before making connections or jogs. For example, draw transistors as in Figure E2.

   Figure E2: Component leads. Click image to enlarge.

   
   

7. Leave some space around circuit symbols; e.g., don't draw components or wires too close to an op-amp symbol. This keeps the drawing uncluttered and leaves room for labels, pin numbers, etc.
8. Label all boxes that aren't obvious: comparator versus op-amp, shift register versus counter, etc. Don't be afraid to invent a new symbol.
9. Use small rectangles, ovals, or circles to indicate card-edge connections, connector pins, etc. Be consistent.
10. The signal path through switches should be clear. Don't force the reader to follow wires all over the page to find out how a signal is switched.
11. Power-supply connections are normally assumed for op-amps and logic devices. However, show any unusual connections (e.g., an op-amp run from a single supply, where V- = ground) and the disposition of unused inputs.
12. It is very helpful to include a small table of IC numbers, types, and power-supply connections (pin numbers for Vcc and ground, for instance).
13. Include a title area near the bottom of the page, with name of circuit, name of instrument, by whom drawn, by whom designed or checked, date, and assembly number. Also include a revision area, with columns for revision number, date, and subject.
14. We recommend drawing schematics freehand on coarse graph paper (nonreproducing blue, 4 to 8 lines per inch) or on plain paper on top of graph paper. This is fast, and it gives very pleasing results. Use dark pencil or ink; avoid ball-point pen.