Sketching circuits
Before the program there is the pencil. A clear sketch solves half the problems of a design, and it teaches you to think of the circuit in blocks before drawing a single wire.
01Why sketch before opening the program
A sketch is a freehand drawing, proportioned but not to scale, made to think. It does not compete with the CAD drawing: it comes before it.
- It is faster: you can try three arrangements in the time it takes to place one symbol on the computer.
- It forces you to decide the structure—which blocks there are and how they are linked—before getting lost in the details.
- It is useful for talking: a sketch on a piece of paper settles a workshop discussion in two minutes.
- It leaves a record of what is there: reverse-engineering the schematic of a piece of equipment without documentation always starts with a sketch.
A sketch is not measured off, but it respects proportions: if a part is twice as long as it is wide, it is drawn that way. And it carries the real dimensions written down, taken with a caliper or a tape measure. With those two things, you can later move to CAD without measuring again.
02Freehand technique
- Long strokes in one go, moving the forearm and not the fingers; no fuzzy lines made of short overlapping strokes.
- Mark the endpoints first with two dots and join them looking at the destination point, not at the pencil tip.
- A mental grid or squared paper to keep the angles square.
- Circles built from a square and its diagonals, marking the four midpoints.
- A soft pencil for construction lines and a firmer one for the final outline.
All of this applies equally to a mechanical sketch and to a circuit sketch. What changes is what gets organized: in the mechanical one, the views; in the circuit, the topology.
03Organizing a schematic
The same circuit can be drawn in endless ways, and all of them are electrically identical. But only some are understandable. These are the rules that turn a scribble into a readable schematic:
| Rule | Why |
|---|---|
| Signal from left to right | It reads like text: input, processing, output |
| Positive at the top, ground at the bottom | It shows at a glance where the power comes from |
| Only horizontal and vertical lines | Diagonals tire the eye and hide junctions |
| Minimize crossings | Every crossing is a chance for a misreading |
| Ground and power symbols | They avoid long wires that run across the drawing |
| Group by functional blocks | You understand what each part does before looking at values |
The three versions are electrically identical: only how they are drawn changes. Step through them and see what is gained in each one.
04Thinking in blocks
Before drawing a single component it is a good idea to make the block diagram: rectangles with the name of each function and arrows between them. For a regulated power supply they would be transformer, rectifier, filter, regulator and load; for a measuring instrument, sensor, signal conditioning, analog-to-digital converter, microcontroller and communication.
Besides organizing the sketch, it is the first page of the final documentation and the first thing looked at by someone who has to repair the equipment ten years from now. It takes five minutes and saves hours.
When the circuit grows, each block becomes a sheet of the schematic, and the signals that cross from one sheet to another are identified with labels. That is how any real project of more than twenty components is worked.
05Sketching parts and enclosures
For the mechanical side of the work—a cover that has to be drilled, a bracket that has to be bent—the sketch follows a different order:
- Choose the main view and draw its outlines with a light stroke.
- Add the other views, aligned, without losing the reference.
- Mark the axes and centers of the holes.
- Measure the real part and write down the dimensions, from reference edges and not as a chain.
- Only then go over the final outlines.
A sketch like this, made of an enclosure cover with a caliper, is enough for the workshop to machine it or to move it to CAD without having the part in hand again.
06In the lab
Take a badly drawn schematic from the internet—with diagonals and crossings—and redraw it applying the six rules. Compare how long each version takes to understand.
With the multimeter in continuity mode, reconstruct the schematic of a simple board. First a rough sketch following the traces, then a tidy one.
Sketch freehand the cover of an enclosure with all its holes and dimension it with a caliper. Hand it to another group and see whether they can manufacture it without further information.
07Common mistakes
- Drawing the schematic following the physical location of the components on the board.
- Using a ruler for a sketch. A sketch is freehand; if you need a ruler, it is a drawing.
- Sketching without writing down the dimensions and having to measure everything again.
- Fuzzy lines made of many short strokes.
- Leaving long wires that cross the drawing instead of using ground and power symbols.
- Starting with the details instead of with the block diagram.
08Self-assessment
Is a sketch drawn to scale?
No, but it is proportioned, and with the real dimensions written down.
In which direction is the signal flow drawn?
From left to right, with power at the top and ground at the bottom.
Why is it best to avoid diagonals in a schematic?
Because they make it harder to follow the conductors and make junctions ambiguous. With right angles the eye follows the layout effortlessly.
What is a block diagram and what is it for?
A diagram of functions and their links, without components. It organizes the design and is the first page of the documentation.
How do you sketch a circle freehand?
By first drawing a square and its diagonals, marking the midpoints of the sides and joining them with four arcs.
Why use ground symbols instead of a common wire?
Because they avoid long lines that cross the whole drawing and let you see the structure of the circuit. Electrically they are the same.
09Further reading
- Thomas E. French and Charles J. Vierck. Engineering Drawing and Graphic Technology. (Spanish edition, “Dibujo de ingeniería y tecnología gráfica”), McGraw-Hill, 1988. The chapter on sketching, with the freehand stroke technique step by step.
- Paul Horowitz and Winfield Hill. The Art of Electronics. 3rd ed., Cambridge University Press, 2015. Their own rules for drawing readable schematics, with examples of what not to do.
- Frederick E. Giesecke et al. Technical Drawing with Engineering Graphics. 14th ed. (Spanish edition, “Dibujo técnico con gráficas de ingeniería”), Pearson, 2013. Technical sketching and its relationship to the final drawing.