Customizing Elements

Grouping Elements

If a set of circuit elements are to be reused multiple times, they can be grouped into a single element. Create and populate a drawing, but don’t call draw on it. Instead, use the Drawing to create a new schemdraw.elements.ElementDrawing, which converts the drawing into an element instance to add to other drawings.

d1 = schemdraw.Drawing()
d1 += elm.Resistor()
d1.push()
d1 += elm.Capacitor().down()
d1 += elm.Line().left()
d1.pop()

d2 = schemdraw.Drawing()   # Add a second drawing
for i in range(3):
    d2 += elm.ElementDrawing(d1)   # Add the first drawing to it 3 times
d2.draw()

Defining custom elements

All elements are subclasses of schemdraw.elements.Element or schemdraw.elements.Element2Term. For elements consisting of several other already-defined elements (like a relay), schemdraw.elements.compound.ElementCompound can be used for easy combining of multiple elements. Subclasses only need to define the __init__ method in order to add lines, shapes, and text to the new element, all of which are defined using schemdraw.segments.Segment classes. New Segments should be appended to the Element.segments attribute list.

Coordinates are all defined in element cooridnates, where the element begins at (0, 0) and is drawn from left to right. The drawing engine will rotate and translate the element to its final position, and for two-terminal elements deriving from Element2Term, will add lead extensions to the correct length depending on the element’s placement parameters. Therefore elements deriving from Element2Term should not define the lead extensions (e.g. a Resistor only defines the zig-zag portion). A standard resistor is 1 drawing unit long, and with default lead extension will become 3 units long.

Segments include schemdraw.segments.Segment, schemdraw.segments.SegmentPoly, schemdraw.segments.SegmentCircle, schemdraw.segments.SegmentArc, schemdraw.segments.SegmentText, and schemdraw.segments.SegmentBezier.

The subclassed Element.__init__ method can be defined with extra parameters to help define the element options.

In addition to the list of Segments, any named anchors and other parameters should be specified. Anchors should be added to the Element.anchors dictionary as {name: (x, y)} key/value pairs.

The Element instance maintains its own parameters dictionary in Element.params that override the default drawing parameters. Parameters are resolved by a ChainMap of user arguments to the Element instance, the Element.params attribute, then the schemdraw.Drawing parameters, in that order. A common use of setting Element.params in the setup function is to change the default position of text labels, for example Transistor elements apply labels on the right side of the element by default, so they add to the setup:

self.params['lblloc'] = 'rgt'

The user can still override this label position by creating, for example, Transistor().label(‘Q1’, loc=’top’).

As an example, here’s the definition of our favorite element, the resistor:

class Resistor(Element2Term):
    def __init__(self, *d, **kwargs):
        super().__init__(*d, **kwargs)
        self.segments.append(Segment([(0, 0),
                                      (0.5*reswidth, resheight),
                                      (1.5*reswidth, -resheight),
                                      (2.5*reswidth, resheight),
                                      (3.5*reswidth, -resheight),
                                      (4.5*reswidth, resheight),
                                      (5.5*reswidth, -resheight),
                                      (6*reswidth, 0)]))

The resistor is made of one path. reswidth and resheight are constants that define the height and width of the resistor zigzag (and are referenced by several other elements too). Browse the source code in the Schemdraw.elements submodule to see the definitions of the other built-in elements.

Flux Capacitor Example

For an example, let’s make a flux capacitor circuit element.

Since everyone knows a flux-capacitor has three branches, we should subclass the standard schemdraw.elements.Element class instead of schemdraw.elements.Element2Term. Start by importing the Segments and define the class name and __init__ function:

from schemdraw.segments import *

class FluxCapacitor(Element):
    def __init__(self, *d, **kwargs):
        super().__init__(*d, **kwargs)

The d and kwargs are passed to super to initialize the Element.

We want a dot in the center of our flux capacitor, so start by adding a SegmentCircle. The fclen and radius variables could be set as arguments to the __init__ for the user to adjust, if desired, but here they are defined as constants in the __init__.

fclen = 0.5
radius = 0.075
self.segments.append(SegmentCircle((0, 0), radius))

Next, add the paths as Segment instances, which are drawn as lines. The flux capacitor will have three paths, all extending from the center dot:

self.segments.append(Segment([(0, 0), (0, -fclen*1.41)]))
self.segments.append(Segment([(0, 0), (fclen, fclen)]))
self.segments.append(Segment([(0, 0), (-fclen, fclen)]))

And at the end of each path is an open circle. Append three more SegmentCircle instances. By specifying fill=None the SegmentCircle will always remain unfilled regardless of any fill arguments provided to Drawing or FluxCapacitor.

self.segments.append(SegmentCircle((0, -fclen*1.41), 0.2, fill=None))
self.segments.append(SegmentCircle((fclen, fclen), 0.2, fill=None))
self.segments.append(SegmentCircle((-fclen, fclen), 0.2, fill=None))

Finally, we need to define anchor points so that other elements can be connected to the right places. Here, they’re called p1, p2, and p3 for lack of better names (what do you call the inputs to a flux capacitor?) Add these to the self.anchors dictionary.

self.anchors['p1'] = (-fclen, fclen)
self.anchors['p2'] = (fclen, fclen)
self.anchors['p3'] = (0, -fclen*1.41)

Here’s the Flux Capacitor class all in one:

class FluxCapacitor(elm.Element):
    def __init__(self, *d, **kwargs):
        super().__init__(*d, **kwargs)
        radius = 0.075
        fclen = 0.5
        self.segments.append(SegmentCircle((0, 0), radius))
        self.segments.append(Segment([(0, 0), (0, -fclen*1.41)]))
        self.segments.append(Segment([(0, 0), (fclen, fclen)]))
        self.segments.append(Segment([(0, 0), (-fclen, fclen)]))
        self.segments.append(SegmentCircle((0, -fclen*1.41), 0.2, fill=None))
        self.segments.append(SegmentCircle((fclen, fclen), 0.2, fill=None))
        self.segments.append(SegmentCircle((-fclen, fclen), 0.2, fill=None))
        self.anchors['p1'] = (-fclen, fclen)
        self.anchors['p2'] = (fclen, fclen)
        self.anchors['p3'] = (0, -fclen*1.41)

Try it out:

FluxCapacitor()

Segment objects

After an element is added to a drawing, the schemdraw.segments.Segment objects defining it are accessible in the segments attribute list of the Element. For even more control over customizing individual pieces of an element, the parameters of a Segment can be changed.

n = d.add(logic.Nand())
n.segments[1].color = 'red'
n.segments[1].zorder = 5  # Put the bubble on top
d.draw()

Matplotlib axis

When using the Matplotlib backend (the default), a final customization option is to use the Matplotlib figure and add to it. A schemdraw.Figure is returned from the draw method, which contains fig and ax attributes holding the Matplotlib figure.

d = schemdraw.Drawing()
d.add(elm.Resistor())
schemfig = d.draw()
schemfig.ax.axvline(.5, color='purple', ls='--')
schemfig.ax.axvline(2.5, color='orange', ls='-', lw=3);
display(schemfig)