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 set show=False. Instead, use the Drawing to create a new schemdraw.elements.ElementDrawing, which converts the drawing into an element instance to add to other drawings.

with schemdraw.Drawing(show=False) as d1:
    d1 += elm.Resistor()
    d1 += elm.Capacitor().down()
    d1 += elm.Line().left()

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

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.

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

class Resistor(Element2Term):
    def __init__(self, **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.

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.

Parameters and Defaults

Element subclasses may have an _element_defaults class attribute dictionary to specify default parameters used for drawing the element. This dictionary will be ChainMapped with the _element_defaults from all its parent classes into the Element.defaults dictionary the user may change to modify default behaviors.

To access any of these parameters when defining the element, use the self.params dictionary, which ensures the correct parameter, whether a default value, a default from a parent class, or a parameter overriden by the user, is obtained. Any non-None named arguments provided to the Element will be inserted into self.params automatically (by the Element.__new__ method).

Parameters that need to be set dynamically during instantiation should be set in the self.elmparams dictionary, so they may still be overriden by the user.

For example, consider the Dot element:

class Dot(Element):
    ''' Connection Dot

        Keyword Args:
            radius: Radius of dot [default: 0.075]
            open: Draw as an open circle [default: False]
    _element_defaults = {
        'radius': 0.075,
        'open': False}
    def __init__(self,
                radius: Optional[float] = None,
                open: Optional[bool] = None,
        fill = 'bg' if self.params['open'] else True
        self.elmparams['fill'] = fill
        self.segments.append(SegmentCircle((0, 0), self.params['radius']))
        self.anchors['center'] = (0, 0)

It contains two default parameters, radius, and open. The user may override these for every new Dot by setting Dot.defaults[‘radius’] = value. Or to override the defaults for a single instance of Dot, provide the parameter at instantiation: Dot(radius=value).

Inside the Dot.__init__ method, the fill parameter is determined based on the value of the open parameter, read from self.params[‘open’]. The Dot is filled when the dot is not open, but filled with background color (‘bg’) when the dot is open. Because the fill value was added to self.elmparams, the user may sitll specify their own fill color using Dot(fill=color).

Next, a SegmentCircle is added with radius taken from self.params[‘radius’], so that the default radius will be used unless overridden. Finally, an anchor named center is defined at the center of the dot.

When drawn, the parameters for the element are obtained from a ChainMap of the parameters in this order of preference:

  1. Setter methods like .fill() or .color() called after the element is instantiated

  2. Named arguments provided to Element instantiation

  3. Defaults set by the user in Element.defaults (inheriting from parent classes)

  4. Parameters defined in the Element attribute self.elmparams

  5. Parameters defined by Drawing.config

  6. Parameters defined by Schemdraw.config

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, **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__ and/or added to _element_defaults 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, **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:


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.

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

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()
schemfig = d.draw(), color='purple', ls='--'), color='orange', ls='-', lw=3);