How to Design a Circuit

Designing a circuit is usually a multi-step process. First, you start with the idea, next research the available components, and then having decided on the components, design it into a circuit showing how they will be connected. You can then prototype the circuit, making a temporary circuit before creating the final finished circuit. The final circuit could be built on an off-the-shelf board such as stripboard or made into a complete printed circuit board depending upon your budget and the complexity of the circuit.

Each of these stages can be repeated as necessary until you come to the final design. As you move through the stages, the potential cost increases both in terms of money and the time taken, so the earlier you identify any potential problems, the less it will cost. Don’t be afraid to go back to the start rather than trying to continue with a design that isn’t working.

The prototyping stage is a useful stage as this is when you get to see if the circuit you have designed is likely to work. This is often done using a breadboard. You don’t necessarily need to test every single component at this level, but you should test all the basic blocks. For example, when designing the four-light disco lights, I only tested one of the MOSFET switches, and the other three followed the same design, so I did not need to test them separately. The more comprehensive your testing, the better the chance of identifying any potential problems.

There is one additional step that is used in professional circuit design, which is to perform a circuit simulation. Circuit simulation involves computer modeling of the behavior between components to see how they will work together. It can be a complicated process and should not be required for the type of circuits you will be making at this stage.

The idea is something you can come up with yourself, although hopefully some of the ideas in this book may help with getting the inspiration. This will look at some of the sources of information, and the tools you can use help design a circuit.

Datasheets

When designing circuits, you need to understand the particular characteristics of the components we use. Components that look similar to each can behave in different ways when used in practice. To get this information, manufacturers provide a datasheet which explains the characteristics of their components. Interpreting the information from a datasheet is one of the key skills when designing your own circuit.

Datasheets for different types of components may be completely different, but there are a number of sections that are often included. Rather than covering just one type, I have created a few examples of the sort of thing that you may see on a datasheet. I suggest you download some real datasheets so that you get familiar with their content. The best place to download the datasheets is normally from your component supplier. Some companies such as Farnell/Element 14 provide links to the datasheets in the component listings, some other suppliers may include practical examples and tutorials, but some suppliers do not provide any information. If you don’t find what you are looking for, then search for datasheet followed by the component name using an Internet search engine.

The first thing in a datasheet is a descriptive title, the component model number, and the name of the manufacturer. This is normally followed by a paragraph explaining what the component is and where it is useful. In some cases, a single datasheet may include multiple components (such as the TSOP2438 infrared receiver which shares a single datasheet with 23 other similar receivers).

There is often a diagram showing the pin layout. Obviously, this will look different for a discrete component such as a transistor compared to an integrated circuit. For a transistor or similar, it may be a 3D picture labeling the leads, or in the case of an integrated circuit, it may look like the example shown in Figure 13-1.

This is the MCP3004 analog-to-digital integrated circuit that we used in Chapter 6. This has been chosen as it is a good example of the different types of pin labels that you may come across.

The numbers on the inside of the IC are the pin numbers. Pin 1 is indicated at the top left which is normally physically identified on the chip by a semicircle or small dot on the side with pin 1. CH0–CH3 are the input channels (analog inputs). NC means not connected; this normally means that it is not used and so the pin should be left without physically connecting to the rest of the circuit. Normally, GND is used to signify the pin that needs to be connected to the 0V ground part of the circuit. In this case, it has a separate ground connection for the digital and analog parts of the circuit indicated by DGND and AGND, respectively. An alternative is V_SS which is used by some manufacturers to refer to ground instead.

The pin marked CS/SHDN is used for chip select or to shut down the input. An alternative that you may see for some components is EN which stands for enable and normally means the same as chip select. You will see that there is a line above CS. This indicates that it is inverted, so a high input has the opposite meaning as the wording. In this case, “chip select” happens when it receives a low signal rather than a high signal.

The pins labeled as D_IN and D_OUT are the data in and out pins used to communicate with the Raspberry Pi using I2C. CLK stands for clock where a timing signal is required. The V_REF pin is used for the analog part of the circuit as a reference voltage that the input is compared against. Finally, the V_DD pin is connected to the positive power supply; on some circuits, this may be V_CC instead.

The pin layout may also show different variants of the component. This may include an option for a surface mount device (SMD) or through the hole device. You will normally want to avoid the SMD components when first creating your own circuits as they can be difficult to solder.

The next part of the datasheet is often a table of absolute maximum ratings, which are values that should not be exceeded. Going outside of these ranges will mean that the component may not operate correctly or may cause permanent damage to the component.

The following example in Figure 13-2 shows part of the information for the IRL520 MOSFET.

This shows the maximum voltages between different pairs of pins as well as the continuous and pulsed currents. Exceeding these values is likely to damage the MOSFET. Also note that some values may depend upon certain conditions, and in this case, the maximum drain current is less as the temperature increases.

There are then often several tables providing more information about the behavior of the component under certain conditions. For logic circuits, this will include the valid voltage ranges for a signal to be considered as a true or false; for a transistor, it will include the typical gain; and for an LED, it may include the luminous intensity. There may be other information that is useful under certain circumstances such as timing information or effective capacitance.

Sometimes it is not possible to put the information into a table as the information may vary depending upon the input, temperature, or other characteristic. In this case, the information may be presented as graphs or timing diagrams.

Another common section is a circuit diagram showing the internal characteristics of the inputs or outputs. These are used when deciding what additional components may be needed to connect the circuit to another component. For instance, if a circuit has an open collector output as shown in Figure 13-3 and then if connecting to a logic circuit, then it would need a pull-up resistor to ensure that the input to the next stage was not floating when the transistor was switched off.

Another example is the diagram in Figure 13-4 which is from the SN754410 H-bridge IC, showing the effective output circuit from the chip, including the protection diodes.

The datasheet may also include examples of how the component can be used in a circuit and often includes the dimensions of the component and how it connects to a printed circuit board.

Some datasheets will be more useful than others, but as you can see, it can be an invaluable source of information when designing a circuit.

Introduction to Fritzing

If you want to run Fritzing on something other than a Raspberry Pi, then you can pay to download the software, which will allow you to download the latest version and to help support future development of the software.

Designing a Circuit Diagram/Schematic

The breadboard view is the first tab for designing circuits in Fritzing, although I recommend starting with a circuit diagram, which is called a schematic in Fritzing. This allows you to design the circuit as you would like it to connect logically, and it will then guide you to creating the physical layout for a breadboard or PCB. The initial schematic view is shown in Figure 13-5.

The main part of the screen contains an editor area, which is a white background with a grid for laying out the circuit design. The area to the top right is labeled parts which contains the available components and circuit boards, and the inspector area at the bottom right will allow you to change any of the parameters relating to the current component.

In the parts selector, most of the standard components are included under the CORE tab. This includes common components such as a resistor and PCB, but also labels, wires, and even an entire computer in the form of the Raspberry Pi. The components can be dragged across to the edit area. If the component has some options available, then these can be selected in the inspector area. This can be used to select values such as resistor values and different component types such as THT (through-hole technology) or SMD (surface mount devices). When presented with the choice of THT and SMD, you will usually want to choose the THT components, which are components with leads that can pass through a printed circuit board or mounted into a breadboard. SMDs are very small components that are mounted to only one side of a PCB and are very difficult to solder by hand without additional tools. The inspector panel for a resistor is shown in Figure 13-6.

The components are then connected together by dragging the mouse from the lead of one component to the other. Each time you create a connection between the components, it is called a net. You can connect more components to the same net by right-clicking a connection and choosing add bendpoint. You could also connect to a net label (under the core components), and any other net labels with the same name are connected together. A complete circuit for the basic transistor LED circuit is included in Figure 13-7.

I haven’t included the Raspberry Pi on this diagram, although you can include one by dragging it from the core components. In this case, I have used a net label at the point where the circuit connects to the GPIO, instead of showing the Raspberry Pi on the diagram.

Creating a Breadboard Layout

Now that we have a circuit diagram, we can convert this to a breadboard layout by switching to the breadboard tab. If you do so, then it will likely look a bit of a mess, such as the diagram in Figure 13-8.

Not only does this look a mess, but for some circuits, components will be on top of each other hiding those below them. First, you should click the breadboard and use the inspector to set it to the appropriate type of breadboard. The most common breadboard is the half+ breadboard.

You can now drag the components to an appropriate position, rotating them as necessary. When you place a component onto the breadboard, the row of pins that it will connect to will change to green. The dashed lines represent the nets that we created in the circuit diagram. Clicking one of these will turn it into a wire that you can place as appropriate. You will also need to add connections for the positive and ground power supplies, which are normally connected to one of the appropriate red or blue strips along the breadboard (not all breadboards include the colored lines). The final layout is shown in Figure 13-9.

The resistor going to the base of the transistor would normally connect to the Raspberry Pi, but I have not shown the Raspberry Pi in this view.

Designing a PCB

Fritzing can also be used for creating a professional printed circuit board (PCB). In the past, it was expensive to get custom PCBs made, but it has come down considerably in price and can provide a professional look for your project. The price for PCBs varies depending upon manufacturer, lead time, and quantity. The quantity is an important factor as buying in bulk can be much cheaper, but you will want to test your PCB design first.

This is a very simple circuit, so it doesn’t really need its own PCB, but it is useful for illustrating the process of designing a PCB. Clicking the PCB tab shows a gray area for the PCB board. You will find all your components stacked up on the top corner which will need to be moved around.

First, set the size of the PCB and move the components to appropriate positions on the board. Before you go about working out how the copper will be laid out, you need to add some way of connecting to the power supply and to the Raspberry Pi. For the power supply, you can use one of the included power connectors, and for the Raspberry Pi, you may want to include a 30-way connector, but both these would be overkill for such a simple design, so I instead used three terminals, two for the power supply and one for the GPIO connection. I used the part for a terminal connector, but you could just solder wires directly to the pins rather than using a physical connector.

Under the core parts, look for connections and “Generic Female Header – 2 pins”. The number of pins can then be changed in the inspector tab. The PCB so far is shown in Figure 13-10.

The next stage is to add the copper tracks which go between each component. There is an Autoroute option, but you can do this manually similar to the breadboard layout using the dotted lines as a guide. There will be no connections shown to the connector terminal, so you will need to add these as well. Before you start dragging out the connections, you may want to specify which layer to run the connections on. While technically it doesn’t really matter whether these are on the top or the bottom, I normally prefer to put as many on the bottom as possible; this is more from my experience creating single layer boards in the past. To do this, look for the icon on the bottom that says “Both Layers” and choose “set bottom layer clickable”. Any tracks you draw will be orange, indicating that they are on the bottom; if you change it to the top layer, then they will be colored yellow.

Once the tracks are connected, you should have something looking similar to Figure 13-11.

If you now switch to the schematic view, you will see that the connector component has been added and that it has dashed connections to the power supply and resistor RB. This is one of the features of Fritzing as you can add a component in any of the different views, and it is added to all the others. You may want to tidy the other diagrams up by repositioning the connector and wires. Alternatively, if you didn’t want to show the wires, you can use the “Delete ratsnest line” to hide the dashed line. Take care when using this option as it is a useful way to check that you have connected up your circuits correctly.

It’s a good idea to rearrange the text labels for the components at this point and then label up the connections for J1. You can add text using the logo part, which can be used for adding any kind of text, not just logos. The updated PCB is shown in Figure 13-12.

This is almost ready to have made up as a printed circuit board. The final thing you should do to the design is to add a ground or copper fill. This will fill most of the PCB with copper, and in the case of a ground fill, it will be connected to ground which can help with avoiding electromagnetic interference. To add a ground fill, right-click the Gnd connection and choose Set Ground Fill Seed, and then from the Routing menu, choose to apply a ground fill. The entire board will then be filled in with copper, except for a gap around each of the connections. If you look at the bottom layer, you should see that the ground connection from the connector to Q1 is merged into the ground fill.

The PCB is now ready to be sent for manufacturing. Before actually sending it off, it is a good idea to perform some checks on the finished layout. First, run the Design Rules Check (DRC) from the routing menu, which will check for any common problems. Then export the PCB layout using the File ➤ Export for Production and choosing PDF. Print off the component layout and check that the components fit. Double-check that all the components oriented correctly on the PCB layout.

Once you’ve checked everything twice, check it once more before you send the files to the manufacturer. Remember the extra cost and time if there is a mistake. If you want to use the Fritzing lab (which funds the development of Fritzing software), then you can use the Fabricate button which will load a web page for submission. There are a couple of advantages to using Fritzing. The first is that you can just send the Fritzing file as it is, and the second is that they will perform some additional checks. It can be more expensive than some other manufacturers.

If you would rather send to a different PCB manufacturer, then you can export for production as extended Gerber files. You may need to fine-tune some of these files and check the submission rules, especially what files they expect to receive.

I didn’t actually have that circuit made as a PCB as it was easy enough to create on stripboard, but I have created a more complex PCB that I have used in one of my own projects. Figure 13-13 is a circuit that combines the disco lights and NeoPixel circuits onto a single board. This is a circuit that I have made up and used at a disco.

Powering the Raspberry Pi

The Raspberry Pi is normally powered from a dedicated power supply through either a USB-C or micro-USB connector. This is fine for normal use, but what if you want to include the Raspberry Pi into a complete project that already has a power supply? When I created the disco light project which runs on 12V, I didn’t want to have to add a second power supply for the 5V needed by the Raspberry Pi. You can make a power supply yourself; this is useful to understand how you can add a power supply directly into your own circuit. You can however buy more efficient solution that can be easily connected into the circuit.

78xx Linear Voltage Regulator

The 78xx series of voltage regulators are small components that can be easily included directly onto a PCB or stripboard circuit. The regulators take a higher voltage on their input and provide a constant lower voltage for their output. In this case, from 12V on the input to 5V for the output. The only extra components required are a capacitor at both the input and output to smooth out any noise. The circuit diagram is included in Figure 13-14.

The voltage regulator comes with a variety of different output voltages, which are denoted by the last two digits. For example, the 5V regulator suitable for the Raspberry Pi is the 7805. The input voltage needs to be higher than the output voltage and in the case of the 7805 needs to be at least 7.3V.

One disadvantage of a linear regulator is that the voltage difference between the input and output is turned into heat which wastes power. If this is supplying 600mA (a typical current draw for a Raspberry Pi 3) using a 12V power supply, then the amount of power lost would be 4.2 watts (7V dropped x 0.6A). This is quite a significant amount of heat, and a heat sink would be required to prevent the regulator overheating.

Buck Converter

A more efficient way of powering the Raspberry Pi from a 12V power supply is a buck converter, also known as a DC-DC converter or a DC switching regulator. I used one of these to power the Raspberry Pi used in the disco light project. The buck converter is normally bought pre-built on a printed circuit board. The output voltage can sometimes be changed using a trimmer built onto the PCB. You would need to use a multimeter to check the output voltage. A buck converter is shown in Figure 13-15.

A buck converter is available through various suppliers such as Amazon Marketplace or eBay. These are more expensive than the linear voltage regulator, and they take up more space but are better where power efficiency is concerned.

Caution

Always check the output from a buck converter before connecting it to your Raspberry Pi. I learned this from experience when the one I purchased gave too high a voltage, damaging the Raspberry Pi.

Designing More Circuits

In this chapter, you have looked at the information needed to design your own circuits. This has shown how datasheets provide the information needed when designing a circuit and how to extract the relevant information. It then looked at Fritzing and how it can be used to design circuits, including creating a schematic layout, designing a breadboard and stripboard layout, and finally creating a professional printed circuit board.

This chapter provides the basics for getting started with Fritzing, but only scratches the surface of what Fritzing can do. I encourage you to spend more time learning about Fritzing. There are also other circuit design software such as KiCad (which is open source software) and EAGLE PCB design software which is a commercial product but free to use for small circuits. Neither of these provides the ease of use and breadboard layouts that Fritzing does.

Through this book, you have learned about electronic circuits, about how to connect them to the Raspberry Pi, and the steps involved in designing your own circuit. With this information, you can now design your own electronic circuits to have the Raspberry Pi interact with the real world.

Look around at what others have made, come up with your own ideas, and design your own electronic circuits.