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Welcome to the hardware at Midnight Sun! This introductory boot camp is a continuation of Hardware 101 and will go through the basics of our flow for hardware design.
Getting Started
Please drop a message pinging @Hardware Lead start a post in the #hw-onboarding channel forum indicating that you’d like to start your onboarding process! Include the email you would like to have associated with your Altium account, or the email you already have associated with an account. If you do not yet have an Altium account, do not create one by yourself. We will create an account for you with access to the team licenses. Otherwise, you will have to request an educational license on your own.
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Installing Altium
[Coming Soon! Hopefully this was covered during in HW101.]
Navigating the UI
Most UI elements you will interact with are housed in “Panels”. They are synonymous with windows, you can resize them and drag them around, even off to a second screen. You can select which ones are visible using the panels tab in the bottom right corner.
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While you are in the properties panel, you can verify that the units are set to mils, and that the visible and snap grids are both set to 100mils. The default shortcut to change the grid is “g”, and it’s also displayed in the bottom left corner.
Circuit Diagram
During HW101, we assumed the outputs of the comparators had 2 states: high (output 12V) or low (0V). This is known as an “active high” output, meaning the output will be driven my a power supply. However, often times the outputs are “open collector” as it makes the design of the IC simpler. This type of output also has 2 states: floating (not connected to anything), or low (connected to gnd). With this type of output, a resistor is often used to “pull” the output high when the pin is floating. Otherwise, a floating pin is at an undefined state.
Since we also want to divide the 12V reference by 2 to feed in to the second stage comparator, we will connect the output of the first stage to the center of the resistive divider. Here the switch to ground represents the open collector output, either connected to ground or not (floating).
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Adding Existing Components
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We can start by adding a few standard components to our schematic. Search for the following components below from the components panel. Double clicking a result will lock it to your cursor, where you can then drag and place it onto the page. Note that searches are not caps sensitive, you can type everything in lowercase!
Note that for some components, there seems to be a bug where on some types of components, the “comment” property is visible. The common cases are resistors and LEDs. After double clicking a component from the panel, you can press tab to “pause” the cursor lock, and hide the comment in the properties panel. Un-pause by clicking the icon in the centre of the screen, and press space to rotate the component to update the position of the remaining visible properties.
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2POS Microfit: The Molex Microfit series is our standard connector. We’ll need 2 of these for battery input and output, with each connector having 2 positions (+ and -). You may find 2 results come up, we will use the normal upright one rather than the right angled (R/A) connector.
SOT23 Comparator OC: The comparators we will be using. These are open collector (OC), which means instead of the two output states being high (connected to power) or low (connected to ground), it is either not connected (floating) or low.
0603 Resistors: Grab the resistors needed for the resistive dividers. Note that in the example slides, a 1k/5k divider is used, but 5k resistors are not common. It can be swapped for 2k/10k, which has the same ratio. Websites like Voltage Divider Calculator (ti.com) can be used to calculate which standard value resistors will get you closest to the desired ratio. Try using the E24 series first, if nothing is a perfect fit, you can see if using E96 resistors will get you a lower error.
0.1uF 0603 Caps: These capacitors help to ensure the power rail is smooth, without any unwanted oscillations. Generally we want at least one per set of power/gnd pins. Since we have 2 comparators and each comparator only has 1 set of pwr/gnd, 2 caps are sufficient here.
Clear Red Green 0603 LED: To serve as an indication when our battery is outside operating conditions (over/under voltage).
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When a required part is not already in the library, you will have to add a new one yourself. In this case, you will spec out and create a resistor to add in series with the LED.for your board.
An LED requires some form of current limiting to prevent it from getting too hot and burning out. The simplest way of doing this is with a series resistor. Since the resistor is in series with the LED, the current flowing through both devices is the same. We can select the value of the resistor using Ohm’s law to achieve the desired current.
In this case, the supply voltage is 12V, and 2V is dropped across the LED leaving 10V across the current limiting resistor. The LED we are using has a maximum forward current of 20mA, but often times there is no need to go that high. You can pick the target current using the hour of the time you are reading this (ie. if its 7:30pm you can target 7mA). Add a note (right click the text icon in the schematic toolbar) to the schematic with your target current and calculations.
The ideal resistor value would thus be given by R = V/I. Using numbers from our example, 10V/7mA = 1429 Ohms. In many cases, such as with this example, a resistor with the exact desired value may not exist. Standard values from E24 and E96 resistor series are commonly used, and you can pick a close value from there. Standard Resistor Values: E3 E6 E12 E24 E48 E96. Values are available in all multiples of 10, ie 1.43 is part of the E96 series, so you will be able to find 1.43k, 14.3k, 143k… resistors.
For the purposes of creating a unique part, you can pick the closest value in the E96 series. However, in general for these types of applications where the value doesn’t matter too much as long as it’s within operating conditions, you’d pick a common resistor like 1k, 10k… to minimize the number of different parts on your board and simplify assembly.
Finding a part
Given that 1.43k is the closest standard value resistor to our target, we will find one to put on our board. DigiKey Canada is our main distributor for components. In addition to the value, we are looking for a few other specific properties that align with our standards for resistor selection, namely a 0603 surface mount package and a 1% tolerance. https://uwmidsun.atlassian.net/wiki/spaces/ELEC/pages/3348267009/Electrical+Standards#Resistors.1
I’ll use “1.43k 0603 1%” as my search query, apply the “in stock” and “exclude marketplace” filters, and sort by lowest cost. Generally you’ll pick the lowest cost part that meets all the requirements you’re looking for. You can click into a part you’re interested in, and if it looks, good, copy the supplier part number.
Adding Parts to our Library
In the components panel, you can hit create component, and select the type (resistor in this case).
In the bottom left corner, you can add a part choice. Search for the part using the supplier part number, and select the part. A prompt will appear asking if you want to auto populate part parameters from the online database. You can use this, but only for parameters matching the template (there is a toggle for this). Click the models and parameters tab, and make sure everything is unchecked. We have our own models and won’t be using those from the manufacturer part search.
The resistor name by default is the part number (MPN), but given that its difficult to know what “RC0603FR-071K43L” is when searching for resistors in our library, we have our own naming convention. https://uwmidsun.atlassian.net/wiki/spaces/ELEC/pages/3348267009/Electrical+Standards#Resistors The Digikey “description” follows this most of the time, but not always. You can copy it from there and modify it if needed. Since this is for practice, please append ONB to the start of the part name so we can clean it out easier. If you are warned that the name is a duplicate, you can add a random number to the name. Ie. “ONB 7 RES 1.43K OHM 1% 1/10W 0603”.
The symbol is already populated by the template, but since resistors come in different sizes the footprint is not. We have standard footprints for most of the common packages, including 0603. Click the dropdown arrow, select existing, and navigate to Managed Content → Models → Footprints → Chip → RES 0603.
After that, you are good to save to server by clicking the button in the projects pane. You can now search for the component and place it on your schematic.
Placing Components
Any time you have a component locked to your cursor in drag mode, you can:
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Immediately to the left of the wire icon are the port icons. You can right click to select between power and ground ports. Ground ports generally don’t need to be renamed, while power nets follow our Power Net Naming Standard. Since the battery voltage is variable, we will name it “VBAT”.
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Designators
All components have a letter designator indicating the type, along with a number to act as a unique identifier. At this point the number will be a “?”, so to assign them all numbers, you can use Tools → Annotation → Annotate Schematics. Click Update Changes List, and then Accept Changes → Execute Changes. Once you close the panels the schematic should be annotated.
Title Block
The last thing to do is populate the title block! When you have nothing else selected (clock on empty part of the schematic), you can access schematic parameters from the properties panel. Update the Title, Author, Revision, and use Tools → Annotation → Number Schematic Sheets to update the sheet number.
Validation
Validate the schematic for any errors using Project → Validate. There should be no errors or warnings in the messages panel! You can add some titles to make it look a little neater, here’s an example.
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You should now be ready to start the layout! If nobody has seen your schematic at this point, you should ask a lead to look it over before you start.
PCB Layout
Coming soon! You’re done the schematic already?? Wow!