Tuesday, September 25, 2018

So You Want To Build An Electronics Project?

So you want to do an electronics hardware project at CEDT (or your own place)? You are welcome! However, before undertaking a project, it would be prudent to take the following points in consideration at all times during project planning and development.

1. Aim, objective and deliverables: What is the aim of the project? What is expected from it? Writing this down in black and white is important and helps during the execution phase of the project so that one does not veer off the original objectives. The deliverables should include the final project in physical form, any prototypes if they exist (and have not been cannibalized), record of the testing strategy and test data, a comprehensive report with suitable photographs and possibly a video of the working project. One can also estimate the total time it would take to complete the project as well as time for each part of the project. One can map this on a Gantt Chart for checking dependencies between various parts of the project.

2. Visualization: How do you visualize your project. Visualization of the physical form helps in better planning towards the project. Visualization is often helped by drawing sketches, keeping in mind the physical dimensions of the components expected to be used in the system as much as possible. One can also use some CAD tool to help in visualization. Such tools have the added benefit that they let you visualize from various angles.

3. Schematic and PCB layout:
Electronics will remain the most important part of projects at CEDT and therefore it is very important that great care be exercised in the issues related to schematic capture and PCB layout. We use EagleCAD for schematic and PCB layout. We also have a parts library named ?CEDT? although it may need substantial inputs to include new and emerging parts and more activities related to the use of this software so that everyone is competent in creating quality schematics and PCB layouts. The user is expected to create a Bill of Material (BOM) file during the completion of the schematic and layout files. EagleCAD allows the user to export a BOM file and one could create a word document with further details added to the basic BOM file exported from EagleCAD. The PCB layout should have sufficient space for all the components and should have holes for mounting screws in an enclosure. A separate and detailed note on the aspects related to schematic and PCB layout can be referred to, for details.

(Example of a badly drawn)

(Better schematic)

(Example of a good PCB layout. Notice the mounting holes on the four corners)

4. Circuit Fabrication: Any electronics project can be implemented in 4 ways. Each of the methods has their own advantages and disadvantages as listed below:

* Prototype on a breadboard: The breadboard prototypes should normally be avoided. This method allows fastest testing of an idea but suffers from several issues such as connection problems that can compromise the testing and take more time to fix than anticipated. Breadboard testing that involves high impedance or high frequency or low level signals can be very daunting and challenging. One of the challenges of prototyping with this method is the difficulty/inability to use surface mounted devices (SMD) because of the much smaller pitch as well as lack of insertable pins on such components. SMD components do not have pins but pads that cannot be inserted in a breadboard.

* Prototype on a general purpose zeroboard: This is a good way to test your project. However, it can be a challenge to solder SMD components on a zeroboard. Also, use this approach if you plan to just test an idea that cannot be tested with a breadboard approach and you plan to integrate this test circuit in a bigger circuit later on. Or if you plan to build only one single circuit with no plan towards mass duplication. Typically, the general purpose zeroboard also has a pitch of 0.1 inch and that makes use of SMD components as challenging as prototyping with a breadboard.

* Prototype using a PCB made in the lab: This is a standard practise in the lab (although it has been observed that people rush mindlessly in making a PCB only to dump the PCB after fabrication because they discover some flaw in the design). Please refrain from making a PCB for the sake of a PCB. It wastes common resources and creates pollution that can be minimized. Usually, it is very easy to make a single sided PCB but making double sided PCB is not difficult either and usually involves making two single sided PCBs and then sticking them together, taking great care in ensuring that all the holes align properly.

* Prototype using a PCB fabricated through a PCB vendor: This approach is desirable if most of the components are very small feature size SMD components that cannot be soldered on a PCB made in the lab. This approach would take a lot of time and money before the PCB can be tested. Extreme care should be exercised before the PCB files are sent to the manufacturer for fabrication. Once sent, the exercise cannot be undone or aborted midway without a financial and time penalty.

5. Power Supply:
Electronics projects can be a simple circuit that has just one PCB and few components. But even the simplest of electronic circuit will need a power supply. During the testing phase, one may like to use general purpose power supply for testing. During this phase, it is recommended to measure the voltage being applied. It should be ensured that the polarity of the voltage being applied will not damage the circuit. The current consumed by the circuit should be noted. For low power circuits, the current consumption may be less than the least count of the power supply readout or what if the power supply does not have a display? One could use a digital multimeter (DMM) for recording the consumed current but often, the current reading capabilities on a DMM are compromised, then what? There are alternative methods like using a small series resistor together with a DMM to measure voltage across it as a measure of the current, could be used. The choice of this series resistor is critical and care should be exercised in selecting the value and power of the required resistor.

Eventually the project may need it?s own power supply. For the power supply, the source of energy, any step up/down mechanism and the voltage regulator, all these aspects need to be considered. Also, some projects may require non-stop uninterrupted operation and so ˇuse of a standby battery may be needed. All these points must be kept in mind while designing the power supply. One may also rely upon ready-made power banks to power their project. It is imperative to keep in mind that most power banks have an auto turn-off feature if the current being drawn off the power bank falls below some lower threshold and this may turn the project off.

6. Circuit Soldering:
Once the PCBs are ready, you should physically inspect the PCB for any possible shorts or open connections on the PCB tracks. Using a magnifier glass is often helpful. Start by soldering those components that have the lowest physical profile such as SMD components, resistors etc. Make sure that the soldered connections are done properly and do not exhibit any shorts with neighbouring pins. Check for dry solder joints too. Avoid using extra flux, as the solder wire has sufficient flux inside and when you melt the solder wire on the soldering pads, the flux is released to help clean the surface. For extremely dirty contacts, one may need additional flux and if that is the case, one should clean the PCB with suitable solvent after the soldering is completed.

7. System Wiring:
Sometimes, the project will involve connection to the outside world using wires. Such connections need to be made with great care specially the physical stress and strain the wiring will have to bear. It is strongly recommended that the wiring be securely anchored to the physical enclosure (or the PCB) just prior to the electrical connections being made. In the absence of this ˇanchoring, if the wiring experiences external stress and strain, it may lead to electrical short in the circuit.

8. Testing: Testing while the project is in progress is a good idea. You may test connections by visual examination. After all the soldering is completed and if the project allows, one should start by testing the power supply without engaging the rest of the components, specially ICs. This is easily doable in case the ICs are DIP version you have chosen to use a IC socket. If the system uses SMD components, it may not afford this possibility but certainly the supply voltage could be tested before the SMD components are soldered. One should check if the required voltage appear at all the power supply pins of the components and ICs? Check with a DMM and an oscilloscope. The other tests should include the following. Testing does not merely mean taking a reading. It means keep a written record of the readings.
* Measure the supply voltage at important points of the circuit and keep a record of this.
* Measure the quiescent current consumption. In fact, for CMOS circuits, there is a very well known and popular testing protocol referred to as IDDQ testing. Apart from quiescent current, measure the operating current in various states that the project might operate in. When driving motors or inductive loads, the transient currents when the load (motor etc) is switched on/off need to be watched.
* For timed circuits, measure the time using a stopwatch or an oscilloscope depending upon the resolution.
* Measure the frequency of the circuit if there is dominant frequency. For example, many circuits would employ a microcontroller or a Real Time Clock (RTC). Both these components require a crystal (the microcontroller may be working using an internal RC oscillator and it may not be directable readable), then one should record the frequency values.
* If the circuit employs op-amps, one could measure error voltage at the input terminals.
* If the circuit employs an astable multivibrator (using 555 IC or otherwise), measure the frequency.
* If the system uses a Pulse Width Channel (PWM) signal, measure the frequency of the PWM signal and ensure that it is according to circuit and system design requirements. If the circuit employs a signal filter then the filter should be tested independently for proper operation.

9. Enclosures: It may be imperative that your project is housed in an enclosure. Except for very simple projects, most projects should have a suitable enclosure. There are many ways to get a suitable enclosure for your project.
* Find a suitable ready-made enclosure. There are many vendors that sell such enclosures in a variety of sizes and ˇmaterials - plastic, aluminum etc.
* Make an enclosure using PCB stock, by soldering the PCBs together to build the walls etc. Such an enclosure also works great as a Faraday cage. Enclosures made with PCB material may not be very rugged or sturdy and may break easily.
* Use a 3D printer to build a custom enclosure. This approach is very good for prototyping. However, one must be aware of the large time it takes to 3D print anything.
* Make an enclosure using paper or cardboard. This may be acceptable in some situations.
* Build a custom enclosure using traditional materials such as wood, plastic or aluminum for which you must have suitable fabrication and machining tools and skills.
* Use food grade boxes for enclosures. These days a lot of variety of food grade plastic boxes are available quite inexpensively. This may be quite acceptable for prototyping.

Apart from suitable enclosure, one may need to puts text or other identifier markings on the enclosure. This helps identify the various input and output connectors, inputs and outputs. This is extremely important to avoid making wrong connections. One may use permanent markers or use decals transferred on the enclosures for a more professional outlook.

10. Documentation: Creating complete documentation of the project is an extremely important activity. The documentation should include the title, the motivation, the plan that was envisaged to undertake and complete the project. Further, the documentation should include all the technical details such as the block diagrams, electronic circuitry, the PCB layout, the testing protocol that was employed to test the system supplemented with relevant records and ˇpictures. If the project used a microcontroller, the details of the flowchart and the code would help. At the end, suitable references that were used in the project help a lot. Documentation could also include a video of the operation of the project.