Week 5: Mechanical Design

This week’s task was to design an enclosure for our device using Autodesk Fusion 360 to be 3D printed, as well as to ‘miniturise’ our breadboarded prototype onto perfboard to produce a tangible wireless device as a finalised prototype.

planning

At the start of this sprint, our facilitator ran through an introductory demonstration of Autodesk Fusion 360 while we followed along. Because I’ve previously used SolidWorks, I felt that I could pick up on Autodesk quite quickly, although I had never actually printed anything designed in SolidWorks so I had to learn and keep into consideration the limitations of 3D printing – such as tolerances and minimum thicknesses.

Next I began to plan out my perfboard layout ready to begin soldering. As seen in the sketches below, the easiest method to go about doing this was to draw each component and the potential wiring required. To help optimise the space on the board, I decided to include two rails – one for 3.3V and one for ground.

Initial Layout Sketch
Optimising space planning

Final Wiring Diagram
autodesk fusion 360

After planning out my perfboard layout, I began to draft my Autodesk design. To do this I began by measuring my components with digital callipers in order to accurately draw each component to then insert into the one drawing for the board its self. Following Tim’s advice, instead of drawing each component from scratch, I utilised GrabCAD to find common components already made. In my case, I could only find the mini ESP8266. Fortunately, most of the studio students were using the same components so we were able to share our drawings between us to lessen the amount of work.

soldering the perfboard

To begin soldering my components onto the perfboard, I began to cut various pieces of breadboard wire into appropriate lengths. Initially I found the process confusing as I had never done this type of soldering before. After asking our facilitator for guidance, I learnt that the best method is to solder the wires perpendicular to the header pins. Expanding on this, I decided to try to only use headers for the mini ESP8266, as it was the only component which requires them, and to only use the breadboard wires to make connections for the other three components. During soldering, I learnt very quickly that small sections of wire will move as soon as the solder is melted around them. To overcome this I found that applying solder first, then using tweezers to position the wires was a much easier method to use.

Soldered Perfboard Top View
Soldered Perfboard Bottom View
the battery charger problem

During this sprint the main issue raised was that the IP5306 lithium battery charger board a few of us were going to use was not suitable for smaller batteries such as the one I was using. We could test this as when supplying the board with around 1-2 amps using the lab power supply, it would not limit the current and charge the lithium battery too fast. During this week we took many approaches to try and solve this issue as many of us did not want to change to a larger battery or a different charging board.

As a side tangent related to the battery, there was also the issue of supplying the charging board a power source. Originally we discussed utilising the micro USB port on the mini ESP8266, however our facilitator recommended an easier approach that would not require any diodes. Instead we added a DC power supply jack to be positioned on the outside of our enclosures to supply 5V to the charger board.

The first direction our facilitator took to solve the charger issue was to look at the datasheet as I learnt that usually lithium battery chargers are able to be “re-programmed” to different charge rates by changing a set resistor. After attempting to change what we thought was the correct resistor multiple times, and seeing no change to the current during charging, our facilitator did some more research and found that that particular board cannot be reprogrammed and is only designed for larger batteries.

After making this discovery, another idea which our facilitator suggested was to design a current regulator using transistors, thereby digging deeper into the issue and tackling it at a lower level.

Block diagram of potential method to solve charger issue using transistors

While I was trying to understand transistor circuits, our facilitator brought up another suggestion. After a closer inspection of my battery, he saw that it might potentially have a limiter circuit already attached to it. Following this, we conducted some more testing of my battery alone to see if this was true.

By connecting a current probe at both the output of the charger board and the battery output, and viewing the current on the DSO, we were able to see how much current was passing through the circuit. We were able to force max discharge of the battery using large resistors, and by reading the LP553436 datasheet I found that the discharge limit was 1260mA, meaning that the board’s output was staying within the correct range.

As for charging, my battery’s charge limit was 630mA and by supplying the board with a few amps we could see on the DSO that it remained under 630mA. Therefore this meant that my battery had its own built in current limiting circuit and there was no need to make any additional changes.

In retrospect after this fiasco, it is sometimes better to not go to lower level solutions as it is easy to get stuck and not make any progress when trying to solve an issue. Especially for someone like me with limited experience, I found it very challenging to tackle lower level strategies such as designing circuits using transistors. This sprint has definitely taught me to first take higher level approaches and I will keep this in mind for the last step of my project – calculating the BPM value from my ECG data.

building the enclosure

After double checking my measurements of my perfboard, I began to draw up my enclosure on Autodesk. Following the last road-block, this process was very streamlined. I also learnt many more skills in Autodesk including using projection and adding extruded text to surfaces. After printing my enclosure, the final steps will be to produce a clear acrylic lid using the laser cutter and to added double sided foam tape to the underside of my board to hold everything in place.

S U M M A R Y

accomplished

– miniturised prototype on perfboard
– 3D printed enclosure


learnt

– Learnt basics on Autodesk Fusion 360
– New soldering technique
– Sometimes it is better to take a higher level approach to solve problems
– How to use the 3D printer


what next

– Calculate BPM value from ECG data and publish to MQTT
– Produce lid for enclosure using the laser cutter


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