This is part 2
IR thermometer v1 – the hardware
In my previous blog post I described my motivation to build my own body IR thermometer, included a few sentences about the principles and showed 2 prototypes using a ready-made development kit and a breadboard with PSoC 5LP and PSoC 4 devices. I also mentioned, that I wanted to create my first professionally made PCB.
As written before, my approach is to do incremental development, one step at a time. You will definitely see images of electronics made by an amateur. However, my goal is to learn at least a small bit of something new every time I am doing something, so feel free to comment what can I make better next time.
The design software
The first step to create a PCB is usually creating schematics. Why usually? I have a friend that has a good memory and a sense of routing signals, so he usually draws PCBs with up to ca. 300 components (yes, three hundred) in his design software straight away.
I am a fan of more planning (drawing schematics) and only after that, placing the components and drawing tracks. About 10 years ago, almost every amateur I met (physically or on the internet) used EAGLE. I never really got used to the controls, so I tried about 10 design programs and then found Target 3001! The fun part is, I tried EAGLE, KiCAD and some software packages offered by component distributors and ended up with Target 3001! again. I don’t want to make any advertisement and I have no interest in doing so, but I like the software for two things. First, it gets the job done quickly, the controls are quite intuitive for me. Second, it is being developed almost three decades by a family company. If you write them an email and ask for support, it is not unusual you get an answer from the founder or his son. Pretty rare these days.
It won’t be surprising how I started drawing. I found the schematics of the PSoC® 4 CY8CKIT-049 4200 Prototyping Kit, removed what wasn’t necessary and added all the components from the breadboard.
Just a few notes about the design:
J1 is a power connector and the idea is to power the device directly by 2 AA or AAA alkaline batteries.
J5 is a connector intended for UART. I might find it useful in the future, but it is not implemented at the moment.
There are two push buttons and one LED. The LED has no function and only one of the buttons starts the measurement. The other button has no function. Both buttons have unpopulated pin header in parallel with them. My idea was to embed the device into a box, but leave the possibility to control the thermometer without the box, too.
Here comes the full schematics. I know, it is kind of chaotic.
And the PCB I drew based on the schematics. My first PCB sent to a PCB manufacturer ever
I ordered the PCB at a smaller Czech company to show some support to local companies. My friend helped me to prepare the data (especially for the pool service, which makes the PCB cheaper). I thought exporting should be easy, but it is worth asking the manufacturer about formats etc. to avoid additional costs of manual adjustments at the manufacturing facility.
One week passed and this is what I got:
I also knew that I wouldn’t be able to solder the chip with my current soldering iron. So, this was a perfect excuse to buy a new one, with a finer handle and a tip. Plus, better solder and flux. I have seen many videos on YouTube and they have one thing in common. Professionals looking for a professional outcome use a good quality soldering iron, solder and flux.
By the way, have you seen this series of soldering in avionics industry? Must see for everyone looking for a professional soldering result. https://www.youtube.com/watch?v=0SnOW2VdCTI . I don’t say all should do it this way, but it is worth knowing all the steps.
Here is my soldering. On the left is the thermometer PCB, on the right is a simple practice board bought for a few Euros. I highly recommend practicing after longer periods of not soldering.
As you can see, there is a jumper wire between the power connector and ground of the remaining board. I made an error concatenating signals, which was discovered after assembly
And last, finalized assembly of version 1.
Did it work on the first power up? No – it took me a week to figure out I rotated the chip by 180 degrees. The chip has two circular markings at the edges and when I soldered the chip, I haven’t noticed it. The smaller one is the correct (and mentioned in the datasheet). Fun fact: according to the datasheet is the chip label irrelevant to the chip orientation.
Fortunately, I was able to remove the chip using a common hot air gun (not for soldering) and solder it again.
Did it work on the second power up? No – it took me another week to find a missing ground connection.
- If the design software features a design rule check, use it. If I had used it properly, I wouldn’t have soldered the jumper wire between two different ground signals.
- Use one size of components if possible. I mixed 1206 and 0805 resistors and capacitors. The less sizes, the better to maintain stock supplies.
- Do not play with mirroring the components on the PCB. I mirrored two connectors (serial port and programming header) and then had to solder them from the other side to keep the layout as printed on solder mask. I was fortunate with this 2,54 mm header, but other connector types would mean a useless PCB.
- Have a proper lightning and make sure all components are placed on the right places.
TO BE CONTINUED