So I thought up this cool little project during my last few weeks during my co-op term at Echologics Engineering. I saw everyone had nice, professional looking business cards, and I needed something to springboard myself into PCB design and manufacture. One and one came together, and this idea came about.
There were quite a few idea revisions in my mind before I actually got around to spinning the PCB. Microcontroller? Basic LED’s? No circuitry at all? Finally I got the idea of using a 555 timer (after seeing something about worldwide 555 timer competitions on the EEVBlog) that would be outputting a clock to LED’s, which would flash depending on some external interaction to the timer.
First idea: a photoresistor of course! The external RC circuit worked perfectly in ambient light with a simple 10k photoresistor. I quickly ran into an issue though; if I wanted to use a photoresistor, I would have to make my PCB through hole. I was not able to find one surface mount photoresistor. Therefore, I had to “fabricate” my own! How does one do that?!
Current in parallel with a normal resistor, of course! A phototransistor could act in place of the photoresistor, limiting the current in the RC circuit control for the 555 timer. Some issues with this, of course, is that phototransistors are quite expensive, and I managed to purchase opaque top photoresisors (which Digikey first sent as Red LED’s… D’OH)! After some trial and error with a scope and a breadboard, a working 555 timer, LED blinking, opto-frequency controlled circuit formed.
Then came the digitization! Here is the PCB layout and electrical schematic for my project. I was going to use SEEEDStudio’s Fusion PCB service to create my PCB’s, so I made sure I was using 14-14 spacing for traces. I also increased tolerances for pads and such, just to keep everything safe. Another folly I will learn from this project; don’t use diode type pads for surface mount protection diodes. These special diodes (that are very expensive mind you) are in the mail for tomorrow, and will be exceedingly difficult to solder on, when I could have easily used a much larger SOD package.
The idea behind the circuit is that you lay a 9 volt battery on the terminals, and the circuit then operates as expected (see video). The only issue is with the phototransistor, as it only operates very well in high intensity light (direct sunlight, or a “low” powered laser). It does vary frequency from 0-5 Hz with a bright light source.
The card features my personal info, as well as one of my favourite Tesla quotes on the backside.
And there it is! Just in time for interviews too, one of which I have tomorrow. I hope these turn some heads!
Here’s the card in action!