Cody Shaw, a first year Co-op Electrical Engineering student at the University of Waterloo, spun up these wonderful electronic business cards for his job-search: set a 9V battery on the contacts and vary the light on the photo-sensor and you get a wicked blinkenlights show!
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.
(via Neatorama)