My projects typically involve many LEDs and microcontrollers. However sometimes I just love the simplest of things with LEDs.
One of the very basic LED circuits is a blinky using astable multivibrator. This is the circuit that I've made - with hand drawn and etched PCB and with incandescent lamps - when I was in high school for a PCB making contest and won. I guess it still has sentimental value for me.
Now with LEDs instead of incandescent lamps, and USB power instead of batteries, here's the USB Blinkly.
I integrated the USB plug into the PCB, so it's simpler as well as economical. You can plug USB Blinky into any USB receptacles - computers, chargers, everywhere.
Step 1: Circuit
The circuit is an ubiquitous astable multivibrator. I used NPN type transistors and used the super-bright LESs. As the values shown in the schematics, the blinking rate is about 2 times/second. You can adjust the R2,3 and C1,2 to achieve different speed. Larger the values, the slower the speed. I recommend keeping the values of R2 and R3 between 1k and 47k ohm. At larger resistance transistors do not get enough bias current to fully turn on the LEDs. At smaller resistance than 1k ohm, too much bias current flow into the transistors. Capacitors can be as small as 1uF (very fast blink, more like flicker), or as large as 1000uF (super slow).
Current draw from the USB port is slightly under 5mA.
Step 2: PCB
I made "built-in" USB plug right on the PCB. In order to fit nicely into USB ports, I used thicker (2mm) material for the PCB. This seems to work quite well, snug enough to give good electric connection, but not too tight.
However if you find your USB Blinky to be too lose in the connector, you can put some solder on the backside of the USB plug - there are two strips of pad that you can coat with solder. By putting a good amount of solder there you can adjust the fit of the USB plug.