Tested some blue LEDs in the aardvark
For low speed USB the data lines use 3.3 volt logic levels. 0-0.3v is a logic low and 2.8-3.6v is a logic high.
3.6v zener diodes are commonly used in V-USB hardware to clip the 5v output to ~3.3v. Blue LEDs can also have a similar effect.
I got this idea from the Sparkfun AVR Stick. I previously used it in my phantom keystroker.
The blue LEDs are installed in opposite polarity to how the zener diode would have been. I chose the lowest brightness (MCD), diffused LED I could find on Digikey. These are 3mm "ice cube" square LEDs. Not a common size. They are still quite bright.
This may not work with any random blue LEDs. They are being used in a way not intended.
Since the USB signal is differential D+ is high when idle and D- is low. So most of the time the LED on D+ will be lit and D- will be off. When data is being transmitted D+ will flicker low and D- will flicker on.
I hooked D+ and D- to a scope and the voltage levels are within USB spec. This graph is V-USB with 3.6 zener diodes. Green is D+ and Blue is D-. The minor graduations in the graph are 0.2v.
This graph is the blue LEDs. Voltage are lower than with the zener diodes, but still within range.
For comparison this is a Teensy 3.2 which has a hardware USB transceiver. The Teensy also transmits at a much higher rate than the slow software driven V-USB.
