Thursday, September 10, 2020

Blue LEDs instead of zener diodes in V-USB

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.

zener diodes

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.

blue LEDs

This graph is the blue LEDs. Voltage are lower than with the zener diodes, but still within range.

Teensy 3.2

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.