I2C data detector gadget?

[e100]

Is there anything equivalent to the old neon in a screwdriver device that was used to show the presence of mains voltage.

I just want to visually show the presence of data on a I2C bus, the problem being that the pulse trains can be very brief so it needs a persistance of vision effect.

[Andy Chee]

Logic probe.

Usually they can detect HI, LO, or pulse train.

https://en.wikipedia.org/wiki/Logic_probe

Of course, if you want to distinguish an RS232 pulse train, from a SPI pulse train, from a I2C pulse train, then you need a protocol analyser.  And I don't know if they exist in "probe" form factor.

[Nominal Animal]

If I didn't have a logic probe, then I'd use a small microcontroller, perhaps an ATtiny.  (For quick checks, I use one of my microcontroller dev boards.)

If you want the indicator to have minimal effect on the I²C bus, then you need some kind of a buffer.  I might try a 74LVC2G17, which is dual Schmitt trigger with 5V tolerant inputs; the input currents are about a microamp, so insignificant considering the typical recommended I²C pull-ups of 1k-10k.

All in all, I'd expect the components for this, including the microcontroller, to cost about USD $1, definitely under $2, at Mouser in singles.