Perhaps it’s a side-effect of getting older, but it seems like reading the color bands on blue metal-film resistors is harder than it was on the old brown carbon ones. So often the multimeter ...

This means that the three individual resistors can be replaced by one resistor of \({3.27}\Omega\). Adding resistors in parallel decreases the total resistance. The current has a choice of paths ...

In other words, the voltages around the circuit add up to the voltage of the supply. The total resistance of a number of resistors in series is equal to the sum of all the individual resistances.

The Bus Pirate solution involves adding external pull-down resistors, which increases board size but otherwise fixes the problem. The Raspberry Pi solution, meanwhile, is to enable an input buffer ...