well. regardless of how useful this is, it does make for a delightful fidget toy.
- simplicity itself: you need one transistor per LED, and a current-limiting resistor for each one too. that's it.
- sensitive: total voltage swing is tiny.
- extremely high input impedance (it's just mosfet gates)
- linearly antialiases itself for you
- can be built with crappy parts and will still work fine
- dubiously linear scale
- needs input at weird offset voltage
- scale varies with transistor characteristics
- sometimes self-oscillates
- we have no idea how it works with other transistors (lower thresholds and on-state resistances will make it behave differently)
- if one transistor fails, the rest of the stack above it will no longer work
we think it's best used for cases where you don't care if it's slightly wrong and are just using it as a visualizer, not a measurement display (and you are, or you'd just be using a digital driver, right?) and where you can hand-adjust it.
additional favorable aspect: you can build it in a corner of a breadboard using no jumpers. put the LEDs across the middle gap, the resistors from the top rail, and the transistors at the bottom with their sources in the ground rail, drains on each LED bus bar, and gates tied together in the other lower rail. perfect!
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