The 3V3 pin is an output
The socket's 3V3 pin is not where you feed the 3.3 V side. It is the output of a regulator on the board, which makes 3.3 V from the header's 5V. So the TK97 needs 5V and GND and nothing else, a 3.3 V block in the socket runs on the TK97's own 3.3 V, and a 3.3 V board's 3V3 pin is never wired to it.
One supply in, one out
The regulator's input is the header's 5V pin, and its enable pin is on 5V too, so it runs whenever 5V is there. Its output is the 3V3 rail: the six transistors' gates, the six pull-ups on the L side, and the socket's 3V3 pin. Nothing else feeds that rail.
So the board needs 5V and GND, and that is all. Both sides then work: the 5 V side from 5V directly, the 3.3 V side from the regulator.
What that means for your wiring
With a 5 V board, such as an Uno, nothing changes from what you would guess: the Uno's 5V and GND to the header, and a 3.3 V block in the socket. The block's VCC meets the socket's 3V3 and runs on the TK97's regulator. An Uno's own 3.3V pin is not used.
With a 3.3 V board, such as an ESP32 or a Pico, there is one wire people get wrong. The board's signals go to L1 to L6 and its GND to GND, as you would expect. Its 5V pin, or VBUS on a Pico, goes round to the header's 5V. Its 3V3 pin goes nowhere: the TK97 already has 3.3 V.
The page this book replaces told you to wire the OUT side's 3V3 to "the 3.3 V device's 3.3 V supply". To a sensor, that is right: the socket feeds it. To a 3.3 V microcontroller, it ties two regulators' outputs together. Pick "Your 3V3 on it too" in the figure for what that does.
Different from the 27-pack's boards
The loose 2, 4 and 6-channel boards in the 27-pack have LV and HV pins, and you feed both: LV from your 3.3 V board, HV from 5 V. Power before signals in the llc book is about those. The TK97 has no LV pin. Its 3V3 looks like one and is the opposite.
When it does not work
Measure 5V against GND on the header. With no 5V there is no 3V3 either, the transistors' gates sit at 0 V, and no channel passes anything. On a Pico the 5 V pin is VBUS, which is 5 V only while the Pico is on USB.
Take it off. The socket's 3V3 is already powered by the board's own regulator, so that wire joins two regulators' outputs, which neither is made for, and gains nothing. Wire your board's 5V to the header's 5V instead.
No. It needs 5V: that is the regulator's input and the 5 V side's pull-ups' supply. With only 3.3 V pushed into the socket's 3V3, no H pin can go HIGH, and the regulator is fed backwards, which its data sheet does not cover.
3.3 V within 2 %, so 3.23 to 3.37 V, by the ME6211's data sheet. A meter reading in that range means the regulator is working.
The regulator is rated 500 mA; from 5 V its package gets too hot at about a third of that.
What the 3V3 pin can give →Edit this page — content/books/logic-level-converter/the-3v3-pin-is-an-output.mdx
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