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logic level converter/Two builds and a check/11. Check it with a meter
Two builds and a check · 11 of 11

Check it with a meter

Six readings with a multimeter tell you whether a converter is powered, the right way round and passing signals — before you suspect the sketch, the library or the part on the other side.

Updated 2026-09-114 min readlogic level converter

Before you start

Power both boards as normal. Set the meter to DC volts. The black probe goes on GND and stays there. On the TXB and TXS, that is the one GND pin, bottom right.

Four wires, six readings, one working converter
2CH / 4CH / 6CH · LV / VA
Board
Red probe on
Expect
3.3 V
Black probe
GND
LV is the 3.3 V board's supply, passed straight through. 0 V means LV is not connected. 5 V means LV and HV are swapped — unplug before anything else.

Power first

  • LV (or VA): 3.3 V.
  • HV (or VB): 5 V.

Anything else, stop and fix it — no signal test means anything until both are right.

Idle channels

On the MOSFET boards and the TXS, pull-up resistors hold each side of an idle channel at its own voltage: A1 reads 3.3 V, B1 reads 5 V. On the TXB, an idle channel can read anything. That is not a fault.

Drive a channel

This is the test that works on every board. Unplug A1 from your 3.3 V board first, so you are not shorting its pin. Then:

  • Tie A1 to GND. B1 should read 0 V.
  • Tie A1 to 3V3. B1 should read 5 V.
The 4-channel converter at an angle, four transistors in a row between two rows of gold pads, with A1, A2, LV, GND, A3, A4 along the near edge and the matching B pins along the far edge.
The meter checks one channel at a time. If A1 to B1 works, repeat with each channel you actually use.

If every reading is right and it still fails

Then the converter is wired correctly, and the problem is somewhere else. The usual suspects: a TXB on an I²C bus (TXB and TXS), a fast signal on a MOSFET board (the slow rising edge), or the sketch itself.

When it does not work

LV reads 5 V

LV and HV are swapped. Unplug the power now: the A side is putting 5 V on your 3.3 V board's pins. Swap the two wires, then check the 3.3 V board still works.

A1 on a MOSFET board reads about 0.6 V

The 5 V side is holding the line LOW and LV is missing, so only the diode inside the transistor is conducting. Check the LV wire.

An idle TXB channel reads something different every time

That is normal. The TXB has no pull-ups, so a channel with nothing connected holds whatever level it last had. Test a TXB by tying A1 to GND or 3V3 and reading B1.

B1 does not follow A1 on a TXB or TXS board

Check OE first. If it is wired to GND, every channel is off. Then check that VA and VB are both there — either chip switches its outputs off when a supply is at 0 V.

Alongside this page
What the meter is measuring when it says 3.3 VThe three power wires, and OE
Where this goes next

The specs for all five boards, the table of which board for which job, and the questions people ask first.

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