Bidirectional I²C level shifter (BSS138)
A 3.3 V ESP32 and a 5 V I²C device share a wire that either side may pull low. One BSS138 MOSFET per line does it: when the 3.3 V side pulls low, the FET turns on and pulls the 5 V side low; when the 5 V side pulls low, the body diode drags the 3.3 V side down until the FET turns on too. Neither side ever sees the other’s high voltage.
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How it works
LV low: V_GS = 3.3 V, the FET is on (≈ 3 Ω) and the 5 V line falls to 0.5 mA × 3 Ω ≈ 2 mV above the switch.
HV low: the body diode pulls the source down to about 0.6 V, which gives the gate 2.7 V of drive, so the channel turns on and the low side follows to near 0 V.
Idle: V_GS = 0, the FET is off, each side sits at its own pull-up.
Try it on the bench
- Open both switches: each line returns to its own supply.
Parts on the bench
- 2 × Bench power supply
- BSS138 N-MOSFET
- 2 × Resistor
- 2 × Toggle switch
In the course
Embedded & IoT, module “Buses and sensors”.