Level Shifting#

Modern boards are rarely a single voltage. A 3.3 V microcontroller must talk to a 5 V sensor, a 1.8 V memory, or an I²C bus shared across domains, and connecting mismatched levels directly either fails to register a logic high or over-volts an input. Level shifting moves a signal safely from one voltage domain to another — and the right part depends entirely on the direction and the drive type.

Match the Part to the Signal#

• One-way, into a tolerant input — an 74LVC buffer with 5 V-tolerant inputs shifts a 5 V signal down to 3.3 V logic simply by being powered from 3.3 V; the tolerant input accepts the higher level. The single-gate 74LVC1T45 adds a direction pin for a push-pull line.
• Bidirectional open-drain buses (I²C) — the TXS0102 / TXS0108 are designed for this: they auto-sense direction and work with the open-drain, pulled-up signaling that I²C uses. Do not use a push-pull translator on I²C.
• Bidirectional push-pull — the TXB0104 / TXB0108 translate driven (push-pull) signals in either direction, for buses like SPI where both ends actively drive. They are not suitable for open-drain lines.
• A single line, cheaply — a lone N-channel MOSFET (the BSS138 circuit) level-shifts one bidirectional open-drain line; a resistor divider works for a slow, one-way down-shift where speed is not critical.

The One Rule#

Get the two questions right before picking a part: which direction does the signal travel, and is it push-pull or open-drain? Nearly every level-shifting failure comes from using a push-pull translator on an open-drain bus, or assuming a resistor divider can keep up with a fast edge. These families come straight out of the low-voltage logic era, where mixed-voltage design became the normal case rather than the exception.