#134 Bidirectional Level Shifter Modules

Review and test some 2-channel and 4-channel bi-directional 5/3.3V level shifter modules.

The Modules

Notes

3.3/5V level-shifting modules are quite common, as the need to interface 5V microcontrollers with 3.3V devices/sensors (or vice versa) is pretty common. Often these are billed for I²C or SPI, although they are non-specific and can be used for level-shifting any kind of digital signal. Note: yes, digital high/low signals only; they do not provide an analog scaling of voltages.

See the LEAP#133 Bi-directional Level Shifter project, which demonstrates the basic FET trick used by these modules.

There are two types of modules commonly found for sale. I’ll test:

“2-channel” level-shifter modules: 2 pairs of input/output signals, using 1 FET for each pair
“4-channel” level-shifter modules: 4 independent input/output signals with an FET each

modules

Test Circuit Design

Designed with Fritzing: see Modules.fzz.

The test circuit establishes a 5V high-voltage and 3.3V low-voltage rail and simulates signals in both directions. the circuit comprises:

a controlling square wave, generated with LEAP#791 555 Breadboard Pulse Generator
the square wave toggles independent n-FET gates
- with drains pulled high with a 4.7kΩ resistor
- and parallel indicator LEDs
the high-side nFET drain is tied to the high-side level-shifter input
the low-side nFET drain is tied to the low-side level-shifter input
a 4-channel oscilloscope is used to monitor the 4 input output signals:
- CH1 (Yellow): signal 1 - low-side input
- CH2 (Blue): signal 1 - high-side output
- CH3 (Green): signal 2 - high-side input
- CH4 (Red): signal 3 - low-side output
The schematic shows separate 5V and 3.3V power supplies. In the breadboard experiments below however, I am using:
- 5V from a bench power supply
- 3.3V rail generated with a buck converter: LEAP#822 LM2596S 1.23V-30V Buck Converter Module

In the ideal case, input and output signals should be perfectly matched, with a full voltage swing on the high and low sides.

Breadboard

The Schematic

2-channel Level Shifters

I purchased a pack of 10 for US$3.39/lot (Jul-2015) “10PCS Two Channel IIC I2C Logic Level Converter Bi-Directional Module 5V to 3.3V” (aliexpress seller listing).

module1

The modules have two pairs of “transmit” and “receive” lines. It turns out that transmit and receive are handled differently:

TXD/TXI uses a standard MOSFET level shifting circuit, so is usable in either direction with accurate levels
RXD/RXI uses a simple voltage divider. This is OK for signals from high side to low side, but not in the reverse direction.

module1_internal_schematic

Testing 2-channel Level Shifters - Single Channel

First I’ll test bi-directional signals connected on the one channel:

TXD/TXI used for signal from low to high side
RXD/RXI used for signal from high to low side

module1_bb_build1

TXD/TXI, used for signal from low to high side, works perfectly:

CH1 (Yellow): signal 1 - low-side input, oscillates perfectly between 0-3.3V
CH2 (Blue): signal 1 - high-side output, oscillates perfectly between 0-5V

RXD/RXI, used for signal from high to low side, is not perfect but probably “good enough” in most cases (due to the resistor divider networks involved):

CH3 (Green): signal 2 - high-side input, oscillates between 0-~4.1V
CH4 (Red): signal 3 - low-side output, oscillates between 0-~2.2V

module1_scope1