Often, it becomes necessary to interface an existing or new 5V AVR /
PIC / MCU project to new devices that use 3.3V logic, like memory cards,
sim cards, etc. Described here is a circuit that converts +5V to +3.3V
logic or +3.3V to +5V logic. This compact circuit makes it perfect for
embedded systems that need to interface with different logic levels. The
design is based on 74LCX 245 IC, which is a low voltage bidirectional
transceiver with 5V tolerant inputs and outputs, from Fairchild.
The 74LCX245 IC contains eight non-inverting bidirectional buffers
with 3-STATE outputs and is intended for bus oriented applications. The
device is designed for low voltage (2.5V and 3.3V) VCC applications with
capability of interfacing to a 5V signal environment. 74LCX245 IC is
fabricated with an advanced CMOS technology to achieve high speed
operation while maintaining CMOS low power dissipation
Schematic of the Logic Level Converter Circuit
The presented circuit features improvements that set it apart from
other published logic level converter circuits. The improvements include
on-board +3.3 low-drop voltage regulator, direction selection jumpers,
easy-to-use input/output terminals and power on indicator LED. The
circuit can be easily interfaced via standard jumper wires. The whole
circuit can be fabricated on a small piece of prototyping board, with
the help of an SOIC/SOP/SSOP – to – DIP adapter. Or you can prepare your
own SMD printed circuit board for the construction task. In order to
configure the circuit as +5V input and +3.3V output, the jumper (JP1)
must be set to position A -> B. For configuring the circuit as +3.3V
input and +5V output, the jumper (JP1) must be set to position B ->
A. The onboard +3.3V can be used to supply power to the connected +3.3V
device(s).
Logic Level Converter Features
- 8- non inverting bidirectional buffers
- On-board +3.3V/500mA low-drop fixed voltage regulator
- User only needs to supply dc power to +5V pin
- Power on LED
- Direction selection Jumpers
- Small and compact circuit
IC1 is configured to enable output (OE pin -pin 19 of IC1- pulled
down), and the default direction is A -> B (DIR pin -pin 1 of IC1-
pulled up by jumper JP1). You can manage the OE and DIR signals by
applying the desired logic value to these signals
Lab Note: Prototype was tested with 74LCX245 IC in
‘Thin Shrink Small Outline Package’ (TSSOP). The prototype on the common
circuit board was built with the help of a 20-pin TSSOP to DIP adapter.
MOSFET I2C Level Converter Circuit
he above circuit is built with the BSS138 MOSFETs and can be used when
you need only the an I2C level converter from 5V to 3.3V or from 3.3V to
5V.
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