3V Devices to 5V CMOS
4 connections:
74HCT125 is a good cheap choice (14 pin package, quad 3-state buffer).
Power at 5V. Connect each of the 4 output enable pins to 0V. The min high input voltage is 2V so 3V3 compatible and the output state matches input state (it’s a straight buffer).
74HCT08 (quad AND gate).
Many other 5V CMOS chips with TTL level input buffers may be used
8 Connections
74HCT245 or 74ACT245 also provides the solution. The HCT & ACT provide an input voltage range of 0.8 – 2V @ 5V supply. As its 5V powered its output voltage range is good for the PIC. Note that an HC or AC will not work as the input voltage range is not the same.
5V CMOS to 3V Devices
A 74LVT245 powered @ 3V3 has 5V tolerant inputs so will do the job. It will only work in this direction though. A LVT may not be powered @ 5V.
A CD74HC4050 can be powered at 2V to 6V and will accept input voltages up to +16V.
74LVC16373 – 16 bit latch, 3V3 device which accepts 5V inputs
5V CMOS to 3V Devices Bi Directional
A 74LVX4245 will do the job and is available from Farnell. Port A is 5V, port B is 3.3V. T/R and OE can be driven from either side.
A 74ACT245 combined with a 74LVT245 should also do the job and is very slightly cheaper, but obviously takes more space.
Single Wire 1.8V To 5V
SN74LVC1G07 – Open drain buffer. Power at 1V8 or 3V (1.65 – 5.5V) and will accept +5.5V on its input or output.
Single Wire 5V To 1.8V
SN74LVC1G07 – Open drain buffer. Power at 1V8 or 3V (1.65 – 5.5V) and will accept +5.5V on its input or output.
Single Wire 3V To 5V
SN74AHCT1G126 – Buffer. Power at 5V, min high level input voltage 2V. OE high for output active.
SN74LVC1G07 – Open drain buffer. Power at 1V8 or 3V (1.65 – 5.5V) and will accept +5.5V on its input or output.
Single Wire 5V To 3V
SN74LVC1G126 – Buffer. Power at 3V. Max high level input voltage 5.5V. OE high for output active. (SN74LVC1G240 is an alternative inverting version)
SN74LVC1G07 – Open drain buffer. Power at 1V8 or 3V (1.65 – 5.5V) and will accept +5.5V on its input or output.
SN74AHC1G08 – Power at 3V3 and will accept up to +5.5V on its input.
1V8 to 3V/5V Bi Directional
74LVC8T245
Power VCCA at 1.65V to 5.5V
Power VCCB at 1.65V to 5.5V
DIR and OE are supplied by VCCA.
I2C Bus
PCA9517A provides I2C bus interfacing if you don’t want to use mosfets to do it:
Power VCC_A with 0.9V to 5.5V
Power VCC_B with 2.7 V to 5.5V
Tie EN to VCC_B to enable
Higher Voltage Interfacing
A 74HC4049 or 74HC4050 can be powered at 2V to 6V and will accept input voltages up to +16V (pin out is same, 74HC4049 just inverts the logic state).
4049 is basically the same but consumes a bit more current and has a min supply voltage of 3.5V.
A SN7417 is a 5V powered logic buffer which has open drain outputs that can drive loads up to 15V @ 30mA. SN7407 will do up to 30V @ 40mA and is cheaper
Logic To Darlington (open drain driving of higher voltages, relay switching etc)
ULN2803A