Saturday, March 24, 2012

3V to 5V and 5V to 3V logic converters

Here is an adapter for converting up to 4 lines of 5V logic to 3V logic and vice versa.  Using two Texas Instruments SN74AHC125N logic buffers, one running at a 5V and one running at 3V, we have bidirectional converters useful for interfacing circuits running at different voltages.

There are three jumpers.  The top jumper has 5V, ground, and 3V inputs from left to right.  The left 8-port jumper is the inputs and outputs for 5V logic, with the inputs on top and outputs on bottom.  The right 8-port jumper is the 3V logic I/O with the outputs on top and inputs on bottom.  The 5V logic flows from left to right on the top four connections of left and right jumpers, and the 3V logic flows from right to left on the bottom four connections of right and left jumpers.

Careful!  Be sure to read the TI datasheet to review current limits, especially when overdriving the inputs on a 3V chip with 5V inputs.  It is allowed for this chip in the AHC logic family as long as you follow the rules.  This is not guaranteed to work with other families of logic chips.

Testing with 5V and 3.3V
Socket wiring

I/O flow

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